Buick LeSabre 1965 Service Manual page 551

BRAKES
POWER BRAKES 9 - 2 9
POWER PISTON
RETURN SPRING
POWER PISTON
BEARING
FRONT
HOUSING
SEAL
MASTER—,
CYLINDER ^
RUBBER
BOOT
POWER
PISTON ASSEMBLY
REAR
HOUSING
CHECK
VALVE & RUBBER
GROMMET
FRONT
HOUSING
Figure 9-19—Power Brake Unit - Exploded View
a. Hard Pedal
(1) Vacuum failure due to:
(a) Faulty vacuum check valve
(b) Collapsed vacuum h o s e to
manifold
(c) Plugged or loose v a c u u m
fittings
(2) Boundup pedal mechanism
(3) Power brake unit trouble
(a) Jammed air valve
(b) V a c u u m l e a k s in u n i t
caused by faulty air valve seal
or support plate seal. Also, a
damaged, floating control valve,
bad seal of master cylinder, or
powar cylinder mounting studs in
housings, bad seal on master cyl-
inder push rod or a bad seal of
the diaphragm bead between the
housings, or at power piston. It
is possible to have faulty vacuum
check valve grommet.
(c) Defective rolling diaphragm
(d) R e s t r i c t e d a i r f i l t e r
elements
(e) Worn or badly-distorted re-
action plate or levers
(f) Cracked or broken power pis-
ton or reaction retainer.
b. Grabby Brakes (Apparent Off-
and-On Condition)
(1) P o w e r brake unit v a l v e
trouble
(a) Sticking air valve
(b) Restricted diaphragm passage
(2) Reaction system
(a) Dislodged reaction levers
(b) Broken air valve spring
(c) Worn or distorted levers or
plates.
c. Pedal goes either to the Floor
(or almost to Floor)
(1) Fluid r e s e r v o i r n e e d s
replenishing
(2) Power brake hydraulic leakage
(a) Defective primary or second-
ary cups
(b) C r a c k e d m a s t e r cylinder
casting
(c) Leaks at wheel cylinder in
pipes or connections
(3) Faulty master cylinder check
valve has permitted air to enter
system, causing spongy pedal.
d. Brakes Fail to Release
(1) Faulty hydraulic check valve
(2) Blocked p a s s a g e in power
piston
(3) Air valve sticking shut
(4) Broken piston return spring
(5) Broken air valve spring
(6) Tight pedal linkage
9-16 REMOVAL OF
POWER BRAKE
UNIT
1. Disconnect brake pipe from
hydraulic master cylinder and
tape end of pipe to prevent en-
trance of dirt.