Chevrolet 1977 light duty truck Service Manual page 391

N O TE: B efore checking the hydraulic p o w e r booster for the source o f trouble, refer
to the trouble diagnosis procedures fo r S tandard Brakes. A fte r these possible causes have
been elim inated, ch eck fo r the probable cause a nd rem edy as o u tlin ed below:
1. On pedal application u n til b o o ster run-out,
slight pow er steering pum p noise m ay be
heard.
2. Pedal application through run-out will n o t
necessarily be sm ooth due to th e internal
ratio change, i t is possible to push the pedal
past run-out because o f th e higher pedal ratio.
A t run-out o f the vacuum booster th e pedal
ju st becom es hard.
P relim inary Check
W ith the engine stopped, depress the brake pedal several times to eliminate all accumulator reserve
from the system.
Hold the brake pedal depressed with medium pressure (25 to 35 lbs.), start the engine. If the unit is
operating correctly, the brake pedal will fall slightly and then push back against the driver's foot,
remaining at about the same position. If the booster is not operating correctly, the trouble may be one
of the following causes:
P ro b a b le C ause
1. Lo o se o r b r o k e n power s t e e r i n g
pump bel t .
2 . No f l u i d i n power s t e e r i n g r e s e r v o i r .
3. L e a k s i n power s t e e r i n g , b o o s t e r o r
a c c u m u l a t o r h o s e s .
4 . L e a k s a t t u b e f i t t i n g s ,
b o o s t e r o r a c c u m u l a t o r c o n n e c t i o n s .
5 .
E x t e r n a l l e a k a g e a t a c c u m u l a t o r .
6. F a u l t y b o o s t e r p i s t o n s e a l
l e a k a g e a t b o o s t e r f l a n g e v e n t .
7 . F a u l t y b o o s t e r i n p u t rod s e a l w i t h
l e a k a g e a t i n p u t ro d en d.
8 . F a u l t y b o o s t e r c o v e r s e a l w i t h l e a k a g e
be tw e en h o u s in g and c o v e r .
9 . F a u l t y b o o s t e r s p o o l plu g s e a l .
1 0 . I n t e r n a l l e a k a g e i n b o o s t e r .
11 . C o n t a m i n a t i o n i n power s t e e r i n g
12. H y d r a u l i c l i n e s r o u t e d i n c o r r e c t l y .
DIAGNOSIS - HYDRO - BOOST SYSTEM
Norm al O p erating Characteristics
Brake pedal application o f the H ydro-boost system
differs in som e respects from a vacuum type pow er
brake system in th e follow ing m anner:
N O BOOST - H ARD PEDAL
power s t e e r i n g ,
c a u s i n g
f l u i d .
3. On th e first full application o f th e brake pedal,
a slight hissing sound m ay be heard. T he hiss
is the accum ulator charging and the noise should
go away in a sh o rt period o f time.
4. On a spike brake application, a slight pedal
kick-back may be felt.
5. If the vehicle is started w ith the pedal de­
pressed, the pedal will fall away slightly then
return back to approxim ately the original
position.
R em ed y
T i g h t e n o r r e p l a c e t h e b e l t .
1
.
F i l l r e s e r v o i r and c h e c k f o r
2.
e x t e r n a l l e a k s .
R e p l a c e d e f e c t i v e p a r t s .
3.
T i g h t e n f i t t i n g s o r r e p l a c e tub e
4 .
s e a t s , i f d e f e c t i v e .
5. R e p l a c e b o o s t e r .
R e p l a c e a l l b o o s t e r s e a l s .
6 .
7 . R e p l a c e a l l b o o s t e r s e a l s .
8 . R e p l a c e a l l b o o s t e r s e a l s .
9 . R e p l a c e a l l b o o s t e r s e a l s .
1 0 . R e p l a c e b o o s t e r .
11 . F l u s h power s t e e r i n g
r e p l a c e w i t h new f l u i d .
12. R e - r o u t e l i n e s .
s y s t e m and