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Hydroboost Braking System

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The Hydroboost System is used on many different chassis

* Dodge Motorhome Chassis (76 and later)
* Chevrolet/GMC P30 Motorhome Chassis
* Ford F53 Motorhome Chassis
* John Deere / Oshkosh / Freightliner Motorhome Chassis

The Hydroboost system utilizes power steering pump fluid pressure to operate booster. The booster assembly contains an open center spool valve which controls pump pressure magnitude during braking, a lever mechanism to control position of valve, and a boost piston to provide the force necessary to operate the master cylinder.  The booster unit also has a reserve system which stores sufficient fluid under pressure to provide at least two braking applications in case fluid flow from power steering pump is not available. Brakes can also be applied manually if reserve system is depleted.

Front Disk / Rear Drum

4 Wheel Disk

4 Wheel Disk with Drum Tag Axle
Note: Please refer to the Tag Axle Break Thread for information about that portion

ln this position, spool valve return spring holds spool valve open. In open position, spool valve provides unrestricted fluid flow between power steering pump and power steering gear. Fluid pressure is blocked from entering boost pressure chamber by lands on spool valve. As fluid pressure increases with steering demand, it has no effect on boost pressure chamber. Boost pressure chamber is vented through spool valve, to pump return port, and back to power steering pump.

As brake pedal is depressed, it moves pedal rod and initiates movement of spool valve. This closes fluid return port to pump from boost chamber, and admits fluid into boost chamber from pressure port. Additional valve movement restricts flow between pump and steering gear, causing pump to increase fluid pressure to maintain flow rate to steering gear. As fluid pressure increases in boost chamber, it forces piston forward actuating master cylinder piston, resulting in brake application. lf fluid pressure is required for steering while braking, pump pressure will rise and spool valve will shift in an open direction allowing more fluid to flow to steering gear.

1) System consists of a charging valve, accumulator valve, and a spring loaded accumulator. Accumulator is integral with booster unit. System is open to pressure port of booster unit. Charging valve has an orfice and ball check. Fluid from pump passes through orfice in the valve, and if pressure exceeds pressure in accumulator, it unseats ball check valve and enters accumulator. Ball check valve prevents reverse flow when accumulator pressure is greater.

2) Accumulator valve is a poppet type valve held closed by pressure stored in accumulator. An actuator on spool valve sleeve opens accumulator valve when a stop with no pump pressure is made that requires use of reserve pressure. Fluid pressure can also enter accumulator from boost chamber through accumulator valve, when boost chamber pressure exceeds accumulator pressure. A pressure relief valve vents accumulator to pump return port when pressure in accumulator exceeds approximately I600 psi.

Chevrolet & GMC (MHC Only)
With brake return spring installed, brake pedal should return hard into rubber stop, and master cylinder and pedal rod lever should be at lull return. Install pre·assembled brake pedal rod assembly (rod, end and boot). Adjust brake pedal rod to 3l.75" and adjust rod end until pedal free play is .06·,36”. Fasten boot to floor pan and compress to an installed height of 2.54". Pedal travel (with engine off and accumulator depleted) should not be more than 6" under a 90 lb. load,

Dodge ("M" Models)
l) Disconnect wiring at rear of stop light switch. loosen lock nut at switch and loosen switch until plunger is no longer contacting pedal. Disconnect pedal return spring. Loosen pedal stop lock nut and remove shouldered push rod end bolt and pedal return spring bracket.
2) Place a .010 - .0l5" spacer between pedal and stop. Turn stop in or out until shouldered bolt can be easily inserted in pedal and push rod. Install return spring bracket and tighten pedal stop nut. Remove spacer and connect return spring. Tighten stop light switch until plunger just contacts pedal and then turn an additional 2 I/2 turns. Tighten switch lock nut and connect wiring.

NOTE: Hydroboost cannot cause noisy brakes, fading brake pedal, or pulling brakes. If one of these conditions exists, other components of brake system are at fault.


Check fluid levels in master cylinder and power steering pump. Check power steering pump belt tension, and adjust if necessary. inspect all power steering hoses for leaks or kinks. lf fluid smells burned, check hoses or cooler for restrictions. Check and adjust engine idle speed. Check power steering hydraulic fluid for operation (shown by bubble in fluid), bleed system it necessary.

NOTE: lf problem cannot be found in preliminary steps, make following tests. lf unit is found to be operating properly, check areas of brake system that might cause condition. See Hydraulic Brake Trouble Shooting in this section.


Check hydraulic brake system for leaks or insufficient fluid in master cylinder reservoir. With transmission in Neutral, stop engine and apply brake pedal several times to deplete all accumulator reserve. Hold brake pedal depressed with medium pressure (approx. 25·35 lbs.) and start engine. If unit is operating correctly, brake pedal will fall slightly and then push back against foot. If no action is felt, hydraulic booster system is not operating.


1) Start engine and operate at idle speed. Turn steering wheel to either ”Stop" and hold in this position for a maximum of five seconds. Return steering wheel to center position and turn off engine. Depress and release brake pedal. Repeat procedure until ¤ hard pedal is obtained. There should be at least two power assisted brake applications with 20-25 lbs. pressure applied to brake pedal.
2) Restart engine and let it idle. Turn steering wheel to either "Stop". There should be a light hissing sound as accumulator is charged. Hold steering wheel lightly against "Stop" for a maximum of five seconds. Return steering wheel to center position and turn oft engine. Wait one hour and apply brake pedal
(DO NOT re-start engine before applying brake). There should still be at least two power assisted brake applications at 20-25 lbs. pressure before a hard pedal is encountered.

NOTE: If Hydroboost is not functioning, insure power steering system is operating normally before replacement or overhaul of Hydroboost unit.


Unit Removal
1) Depress and release brake pedal several times to be sure that all pressure is discharged from accumulator prior to disconnecting hoses from booster. Raise Motor Home Chassis on hoist; all others, leave on ground. Clean all dirt from booster at hydraulic line connections and master cylinder. Remove nuts that secure master cylinder to booster and support bracket. Support master cylinder, and cover exposed end with clean cloth.

NOTE: ln most cases, il is not necessary to disconnect master cylinder hydraulic lines to remove booster unit.

2) On all models except Motor Home Chassis, remove booster pedal push rod cotter pin and disconnect push rod from broke pedal ("C” & "G" models) or from booster bracket pivot lever ("P" models). Remove booster support bracket ("C" & "G" models) or support braces ("P" models). Remove booster bracket-to-firewall or support bracket nuts and remove booster assembly.
3) On Motor Home Chassis, remove cotter pin, nut and bolt that secure operating lever to vertical brake rod, Remove the six nuts and bolts that secure booster linkage bracket to front and rear support brackets, and remove booster from vehicle by sliding booster off rear support studs. Remove cotter pin, nut, and bolt that secures operating lever to pedal rod. Remove brake pedal rod lever nut and bolt and then remove lever, sleeve and bushings.

To install, reverse removal procedure noting the following:
Lubricate pedal rod and linkage pivot bolts,pins, sleeves and bushings with suitable lubricant (Delco Brake Lube 5450032).
Bleed booster/power steering hydraulic system.

NOTE: This is a separate procedure from bleeding hydraulic brake system. See Bleeding Hydroboost System in this Article. Check brake pedal and stoplamp switch adjustment.

DODGE ( Motorhome Chassis)

Depress and release brake pedal several times to be sure that all pressure is discharged from accumulator. Disconnect and plug hoses from booster. Remove master cylinder nuts ond lay master cylinder to side without kinking lines. Disconnect broke return spring. Remove brake pedal push rod bolt, remove mounting nuts and booster.

To install unit, reverse removal procedure. Tighten all nuts and hose connections. Bleed booster/power steering hydraulic system.

NOTE: This a separate procedure from bleeding hydraulic system. See Bleeding Hydroboost System in this article. Check brake pedal and stop lamp switch adjustment.


NOTE: lf power steering fluid has foamed due to low fluid level, it will be necessary to park vehicle for approximately one hour (reservoir cap loose) so that foam can dissipate.

1) Raise front of vehicle on o hoist so that tires are clear of floor. Check reservoir and fill with suitable power steering fluid (GM Power Steering Fluid).

NOTE: Leave reservoir cap off during entire bleed procedure. Install a remote control starter switch so that engine can be cranked but not started.

CAUTION: Whenever engine is cranked with a remote starter, or other means, distributor primary lead must be disconnected from negative post on coil.

2) Crank engine for four to five seconds while pouring fluid into reservoir. Fill reservoir and crank engine until system will no longer accept fluid. lt is normal that fluid may spill from filler tube when cranking stops. Remove remote control starter switch and reconnect distributor wire. Start engine and run for two seconds. Check and refill reservoir if necessary.
3) Start engine and depress brake pedal several times while rotating steering wheel from "Stop" to "Stop". Turn engine off and then pump brake pedal four to five times to deplete accumulator pressure. Check and refill reservoir if necessary.
Repeat step 3). Install reservoir cap.

DODGE (Motorhome Chassis)
1) Check power steering pump reservoir and fill with suitable power steering fluid (MOPAR Power Steering Fluid). Allow fluid to remain undisturbed for two minutes. Leave reservoir cap off during bleeding operation.
2) Start engine and run for ten seconds. Check fluid level and add fluid if necessary. Repeat procedure until fluid level remains constant. Raise front of vehicle and allow tires to clear floor. Start engine and run at l500 RPM. Apply and release brakes several times, at the same time turn wheels back and forth, lock to lock. Turn off engine ond check fluid level. Add fluid if necessary.
3) Lower vehicle. Start engine and run at l50O RPM. Apply and release brake pedal several limes, at the same time turn front wheels back and forth, lock to lock. Turn off engine and check fluid level. Add fluid if necessary. If fluid level is low, repeat bleeding procedure. Place cover on reservoir.
Hydroboost Unit Diagnosis
CONDITIONCAUSECORRECTIONExcessive Brake Pedal Effort    Loose or broken power steering pump belt.    Tighten or replace the belt. No fluid in power steering reservoirFill reservoir and check for external leaks. Leaks in Hydro-BoostReplace faulty parts. Leaks at Hydro-Boost tube fittingsTighten fittings or replace tube seats, if faulty. External leakage at accumulatorReplace "0" ring and retainer. Faulty booster piston seal causing
leakage at  booster flange ventOverhaul with new seal or input rod and piston assembly. Faulty booster input rod seal with
leakage at input rod end.Overhaul with new seal kit. Faulty booster cover seal with
leakage between housing and cover.Overhaul with new seal kit. Faulty booster spool plug seal.Overhaul with spool plug seal kit. Slow Brake Pedal ReturnExcessive seal friction in booster.Overhaul with new seal kit. Faulty spool action.Flush steering system white pumping brake pedal. Restriction in return line from
booster to pump reservoir.Replace line. Damaged input rod end.Replace input rod and piston assembly. CONDITIONCAUSECORRECTION Grabby BrakesFaulty spool action caused by
contamination in system.Flush steering system white pumping brake pedal. Booster Chatters
Pedal VibratesPower steering pump belt slips.Tighten belt. Low fluid level in power steering
pump reservoir.Fill reservoir and check for external leaks. Faulty spool operation caused by
contamination in system.Flush steering system while pumping brake pedal. Accumulator Leak Down
System does not hold chargeContamination in steering hydro-boost
systemFlush steering system while pumping brake pedal Internal leakage in accumulator system.Overhaul unit using accumulator rebuild kit and seal kit.Combination Brake Light Switch, Metering Valve, Proportional  Valve

Dodge Combination Valve

Chevrolet/GMC Combination Valve

The brake warning switch is used to indicate a failure in either front or rear hydraulic system. The metering valve is used to better balance the hydraulic system during certain braking conditions.

Brake Warning Switch Unit The brake warning switch used in these combination valves are latching types. If a pressure loss occurs in one side of the dual brake system the piston in the valve will move toward the failed side and latch in that position. This will cause the brake system warning light to come on and stay on until the brake system is repaired. After repairing and bleeding the brake system, applying the brakes with moderate force will hydraulically recenter the piston and automatically turn off the warning light.


 Testing Brake Warning Switch Unit

The brake warning light bulb function can be tested by starting the engine. The light will go on when the ignition key is turned to the extreme right START position and will go off as soon as the engine starts.
To test the service brake warning system, raise the car on a hoist and open a wheel cylinder bleeder while a helper depresses the brake pedal and observes the warning light. If the light fails to light, inspect for a burned out bulb, disconnected socket, a broken or disconnected wire at the switch. If the bulb is not burned out and the wire continuity is proven, replace the brake warning switch in the brake line Tee fitting mounted on the frame rail.
Metering Valve Unit (Rear Drum Brakes only)
The metering valve holds of hydraulic pressure to the front disc brakes in the 35 to 135 psi range to allow the rear drum brake shoes to overcome the return springs and begin to contact the drums. This feature helps prevent locking the front brakes on icy surfaces under light braking conditions. The metering valve has no effect on front brake pressure during hard stops.

Proportional Valve

The rear brake proportioner improves front-to-rear brake balance at high deceleration. During high deceleration stops, a percentage of the rear weight is transferred to the front wheels. Compensation must be made for the resultant loss of weight to the rear wheels to avoid early rear wheel skid. The proportioner part of the combination valve reduces the rear brake pressure and delays the rear wheel skid.

 ??? wow ???

Just an FYI for everyone. I've had issues with the brake pedal going to floor and not stopping hardly. I've bled/flushed the 4 disc brakes with no better stopping power. Today I bled the power steering pump per the instructions and I can stop better now! If I double pump them its better. I'll have to bled them again later. Also, I disconnected the tag axle vacuum booster at the rear of the motor and now my vacuum gauge stays at 16 psi when I hit the brakes ( that's a later project). But if you read all of the pages in the manual it WILL fix a lot of stuff.

Indeed..   Hm?



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