Power Brake Diagnosis

Any time a power brake unit has been removed and replaced, or a new unit installed on a car, vacuum and hydraulic leakage tests as well as operational tests should be made to determine whether the unit is operating up to standard.  Road test the brakes by making a brake application at about 20 mph to determine if the vehicle stops evenly and quickly.  If the pedal has a spongy feel when applying the brakes, air is present in the hydraulic system.  Bleed the system at each wheel cylinder.  With the engine stopped and the transmission in neutral, apply the brake several times to exhaust all vacuum in the system.  Depress the brake pedal, hold the foot pressure on the pedal, and start the engine.  If the vacuum system is operating, the pedal will tend to fall away under foot pressure, and less pressure will be required to hold the pedal in the applied position.  If no actin is felt, the vacuum system is not functioning.  Stop the engine and again exhaust all vacuum in the system.  Without starting the engine, depress the brake pedal and hold foot pressure on the pedal.  If the pedal gradually falls away under foot pressure, the hydraulic system is leaking.  A faulty check valve may be tested by shutting off engine and, after waiting a few minutes, applying the brakes; there should be sufficient vacuum pressure reserve for several applications.


Brake Diagnosis:

The following diagnosis applies to both power brakes and standard brakes unless specified.  Brake troubles may be easily diagnosed if the complaint is understood.  The trouble will always show up in one of  more of the four ways listed below.  Related parts of the power brake or standard brake system should be checked before dismantling the brake when a malfunctioning brake system is encountered.

1.  Hard pedal feel may be caused by:

A.     Power brake vacuum failure due to:

a. faulty vacuum check valve

b.  collapsed vacuum hose

c.  plugged or loose vacuum hose or fittings

d.  leaking vacuum reservoir tank.

B.    Bound up pedal mechanism

C.    Glazed linings

D.    Grease on brake drum or linings

E.    Power brake unit trouble due to:

a.  Internal vacuum hose loose or restricted

b.  Vacuum leak in vacuum piston assembly or past leather piston packing.

c.  Leak at power cylinder to master cylinder mounting face (Bendix)

d. Faulty vacuum seal (Moraine)

e.  Restricted air cleaner

f.  Jammed push rod plunger (Bendix)

g.  Jammed air valve (Moraine)

h. Broken counter-reaction spring (Bendix)

i.  Leak past atmosphere poppet (Bendix)

j.  Leak past floating control valve (Moraine)

2.  "Grabby" or severe brakes caused by:

A.  Grease or brake fluid on linings

B.  Scored drums

C. Anchor pins improperly adjusted (new linings)

D.  Power Brake unit trouble due to:

a. Vacuum leakage in reaction diaphragm

b.  Sticking push rod plunger or air valve

c.  Faulty pivot arm and vacuum poppet (Bendix)

d. Restricted diaphragm passage

e.  Master cylinder plunger binding in vacuum piston guide (Moraine)

3.  Pedal goes to floor (or almost to floor) caused by:

A.  Brakes require adjustment

B.  Air in hydraulic system

C.  Hydraulic leak in lines or at wheel cylinders

D.  Fluid reservoir needs replenishing

E. Cracked drums

F.  Power brake hydraulic leakage at:

a.  Compensating valve or seal

b.  Hydraulic plunger seals

c.  Outlet fitting

d. Sand hole or crack in master cylinder

e.  Master cylinder plunger binding in vacuum piston guide (Moraine)

4.  Brakes fail to release ( or slow to release) due to:

A.  Bound up brake pedal linkage

B.  Weak brake shoe return springs or rusty bosses on backing plate

C.  Power Brake unit troubles due to:

a.  Faulty residual check valve

b.  Excessive hydraulic plunger seal friction

c.  Faulty compensating valve

d.  Excessive piston packing friction

e.  Restricted air passage in vacuum piston

f.  Restricted air cleaner

g. Sticky push rod plunger (Bendix)

h.  Sticky air valve (Moraine)

i.  Broken piston return spring

j.  Broken air valve return spring

k.  Broken push rod plunger return spring (Bendix)