Buick LeSabre 1965 Service Manual page 533

BRAKES
TROUBLE DIAGNOSIS 9 - 1 1
indicated if a good brake is ob-
tained after pumping brake pedal
several times.
(2) Fluid Reservoir Empty. If
fluid reservoir is empty, a good
brake cannot be obtained by pump-
ing brake pedal. Fill reservoir
(par. 9-6) and check for springy
pedal action (subpar. b, below).
(3) Air in Hydraulic System. Air
in hydraulic system will cause a
springy action of brake pedal. If
volume of air is large, the pedal
will go to toeboard under normal
pressure. See subparagraph b
below for causes of air in hy-
draulic system. Bleed hydraulic
system (par. 9-6).
(4) Fluid Leak in Hydraulic Sys-
tem. A leak in the hydraulic sys-
tem will allow the pedal to go to
the toeboard gradually under con-
tinued pressure. An external leak
is indicated by loss of fluid in
fluid reservoir. Check all brake
pipe connections and all cylinders
for evidence of fluid loss. If no
leaks are found, remove master
cylinder, disassemble and check
for i n t e r n a l leak. Check for
scoring or other damage to cylin-
der bore, piston, or primary cup
(par. 9-13).
In Power brake cylinder, check
for scoring or other damage to
hydraulic plunger or rubber cup.
(5) Improper Brake Fluid. Im-
proper brake fluid may boil due to
high temperature. Flush system
and refill with specified brake
fluid (par. 9-6).
b. Springy, Spongy Action of
Brake Pedal
(1) Air in Hydraulic System. Air
will enter the hydraulic system if
there is not sufficient reserve
fluid in the master cylinder res-
ervoir. A disconnected pipe per-
mits air to enter the system. A
clogged vent will permit air to be
drawn in past the piston secondary
cup on the return stroke. A leak-
ing check valve causes loss of
static pressure in the system,
thus permitting air to be drawn
in past wheel cylinder cups.
Clean out clogged vent or replace
leaking check valve as required.
Fill reservoir and bleed hydrau-
lic system (par. 9-6).
(2) Brake Drum Out of Round.
Usually indicated by an unstable
action of brake p e d a l . Check
brake drums and true up if neces-
sary (par. 9-11).
c. Brakes Severe on Light
Pedal Pressure
(1) Loose Front Wheel Bearings.
Check for play in bearings with
wheels jacked up and adjust if
necessary (par. 7-10).
(2) Loose Brake Backing Plate.
Tighten all backing plate bolts.
(3) Brake Shoes Not Properly
Adjusted.
(4) Excessive Dust in Brake As-
semblies. When excessive dust is
present, brakes are usually more
severe. Thoroughly clean brake
shoe linings to remove embedded
dirt (par. 9-10).
(5) Faulty Brake Shoe Linings or
Drums. Charred linings or scored
drums cause grabbing action. A
small amount of grease or brake
fluid on linings may cause grab-
bing action. Replace linings and
true up or replace scored drums
(par. 9-11).
(6) Brake Shoe Linings Reversed.
The primary lining is shorter
than secondary lining and of dif-
ferent composition. Install brake
shoes in proper positions, (par.
9-10).
d. Excessive Pedal Pressure
Required
(1) Foreign Substances on Brake
Linings. Check for grease, brake
fluid, or other foreign substances
on linings. Replace grease or
fluid soaked linings (par. 9-10).
Sand off other foreign substances.
(2) Improper Brake Lining. Brake
lining is selected to match car
characteristics. Improper lining
can give poor performance or
feel. Install standard Buick lin-
ing (par. 9-10).
(3) Improper Brake Fluid. Fluid
containing substances injurious to
rubber will cause swelling of rub-
ber cups in master and wheel cyl-
inders. Replace rubber c u p s ,
flush system and r e f i l l with
specified brake fluid (par. 9-6,
9-12, 9-13).
e. Hard or Sticky Initial Pedal
Pressure on Power Brakes
Above conditions may be accom-
panied by failure of pedal to re-
turn to r e l e a s e position when
foot pressure is removed.
(1) Binding Pedal Linkage. Re-
move and lubricate pedal linkage
pivot pins with Lubriplate.
(2) Swollen Hydraulic Cups. Re-
place all rubber parts and com-
pletely flush system to remove all
traces of mineral oil.
f. Power Brake Cylinder
Does Not Boost
(1) Test for power cylinder op-
eration as follows: With engine
stopped, depress brake pedal sev-
eral times to eliminate all vac-
uum from the system. Apply the
brakes and while holding foot
pressure on brake pedal, start
the engine. If power cylinder is
operating, the pedal will move
slightly forward when vacuum
power is a d d e d to the pedal
pressure.
(2) If above test shows that power
cylinder is not operating, check
the following items.
(a) Vacuum Check Valve Stuck
Closed. Remove check valve and
free up or replace it.
(b) Vacuum Hose Broken or Ob-
structed. Replace damaged or ob-
structed hose and make certain
connections are tight.