Power Brake Booster - Bendix Hydro-Boost Chevrolet Cab & Chassis K3500

DESCRIPTION

System uses power steering pump fluid pressure to operate booster. Master cylinder is mounted to output push rod of booster. Booster also has reserve system to provide at least 2 assisted braking applications. If reserve system is depleted, brakes can be applied manually.

BRAKES APPLIED

Pressing pedal moves pedal rod and spool valve. This closes fluid return to pump and admits fluid into boost chamber from pressure port. Additional valve movement restricts flow between power steering pump and power steering gear.

As fluid pressure increases in boost chamber, it forces piston forward, actuating master cylinder piston and applying brakes. If steering is required while braking, power steering pump pressure will rise and spool valve will shift open, allowing more fluid to flow to power steering gear.

BRAKES RELEASED

With pedal released, spool valve return spring holds spool valve open. Spool valve allows fluid flow between power steering pump and power steering gear.

Fluid pressure is blocked from entering boost pressure chamber by lands on spool valve. Boost pressure chamber is vented through spool valve, to pump return port, and back to power steering pump.

RESERVE SYSTEM

System consists of charging valve, accumulator valve and spring-loaded accumulator. Accumulator is integral with booster. System is open to pressure port of booster.

Charging valve has an orifice and ball check valve. Fluid from pump passes through orifice in valve, unseats ball check valve and enters accumulator. Ball check valve prevents reverse flow.

Accumulator valve is poppet-type valve held closed by accumulator pressure. If no pump pressure is available, an actuator on spool valve sleeve opens accumulator valve. Fluid pressure can also enter accumulator from boost chamber through accumulator valve, when boost chamber pressure exceeds accumulator pressure.

Pressure relief valve vents accumulator to pump return port when pressure in accumulator exceeds approximately 1600 psi (112kg/cm 2 ).

BLEEDING HYDRO-BOOST SYSTEM

1. Fill power steering pump reservoir and leave undisturbed for at least 2 minutes. Start engine and run momentarily. Add fluid if necessary. Repeat until fluid level remains constant with engine running.
2. Stop engine. Raise front wheels off the ground. Turn steering wheel from stop-to-stop. Add fluid if necessary. Lower vehicle.
3. Start engine. Press brake pedal several times while turning steering wheel from stop-to-stop. Turn engine off. Press brake pedal several times to exhaust accumulator pressure. Check fluid level. Add fluid (if necessary).
4. If fluid is foamy, let vehicle stand for several minutes, then repeat step 3). Air in system will cause fluid level to rise with engine off. Continue to bleed system until all air is expelled. If all air is not expelled after bleeding system, check power steering system for problem.

TROUBLE SHOOTING

For trouble shooting information, refer to trouble shooting charts. (Scheme 6)- (Scheme 9).

TESTING

Note. Inoperative Hydro-Boost cannot cause noisy brakes, fading brake pedal or pulling brakes. If one of these conditions exists, other components of brake system are at fault.

PRELIMINARY CHECKS

1. Check engine idle speed. Check all power steering hoses and brake lines for leaks or restrictions. Fill master cylinder with brake fluid. Fill power steering pump reservoir with power steering fluid.
2. Check for air in power steering fluid reservoir. Check power steering belt tension and condition. Check power steering pump pressure.

ACCUMULATOR LEAK-DOWN TEST

1. Start engine. Charge accumulator by applying brakes or turning steering wheel from stop-to-stop. Turn engine off. Allow vehicle to sit for one hour. Apply and release brake pedal. There should be at least 2 power-assisted applications. This indicates the accumulator is retaining a charge.
2. If accumulator does not retain charge for one hour but functions normally immediately after charging, replace accumulator valve. See «OVERHAUL»(/chevrolet/cab-chassis-k3500/1989-2000/remont/mechanical-hydraulic/#power-brake-booster-bendix-hydro-boost) . If accumulator can be heard charging and discharging but does not hold a charge, replace accumulator valve. See «OVERHAUL»(/chevrolet/cab-chassis-k3500/1989-2000/remont/mechanical-hydraulic/#power-brake-booster-bendix-hydro-boost) .
3. Discharge accumulator by pressing brake pedal several times. Attempt to rotate accumulator can in respect to housing. If it is possible to rotate can, accumulator has lost its gas charge. Replace accumulator assembly.

BOOSTER FUNCTIONAL TEST

With engine off, press brake pedal several times to deplete accumulator reserve. Press and hold brake pedal with about 40 lbs. (18 kg) pressure. Start engine. Brake pedal should fall slightly, then push back against foot. If no action is felt, booster is not operating properly.

Removal

1. Press brake pedal several times to exhaust pressure from accumulator. Clean dirt from hydraulic line connections at booster and at master cylinder. Disconnect hydraulic lines from booster and plug all openings. NOTE: In most cases, it is not necessary to disconnect master cylinder brake lines to remove booster.
2. Remove and support master cylinder. Disconnect booster push rod from brake pedal. Remove booster-to-firewall nuts from inside vehicle. Remove booster and gaskets.

Installation

To install, reverse removal procedure. Lubricate pedal rod and linkage pivot bolts, pins, sleeves and bushings. Bleed hydro-boost system. See BLEEDING HYDRO-BOOST SYSTEM . Check stoplight switch adjustment.

Disassembly

1. Install Accumulator Piston Compressor (J-26889) on master cylinder mounting stud and retain with nut. (Scheme 1)and (Scheme 2). Compress accumulator using "C" clamp.
2. Insert punch into hole in housing. Release clamp and remove accumulator piston compressor. Remove accumulator and "O" ring. Remove retainer, spool plug, "O" ring and spool return spring from spool plug bore.
3. Remove baffle, output pushrod, spring and baffle retainer, piston return spring and spring retainer. Saw off eyelet on pedal rod. Remove pedal rod boot, bracket nut and bracket. Remove housing cover bolts. Separate cover from housing.
4. Remove seal from end of rod on piston assembly. Remove housing seal. Remove piston assembly and piston seal. Remove spool valve. Remove accumulator valve using wire hook. (Scheme 3) Remove return port fitting and "O" ring.

Scheme 1

Scheme 2

Scheme 3

Cleaning & Inspection

1. Clean all parts with power steering fluid. DO NOT use any other cleaner. If any accumulator valve components are damaged or lost, replace all valve components.
2. Inspect tube seats from burrs or nicks. If damaged, remove tube seat using a No. 4 bolt extractor. Install tube seat using hammer and Tube Seat Installer (J-6217). (Scheme 4)

Scheme 4

Scheme 5

1. Lubricate all seals, "O" rings and metal friction points with power steering fluid. Install "O" ring and return port fitting. Install accumulator valve and spool valve.
2. Lubricate Seal Protector (J-24551-A) or (J-25083) with power steering fluid. Install piston seal and piston assembly. (Scheme 5) Install seal on end of piston assembly. Install housing seal in groove.
3. Mate housing to cover. Install housing-to-cover bolts and tighten to 22 ft. lbs. (30 N.m). Install bracket and bracket nut. Tighten bracket nut to 110 ft. lbs. (149 N.m). Install boot.
4. Install spring retainer, piston return spring, baffle and output push rod using seal protector. Install spring and baffle retainer. Install "O" ring on spool plug. Install spool return spring, spool plug and retainer in spool plug bore.
5. Install "O" ring on accumulator. Install accumulator using Accumulator Piston Compressor (J-26889) and "C" clamp. (Scheme 1) Install accumulator retaining ring. Install jam nut and eyelet from repair kit onto pedal rod.

TORQUE SPECIFICATIONS

TORQUE SPECIFICATIONS

Scheme 6

Scheme 7

Scheme 8

Scheme 9

See also:
• OVERHAUL
• BLEEDING HYDRO-BOOST SYSTEM