Testing and Inspection for Firebird V8-305 Vacuum Brake Booster
Vacuum Brake Booster: Testing and Inspection
Checking Complaints
Quick Check
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.
Pressure Gauge Check
Another check begins with installation of a suitable pressure gauge in the brake hydraulic system. 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
Pedal free travel and total travel are critical on cars equipped with power brakes. 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.
Vacuum Boost
On models with vacuum boost brakes, take a manifold vacuum reading or check operation of the external vacuum pump if the power unit isn't giving
enough assistance. Remember, though, currently produced emission controlled engines, manifold vacuum readings may be less than 15 inches Hg 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.
Hydro-Boost
On models with hydro-boost brakes, check operation of the power steering pump and check for improperly adjusted drive belts, defective pressure
relief valve, leaking hoses and fittings, or defective accumulator. Most causes of failure on these systems are not within the power assist unit itself.
However, if all other components are satisfactory but a malfunction persists, the power unit should be disassembled and inspected for damage, wear
and contamination.
Brake Release Failure
Failure of the brakes to release in most instances is caused by a tight or misaligned connection between the power unit and the brake linkage. If this
connection is free, look for a broken piston, diaphragm or bellows return spring on models with vacuum boost brakes. On models with hydro boost
brakes, check for contamination of the fluid which results in restrictions within the spool valve, sticking assist piston within the power unit or defective
accumulator. The power unit will have to be disassembled for this check and, if the spring is in good condition, continue going through the power unit,
checking the fluid, air and vacuum valves.
Hydraulic System Check
A simple check of the hydraulic system should be made before proceeding. Loosen the connection between the master cylinder and the brake
booster. If the brakes release, the trouble is in the power unit; 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 within the brake lines and wheel cylinders to give better pedal response. If it is sticking, the brakes may not
release.
Hard Pedal
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.
Grabbing Brakes/Reaction Control
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. When the trouble has been traced to the power unit, check for a damaged reaction control. The reaction control is usually
made up of a diaphragm, spring and valve that tends to resist pedal action. It is put in the system to give the pedal ``feel.''
