Suburban 3/4 Ton 4WD V8-305 5.0L (1986)
Vacuum Brake Booster: Testing and Inspection
Checking Complaints
Complaints about power brake operation should be handled as if two separate systems exist. Check for faults in the regular brake system first. If it is
okay, start looking over the power brake circuit. For a quick check of proper power unit operation, press the brake pedal firmly and then start the engine.
The pedal should fall away slightly and less pressure should be needed to maintain the pedal in any position. On vacuum suspended power units, air will
rush into the air intake when the brakes are applied.
Another check begins with removal of the stop light switch and installation of a pressure gauge. Take a reading with the engine off and the power unit
not operating. Maintaining the same pedal height, start the engine and take another reading. There should be a substantial pressure increase in the second
reading.
Pedal-free travel and total travel are critical. As a general rule, brakes should be adjusted or relined if the pedal is closer than 1-1/2 in. from the floor
with the brakes applied. Free travel should be kept strictly to specifications.
Take a manifold vacuum reading if the power unit isn't giving enough assistance. Remember, though, that some of the new V-8s have less than 15 in.
of vacuum at idle. If manifold vacuum is abnormally low, tune the engine and then try the power brakes again. Naturally, loose vacuum lines and clogged
air-intake filters will cut down brake efficiency. Most units have a check valve that retains some vacuum in the system when the engine is off. A vacuum
gauge check of this valve will tell you when it is restricted or stuck open or closed.
A simple check of the hydraulic system should be made before proceeding. Loosen the connection between the power unit and the wheel cylinder
lines. If the brakes release, the trouble is in the power unit hydraulic circuit. If the brakes still will not release, look for a restricted brake line or similar
difficulties in the regular hydraulic circuit.
A residual pressure check valve is usually included immediately under the brake line connection on hydraulic-assist power brakes. This valve
maintains a slight hydraulic pressure on the brake lines and wheel cylinders to give better pedal response. If it is sticking, the brakes may not release.
Power brakes that have a hard pedal are usually suffering from a milder form of the same ills that cause complete power unit failure. Collapsed or
leaking vacuum lines or insufficient manifold vacuum, as well as punctured diaphragms or bellows and leaky piston seals, all lead to weak power unit
operation. A steady hiss when the brake is held down means a vacuum leak that will cause poor power unit operation.
Do not immediately condemn the power unit if the brakes grab. First look for all the usual causes, such as greasy linings or scored drums. Then
investigate the power unit.
