Chevrolet 70 Series 1970, Service Manual

Page: 351 / 841

BRAKES 5-60
a" Figure 36—Wedge Cavity in Housing (“ Slots” ShownI
NOTE: Installation tool shown in figu re 33 can
be made lo ca lly according to dim ensions shown in
figu re 34.
3. In sert plunger guide with new w asher inside
s lo ts in anchor p lu n g ers.
4. In stall autom atic adjuster com ponents a s
detailed in applicable p rocedure.
5. In stall wedge a ssem b ly in actuation housing
(refer to fig . 35). B e su re r o lle r s a r e sea ted c o r ­
re c tly in retain er cage and make good contact with
p lu n gers.
NOTE: Spring retain er w ash er has "ears" on
each sid e to be u sed as guides to a ssu r e co rrect
in sta lla tio n (se e fig . 31). Housing has "slots" to
match "ears" on w asher (s e e fig . 36).
6
. In stall brake sh o es a s p reviou sly d escrib ed
under "Brake Shoe Installation" in th is sectio n .
7. In stall brake cham bers a s d escrib ed p r e ­
v io u sly .
8
. Connect a ll a ir lin e s into both cham bers.
9. B uild up a ir p r e s s u r e in sy ste m and push
parking brake knob in to admit a ir into outer
cham bers (with "F ail-Safe" only).
10. With a ir in outer cham bers, turn r e le a s e
bolt at each cham ber (if "Fail-Safe") cou n terclock ­
w is e a s far a s it w ill go (approxim ately 18 turns -
s e e fig . 39). B rak es are now in operating condition,
either for s e r v ic e brakes o r parking.
11. A fter brakes are in operating condition,
adjust brakes and repeat operating and leakage
te s t s a s p reviou sly d escrib ed .
STANDARD "FAIL-SAFE” BRAKE CHAMBERS
GENERAL
The Stopm aster Standard "Fail-Safe" is a
m echanical actuation feature for power b rak es and
i s tw o-fold in p urpose. It is used a s an a ir -r e le a s e d ,
sp rin g-ap p lied parking brake, a s w ell a s a safety
featu re in the event of a ir brake failu re.
DESCRIPTION AND OPERATION
BRAKE CHAMBERS (F ig . 37)
Two brake cham ber a sse m b lie s a re u sed at
each w heel. Type used d iffers by m odel as shown
in figu re 24. The "F ail-Safe" brake cham ber a s ­
sem b ly c o n sists of an inner and outer cham ber.
The inner cham ber, containing a diaphragm and
diaphragm plate rod, s e r v e s a s the s e r v ic e brake
cham ber and op erates in the sam e m anner a s a
conventional brake cham ber. M ovement of dia­
phragm plate rod i s tran sm itted to the brake sh oes
through the m echanical actuating com ponents as
d escrib ed la te r .
The outer cham ber contains a sp rin g-load ed
p iston which a c ts against the s e r v ic e brake cham ­
b er diaphragm p late rod. D uring norm al operation,
constant a ir p r e ssu r e is applied to the outer cham ­
ber; th is p r e s s u r e , acting on the piston , holds the
sp rin g c o m p r esse d . When a ir p r e ssu r e is r e le a se d
from the outer chamber by pulling out the knob on
the parking brake control valve, spring p r e ssu r e
fo r c e s the p iston toward the s e r v ic e brake cham ­
ber; p iston m ovem ent is tran sm itted through the
s e r v ic e brake diaphragm plate rod to the brake
actuating m echanism , applying the b rak es.
T his sa m e action w ill take p la ce in the event
a ir p r e s s u r e is lo st from the sy ste m .
NOTE: The a ir tank supplying a ir p r e s s u r e
to the parking brake cham bers is p rotected from
the main a ir sy ste m by a one-w ay check valve;
th is tank w ill contain enough p r e ssu r e for at le a st
one parking brake r e le a s e in c a se p r e s s u r e is lo s t
from the m ain air sy ste m .
In c a se of com plete p r e ssu r e lo s s , and p r e s ­
su r e in the p rotected tank i s depleted, b rakes w ill
rem ain applied until air p r e s s u r e is r e sto r e d . If
n e c e ssa r y to m ove the veh icle b efore a ir p r e ssu r e
can be resto red , brakes can be r e le a se d by turn­
ing the r e le a s e b olts clo ck w ise a s far a s p o ssib le
(approxim ately 18 turns) to co m p r e ss the sp rin g s,
a s shown in figu re 39. (T his m ust be done at a ll
"F ail-Safe" cham bers at each w heel.) A fter air
p r e s s u r e has been resto red , s e r v ic e brakes w ill
be operative im m ediately; how ever, the parking
brake w ill rem ain inoperative until the sp rin g r e ­
le a s e b olts a re backed out (counterclockw ise) as
CHEVROLET SERIES 70-90 HEAVY DUTY TRUCK SHOP MANUAL