Chevrolet Light Duty Truck 1973 Service Manual page 334

5 -6 8 BRAKES
Probable Cause
1. External le ak: 1eakin g-pipe connections, hose, wheel
c y li n d e r s , stop l i g h t s w itc h , master c y li n d e r head nut,
e t c .
2. In te rn a l le aks: past secondary seals in to power u n i t .
Check vacuum hose f o r f l u i d .
Probable Cause
.
1
Bent, broken, obstru cted tube.
2. Stuck check valve.
3. A i r i n l e t blocked.
4. A i r valve stuck.
5. F aulty diaphragm.
6. F aulty p is to n s e a l .
7. Leaks i n t e r n a l l y .
DIAGNOSIS
NOTE The same types o f brake troub le s are encountered
w ith power brakes as w ith standard brakes.
ing the power brake system f o r source o f t r o u b le , r e f e r
to tro u b le diagnosis o f standard h y d ra u lic brakes in t h is
manual. A f t e r these p o ssib le causes have been e lim in a te d ,
check f o r cause as o u t lin e d below.
BRAKE SYSTEM LOSES FLUID
NOTE: Make the f o llo w in g t e s t before checking hard pedal
f o r the cause. With the, engine stopped, depress the brake
pedal several times to e lim in a te a l l vacuum from the system.
Apply the brakes, and w h ile holding the f o o t pressure on the
brake pedal, s t a r t the engine.
c o r r e c t l y , the brake pedal w i l l move forward when the engine
vacuum power is added to the pedal pressure.
shows t h a t the power u n i t is not o p e ra tin g , the t r o u b le may
be one o f the f o ll o w i n g :
N O B O O S T -H A R D PEDAL
Collapsed hose.
Before check­
Remedy
1. Clean p a rts . Tig hte n.
2. Rebuild master c y l i n d e r .
I f the u n i t is operating
I f t h i s t e s t
Remedy
.
1 Replace d e f e c t iv e p a r ts .
2. Replace valve.
3. Replace f i l t e r . Open passages.
4. Disassemble u n i t - clean - replace d e f e c t iv e
5. Replace diaphragm.
6. Replace s e a l .
7. Rebuild as o u t lin e d in the "Overhaul" Manual.
Tighten b o l t s .
Replace d e f e c t iv e parts,
LIGHT DUTY TRUCK SERVICE MANUAL