US2289491A - Brake mechanism - Google Patents

Description

July 14, 1942. E. FITCH BRAKE MECHANISM Filed March 28, 1941 Patented July 14, 1942 BRAKE MECHANISM Ellerylt. Fitch, Pittsburgh, Pa., assigner to Bendix-Westinghouse Automotive Air Brake Company, Pittsburgh, Pa., a corporation of Dela- Application March 28, 1941, Serial No. 385,743

19 .Claims This invention relates to fluid pressure apparatus and more particularly to a fluid pressure receiving system adapted for use in connection with fluid pressure braking systems for automotive vehicles.

In conventional fluid pressure receiving systems, the fluid compressor, mai-n reservoirs, and the auxiliary reservoirs are so interconnected that the discharge conduit of the compressor is freely open to the relatively large Volume of such reservoirs. Therefore,'it is necessary to operate the compressor asubstantially long period before the fluid supplied to the reservoirs builds up` to a pressure of sufllcient value to effectively apply the vehicle brakes. In order to safely operate` a vehicle utilizing such a system, it is obvious thatthe engine thereof must be operated to drive the compressor, while the vehicle is at rest., until the vfluid supplied to the system builds up to a value to maintain an application of the vehicle brakes.

Therefore, one `of the objects of the present invention is to provide a novel fluid pressure receiving system which voperates to overcome the inherent disadvantages of conventional fluid pressure receiving systems.

Another object of the invention is to provide a fluid pressure receiving system whereina source of fluid pressure for effectively applying the vehicle brakes is available afterthe fluid compressor is operated a substantially short time.

Still another object is to provide a fluid pressure receiving system wherein a source of fluid pressure, of sufficient volume and under sufficient pressure-to maintain an effective application of thevehicle brakes, is available after the fluid compressor has operated a substantially short time.

It has been found that the high temperatures and pressures presen-t during the compression of fluids cause a certain percentage lof moisture to be contained in the compressed fluid, and that such percentage increases as the pressure of the compressed fluid increases. Therefore, when the pressure of the compressed fluid is 10W, substantially little or no moisture will be present, while, on the other hand, when the pressure of the compressed fluid is high, a substantial amount of moisture may be contained therein. If the moisture is not removedv from the fluid pressure before the same is utilized in the system, the moisture may condense from the fluid and collect in the system, and thus cause parts of the system to function improperly.

Accordingly, a further object of the invention is to provide a fluid pressure receiving system wherein fluid under high pressures, having moisture therein, flows through a plurality of reser- A still further object is to provide a fluid pressure receiving system so constructed that fluid under pressures below a predetermined value, having substantially little or no moisture content, is supplied to one reservoir and utilized directly therefrom, while fluid under pressures, above said predetermined value, having a substantial moisture content, is utilized after passing through a plurality of reservoirs whereby the moisture is condensed therefrom.

A still further object is to provide a fluid pressure receiving system so constructed as to automatically isolate a source of fluid pressure when a rupture occurs in certain portions of the system.

Other objects and novel features of the invention will appear more fully hereinafter from the following detailed description when taken in connection with the accompanying drawing. It is to be expressly understood, however, that the drawing is employed for purposes of illustration only and is not designed as a definition of the limits of the invention, reference being had for this purpose to the appended claims.

With reference more particularly to the drawing, a fluid pressure brake system embodying the present invention is shown therein as having a fluid compressor I0 controlled by a pressure governor I I and driven by an engine of the vehicle, not shown, for supplying fluid under pressure to reservoirs I2, I3 and I4, connected in series relation by means of conduits I5, I6 and Il. A conduit Iii is provided for supplying fluid under pressure from the reservoir I2 to a brake Valve I9,

, preferably of the self-lapping type as disclosed in the patent -to W. J. Andres and R. S. Sanford, No. 2,133,275, dated October 18, 1938, for Control mechanism, which is adapted to control the flow of fluid under pressure toa pair of front brake actuators 20, 20 through conduits 2|y and 22 and to a pair of rear brake actuators 23, 23 through conduits 24 and 25. A control lever 26 is provided on the brake valve I9 to operate the latterto close the normally open connection between an atmospheric port 21 and conduits 2| and .24 and to open a lluid connection between the latter conduits and conduit I8.

One of the features of the invention is to provide, in `a fluid pressure receiving system, means for securing a source of fluid, of sufficient volume and under sufllcient pressure to maintain an application of the Vehicle brakes, after the fluid compressor has operated a substantially short time. Such an arrangement dispenses with the necessity of operating the compressor a substantially long time before the system is charged with fluid pressure of suillcient value to effectively apply the vehicle brakes. More particularly, the reservoir I2 is constructed to be of suilicient size to contain the proper volume of fluid, under a predetermined pressure, to supply energy to the brake actuators for maintaining at least one medium brake application. It is to be understood, however, that the reservoir I2 may bc constructed in Various sizes, depending upon the energy desired for instant use, during initial operation of the compressor I0.

During certain phases of operation of the system, namely, during initial operation of the compressor II), or when the fluid pressure discharged therefrom is below a predetermined value, the reservoir I2 is supplied with fluid under pressure directly from a discharge conduit 28 of the compressor I9. As shown, such fluid connection comprises conduit 29, a valve device 39, and conduits 3l and I1.

The valve device 30 is constructed to function as a check valve to prevent the flow of fluid under pressure from the reservoir I2 to conduit 28, and also to provide means for maintaining a differential between input and output pressures thereof, for a purpose described more fully hereinafter. More particularly, the valve mechanism includes a cylindrical casing 32 having one end closed and a cover plate 33 secured to the other end thereof, thus forming a chamber 34 in which a valve member 35 is adapted to reciprocate. Conduit 29 supplies-fluid under pressure to the chamber 34 above the valve member 35, while conduit 3I communicates with a port 36 in casing 32. Communication between conduit 29 and port 36 is normally prevented by the valve member 35, due to the action of a spring 31, positioned in the chamber 34, that holds the valve member in the position shown in the drawing. The spring 31 is compressed sufficiently to prevent downward movement of the valve member 35 until the pressure of the fluid in conduit 29 reaches a predetermined value. Fluted guides 38 formed on the valve 35 serve to guide the valve and permit equalization of pressure throughout the chamber 34.

In operation of the valve device 30, the valve member 35 remains in the normal position thereof shown in the drawing until the pressure of the fluid in conduit 29 builds up to a value greater than the compression of the spring 31. When such pressure is attained, the valve member moves downwardly against the spring 31, thus establishing a fluid connection between conduits 29 and 3|, through which fluid under pressure flows to the reservoir I2. The fluid under pressure thus supplied to the reservoir I2 is conducted to the chamber 34 to act therein against the valve member 35 to oppose the pressure of the fluid acting on the opposite side thereof. Since there are equal pressures exerted on opposite sides of the valve member 35, the spring 31 will move the valve member to the position shown in the drawing, wherein the fluid connection between the conduits 29 and 3| is closed. In order to move the valve member 35 downwardly to again form a fluid connection between the conduits 29 and 3I, the pressure of the fluid in the conduit 29 must be greater than the pressure of the fluid in the chamber 34 by an amount equal to the compression of the spring 31. Therefore, a differential will be present between the pressures of the fluid in the conduit 29 and in the reservoir I2 that will be substantially equal to the compression of the spring 31.

In order to permit flow of fluid pressure from reservoir I4 to reservoir I2, while preventing flow from reservoir I2 to reservoir I4, a check valve 39 of any suitable construction is interposed in the connection between conduits I1 and I6.

As heretofore stated, another feature of the invention is to provide means for supplying a reservoir, which is connected to the brake valve, with fluid under pressure through a plurality of reservoirs arranged in series relation, when the pressure of the fluid in the compressor discharge conduit reaches a predetermined value. As also mentioned heretofore, such an arrangement is advantageous since the fluid under high pressures is utilized from the system after the same has passed through a plurality of reservoirs whereby the moisture in the fluid is caused to condensate therefrom, and, therefore, dry fluid is supplied to the brake valve and subsequently to the brake actuators.

Such means, as shown, includes a pressure responsive device 48, which will be more fully described hereinafter, that is adapted to control the flow of fluid under pressure from a conduit 49, connected to the conduit 28, to a conduit 50 that forms a fluid connection with the reservoir I3. More particularly, the device 48 comprises casings 5I and 52 which are adapted to be clamped together with a diaphragm 53 therebetween, thus forming chambers 54 and 55. The conduit 49 is connected to the casing 5I and fluid under pressure therefrom is allowed to flow to the chamber 54. The conduit 59 is also connected to the casing 5I, but fluid communication between the conduit 50 and the chamber 54 is normally prevented by a valve member 55 which contacts a valve seat 56 and thus closes the end of the conduit 53. The valve member 55 is secured to the diaphragm 53 by any suitable means and is held in Contact with the valve seat 56 by the action of a spring 51 which exerts a downward force on the diaphragm 53. One end of the spring 51 is enclosed by a flanged plate 58 secured to the diaphragm 53, while the other end thereof is enclosed by another flanged plate 59. A member 69, threadedly mounted in the casing 52, has one end thereof contacting the plate 59 and the other end extending outside the casing and provided with a control member 6I so that the member 6E) may be rotated in one direction or the other to regulate the tension of the spring 51, and thus determine the pressure of the fluid that is required in the chamber 54 to move the diaphragm 53 upwardly to allow the flow of fluid under pressure from the chamber 54 to the conduit 50.

In operation of the fluid pressure system heretofore described, assuming that no fluid under pressure is present in the system and that the spring 31 of the valve mechanism 36 is compressed so that a predetermined pressure of fluid in conduit 29 is required before the valve operates to allow the flow of lluid under pressure to the reservoir I2, thereby maintaining a differential between the pressures of the lluid in the conduit 29 and the reservoir I2 that is equal to the compression of the spring, and that the spring 51 of the pressure responsive device 43 is so adjusted that a predetermined pressure of the fluid in the chamber 54 that is greater than the predetermined pressure referred to above with regard to the valve mechanism 3B is required to move the diaphragm upwardly to allow iluid communication from the conduit 49 to the conduit 50, it will be apparent that, as the compressor I9 begins to operate, fluid under pressure discharged therefrom is supplied to the valve mechanism 39 through conduits 28 and 29, and to the chamber 54 of thepressure responsive device. 48` throughfconduits 28.` and 49; When the compressor III has operated for a sufficient period to'increase the pressure of the uid in the conduit 28to a value greaterthan `the compression of the spring `31, the valveA device 30 will operate inY a manner fully described heretofore to forma uid connection between the conduits 291and 3|-, and fluid under pressure isthussupplied to thereservoir I2. The valve device 39, being in thenormal-position thereof, will prevent the ow of fluid under pressure from the reservoir I12'to` the reservoir I4, and, therefore, as the pressure of the fluid in the conduit 28f increases, the pressure of the fluid in the reservoir I2 will also increase. However, it will be understood that the pressure of the fluid in the reservoir I2 will be less than the ypressure of the fluid in the conduit 28 by an amount equal to the compression of the spring 31. Thus, during the initial operation of the compressor IS, fluid under relatively low pressure is supplied to the reservoir I2, the volume of which is such that the fluid contained therein will be ample to maintain at least one medium brake application which may be accomplishedY by operating the brake valve I9 to supply iiuid under pressure to the brake actuators from the reservoir I2.

When the pressure of thefluid in the reservoir I2 builds up to a sumcient value to maintain an effective application of the vehicle brakes, the .s

in the reservoir I2 by an amount substantially equal to the compression of the spring 3l of the valve 30.

When the pressure'of the fluid in the chamber 54 builds up to such value, the diaphragm 53 will move upwardly against the spring 51 to move the valve member 55 away from the valve seat and allow the iiow of fluid under pressure from the conduit 49 to the conduit 58, through the chamber 54. When the pressure responsive de vice 48 operates in the above manner, there will be no further increase in the pressure of the fluid in the reservoir l2 by way of valve tu. Fluid under pressure in the conduit 5) ows to the reservoir I3 and therefrom to supply fluid pressure to the conduit l5, the reservoir i4 and the conduit I6.

Asrthel compressor I0 continues to operate, the pressure of the fluid in the reservoirs i3 and lil is increased, and, when the same is' slightly greater in value than the pressure of the fluid in f,

the reservoir I2, the valve device 39 is caused to operate to allow the iiow of fluid under press-ure from the reservoir I4 to the reservoir I2 through conduits i6 and I'I. Further operation of the compressor will increase the pressure of the iluid in the reservoirs I2, I3 and I4 Iby way of conduit 5I), the differential pressure action of the valve device 33 preventing the flow of fluid under pressure to the reservoir I2 through the conduit 29.

When fluid under .pressure in the reservoir l2 is utilized for a brake application, the supply thereto will always be through the reservoirs I3 and I4, if the ypressure of the fluid in, the discharge conduit 28 is above a predetermined value which is necessary to maintain a fluid connection between the con-duits 49 and 5d, Since the valve device Sil' operates to maintain a difieren tial between the pressures of the fluid in the reservoir I2 and in the conduit 29, the flow of fluid pressure to the reservoir I2, by way of the conduit 29, will be prevented. because thepressure of the fluid is equal throughout the system after such predetermined pressure is present therein. Therefore; when the pressure of the fluid discharged vfrom the compressor Ilfisabove a predetermined va'lue, such fluid pressure is supplied tolthefbrakevalve I9 through a plurality of reservoirs arrangedlin series relation, whereby the moisture inthe fluid, present therein, when the pressureof the fluid. is high, iscondensed therefrom when` flowing through the series of reservoirs.

As discussed heretofore, means are providedfby the present invention for automatically isolating a source of fluid under pressure from the lsystem inthe event 'that sonie'connections therein might rupture. The valve devices (ifand 39 are so ,positioned withrespect to the reservoir I2, and are so constructed that, if a rupture occurs in` any part` of the system that supplies fluid under pressure to these valve devices, they automatically operate to prevent the flow of fluid' under Ipressure from the reservoir I2 through such rupture. Therefore, a source of fluid under pressure is retained that may be utilized for applying the vehicle brakes.

Thus, the present invention provides a fluid pressure receiving system that may be utilized in connection with iiuid pressure `brake systems for automotive vehicles that includes means for supplying fluid under low pressure, during the initial operation of the compressor, directlyV from the compressor to a reservoir designed to retain a sufficient volume of fluid under a predetermined pressure to maintain an effective brake application. Means Varealso provided for supplying fluid atgpressures above a predetermined value to the reservoir through a plurality of reservoirs connected in series therewith, and, at all times after the pressure of the fluid in the reservoir builds up to a predetermined value, fluid delivered tothe brake valve has the moisture therein removed by condensation while flowing through the plurality of reservoirs. The present invention also pro vides automatic means for isolating a source of fluid under pressure for brake applying purposes if a rupture occurs in certain parts of the system.

While only one embodiment of the invention has been shown and described herein, it is to be understood that the invention is not limited thereto but may be embodied in various forms as will `be' understood by those skilled in the art, Without departing from the spirit of the invention. Reference will, therefore, be had to the appended claims for a denition of the limits of the invention.

' What is claimed is.:

l.A In a fluid pressure system comprising a fluid compressor, a plurality of reservoirs, a plurality of lconduits. connecting said reservoirs in series relation, means for supplying uid under pressure from, said compressor to the first reservoir of said series during initial operation of said cornpressor, and means responsive to a predetermined pressure in said first means for thereafter supplying` fluid under pressure to said one reservoir through another reservoir of said series.

2. In a fluid pressure system comprising a fluid compressor, a -plurality of reservoirs, a plurality of conduits connecting said reservoirs in series relation, means for supplying fluid under pressure to one reservoir of said series during initial operation of said compressor, and means responsive to a predetermined pressure in said rst means for supplying fluid under pressure to said one reservoir through other reservoirs of said series.

3. In a fluid pressure system comprising av fluid compressor having a discharge conduit, a plurality of serially connected reservoirs adapted to receive fluid under pressure from said conduit, means for supplying one of said reservoirs with fluid under pressure when the pressure of the fluid in said conduit is between two predetermined limits, and means for supplying fluid under pressure to said one reservoir through another of said reservoirs when the pressure of the iluid in said conduit is above the higher of said limits.

4. In a fluid pressure system comprising a fluid compressor having a discharge conduit, a plurality of reservoirs adapted to receive fluid under pressure from said conduit, means for supplying one of said reservoirs with fluid under pressure, means responsive to a predetermined pressure in said conduit for supplying fluid under pressure to another of said reservoirs, and means for preventing the flow of fluid under pressure to said one reservoir from said conduit when the pressure of the fluid in said conduit is above said predetermined value.

'5. In a fluid pressure system having a fluid compressor, a plurality of reservoirs, and a plurality of con-duits connecting said reservoirs in series relation, means for supplying fluid under pressure below a predetermined value directly from said compressor to one reservoir of said series, means positioned in the conduit between said one reservoir and the adjacent reservoir of said series for preventing the flow of fluid under pressure from said one reservoir to other reservoirs of said series, and means for supplying fluid under pressure in excess of said predetermined value directly from said compressor to another reservoir of said series whereby said one reservoir is supplied with fluid under pressure above said predetermined Value through said other reservoirs.

6. In a fluid pressure system comprising a fluid compressor, a plurality of reservoirs, and a plurality of conduits connecting said reservoirs in series relation, a conduit connecting the compressor to a reservoir positioned at one end of said series, a second conduit connecting said compressor to another reservoir positioned at the other end of said series, means for preventing the flow of uid under pressure from the rst named reservoir to the other reservoirs of said series, means in said second conduit for normally preventing the flow of fluid under pressure to said second named reservoir, and means responsive to a predetermined pressure in said conduit for operating the last named means for supplying fluid under pressure above a predetermined value to said second named reservoir whereby said first named reservoir is supplied wit-h fluid under pressure above a predetermined value throughthe other reservoirs of said series.

7. In a fluid pressure system comprising a fluid compressor, a plurality of reservoirs, and a plurality of conduits connecting said reservoirs in series relation, a conduit connecting the compressor to a reservoir positioned at one end of said series, a second conduit connecting said compressor to another reservoir positioned at the other end of said series, means for preventing the flow of fluid under pressure from the first named reservoir to the other reservoirs of said series, means for supplying fluid under pressure below a predetermined value to said first named reservoir, and means responsive to said predetermined value for supplying fluid under pressure to said second named reservoir whereby said first named reservoir is supplied with uid under pressure above said predetermined value through the other reservoirs of said series.

8. In a uid pressure system comprising a plurality of reservoirs and a plurality of conduits connecting said reservoirs in a series circuit, a fluid compressor, means connecting said compressor to said circuit for supplying fluid under pressure to each reservoir positioned at the ends of said series, means positioned in the conduit connecting one of said end reservoirs to the adjacent reservoir for preventing the flow of fluid under pressure from said one reservoir to other reservoirs of said series, means for normally closing the fluid connection between the compressor and the other of said end reservoirs and means responsive to a predetermined pressure in said first means for operating the last named means to allow the flow of fluid under pressure to said other end reservoir whereby said one reservoir is supplied with fluid under pressures below said predetermined pressure directly from the compressorA and is supplied with fluid under pressures above said predetermined pressure through said other reservoirs.

9. In a fluid pressure system comprising a fluid compressor having a discharge conduit, a plurality of reservoirs, a conduit forming a fluid connection between said discharge conduit and one of said reservoirs, a second conduit forming a fluid connection between said discharge conduit and another of said reservoirs, means for allowing the flow of fluid pressure from said second named reservoir to said one reservoir, means positioned in said second conduit for normally closing the fluid connection to said second named reservoir, said last named means being responsive to a predetermined pressure of the fluid in said discharge conduit for opening the fluid connection to said second named reservoir whereby said one reservoir is supplied with fluid under pressures below said predetermined pressure and is supplied with fluid under pressures above said predetermined pressure through said second named reservoir.

l0. In a fluid pressure system comprising a fluid compressor having a discharge conduit, a plurality of reservoirs, a conduit forming a fluid connection between said discharge conduit and one of said reservoirs, means in said conduit for normally closing the fluid connection to said one reservoir, said means responsive to a predetermined pressure of the fluid in said discharge conduit for opening said fluid connection, a second conduit forming a fluid connection between said discharge conduit and another of said reservoirs, means in said second conduit for normally closing the fluid connection to said second named reservoir, the last named means being responsive to a second predetermined pressure of the fluid in said discharge conduit, greater than said predetermined pressure, for opening the fluid connection to said second named reservoir whereby said one reservoir is supplied with fluid under pressure when the pressure of the fluid in said discharge conduit is above a predetermined value and below a second predetermined value and said second named reservoir is supplied with fluid under pressure when the pressure of the fluid in said discharge conduit is above said second predetermined value.

11. In a fluid pressure system comprising a fluid compressor, a plurality of reservoirs, and a plurality of conduits connecting said reservoirs in series relation, means for supplying fluid under pressure to one reservoir of said series, means responsive to a predetermined pressure in said first means for supplying fluid under pressure to said one reservoir through other reservoirs of said series, and means for preventing the flow of fluid under pressure to said one reservoir through said first means when the fluid under pressure in the latter is above said predetermined value.

12. In a. uid pressure system comprising a plurality of reservoirs, a plurality of conduits connecting said reservoirs in series relation, and a fluid compressor having a discharge conduit, separate conduits` connecting said discharge conduit to the first and the last reservoir of said series, means for normally preventingthe flow of fluid under pressure from said discharge conduit to said last reservoir, means responsive to a predetermined pressure in said discharge conduit for operating said first means to allow the flow of fluid under pressure to said last reservoir, means for preventing the flow of liuid under pressure from said first reservoir to other reservoirs of said series, and means for maintaining the pressure of the fluid in said first reservoir less than the pressure of the fluid in said discharge conduit whereby said first reservoir is supplied with fluid under pressures below said predetermined pressure directly from said discharge conduit and is supplied with fluid under pressures above said predetermined pressure through said other reservoirs.

13. In a fluid pressure system comprising a fluid compressor, a plurality of reservoirs, and a plurality of conduits connecting said reservoirs in series relation, a conduit connecting the compressor to a reservoir positioned at one end of said series, a second conduit connecting the cornpressor to another reservoir positioned at the other end of said series, means responsive to a predetermined pressure in said first conduit for allowing the flow of fluid under pressure to said one reservoir, means in the conduit connecting said one reservoir to the adjacent reservoirs of said series for preventing the flow of fluid under pressure from said one reservoir to other reservoirs of said series, means in said second conduit for normally preventing the flow of fluid under pressure to said second named reservoir, and means responsive to a predetermined pressure in said first conduit greater than said first named predetermined pressure for operating the last named means to allow the flow of fluid under pressure to said second named reservoir whereby said one reservoir is supplied with fluid under pressure through the other reservoirs of said series when the pressure in the system reaches said second predetermined value.

14. In a fluid pressure system comprising a plurality of reservoirs, a plurality of conduits connecting the reservoirs in series relation, and a compressor having a discharge conduit, a conduit connecting said discharge conduit to the first reservoir of said series, a second conduit connecting said discharge conduit to the last reservoir of said series, means in said first conduit for maintaining a differential between the pressure of the fluid in said discharge conduit and in said first reservoir, means for preventing the flow of fluid under pressure from said first reservoir to other reservoirs of said series, means in said second conduit for normally preventing the flow of fluid under pressure to said last reservoir, means responsive to a predetermined pressure of the fluid in said discharge conduit for operating the last named means to allow the flow of fluid under pressure to said last reservoir whereby said first reservoir is supplied with fluid under pressure above said predetermined value through said other reservoirs, said first named means preventing the flow of fluid under pressure to said first reservo-ir through said first conduit when the pressure of the fluid in said discharge conduit is above said predetermined value.

15. In combination with a fluid pressure system having a plurality of reservoirs, a plurality of conduits connecting said reservoirs in series relation, and a fluid compressor having a discharge conduit, separate conduits for supplying fluid under pressure from said discharge conduit to the lirst and the last reservoir of said series, means for normally preventing the flow of fluid under pressure to said last reservoir, means responsive to a predetermined pressure in said discharge conduit for operating said first means for allowing the flow of fluid under pressure to said last reservoir, means for preventing the flow of fluid under pressure from said first reservoir to other reservoirs of said series, and means for preventing the flow of fluid under pressure from said discharge conduit to said first reservoir when the pressure of the fluid in said discharge conduit is above said predetermined value.

16. In a iluid pressure system having a plurality of reservoirs, a plurality of conduits connecting said reservoirs in series relation, means for supplying fluid pressure below a predetermined value directly to one reservoir of said series, and means for supplying fluid pressure above said predetermined Value to said one reservoir through the other reservoirs of said series.

17. In a fluid pressure system having a plurality of reservoirs, a plurality of conduits connecting said reservoirs in series relation, means for supplying fluid pressures below a predetermined value directly to one reservoir of said series, means for supplying fluid pressures above said predetermined Value to another reservo-ir of said series, and means for preventing the flow of fluid pressure through the first means when the pressure of the fluid is above said predetermined value.

18. In a fluid pressure system having a pair of reservoirs, means for supplying fluid pressure directly to one of said reservoirs when the supply pressure is below a predetermined value, and means responsive to an increase in the supply pressure above said predetermined value for supplying fluid pressure to said one reservoir through said other reservoir.

19. In a fluid pressure system having a pair of reservoirs and a source of fluid pressure for supplying fluid to said reservoirs, means for supplying fluid pressure directly from the source to one of said reservoirs, means operable in response to a predetermined increase of pressure at the source for supplying fluid pressure to said one reservoir through the other reservoir, and means associated with said first named means for preventing the supply of fluid pressure directly to the first reservoir when fluid pressure is being supplied thereto through the second reservoir.

ELLERY R. FITCH

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