Initial Inspection and Diagnostic Overview

Take a manifold vacuum reading if the power unit is not giving enough assistance. If manifold vacuum is abnormally low, correct the problem, and inspect 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 inspection of this valve will tell when it is restricted, stuck open or closed.

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 and return spring.

A simple hydraulic system inspection 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.

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, scored rotors or drums, then investigate the power unit.