GMC 1982 Light Duty Truck, Service Manual

Page: 522 / 1435

5-2 BRAKES
wheel brakes at low pressure with initial brake application.
The low pressure fluid quickly provides the displacement
requirements created by the seal retracting pistons into the
front calipers and spring retraction of the rear drum brake
shoes.
O P E R A TIO N OF DISC BRAKE
Upon application of the brakes, fluid pressure behind
the caliper piston increases. Pressure is exerted equally
against the bottom of the piston and also against the bottom
of the piston bore. The pressure applied to the piston is
transmitted to the inner shoe and lining, forcing the lining
against the inner rotor surface. The pressure applied to the
bottom of the piston bore forces the caliper to slide or move
on the mounting bolts toward the inner side, or toward the
vehicle. Since the caliper is one piece, this movement toward
the vehicle causes the outer section of the caliper to apply
pressure against the back of the outer shoe and lining
assembly, forcing the lining against the outer rotor surface.
As line pressure builds up, the shoe and lining assemblies are
pressed against the rotor surfaces with increased force,
bringing the vehicle to a stop.
Lining wear is automatically compensated for by the
outward movement of the caliper and piston. Brake fluid fills
this void as lining wears.
O PER A TIO N OF C O M B IN A T IO N V A LV E
The metering or hold off section of the combination
valve limits pressure to the front disc brakes until a pre­
determined front input pressure is reached, approximating
the pressure to overcome the rear shoe and lining retractor
springs. There is no restriction at inlet pressures below 21
kPa (3 psi) to allow for pressure equalization during non
apply periods.
The proportioning section of the combination valve
proportions outlet pressure to the rear brakes after a pre­
determined rear input pressure has been reached. This is
done to prevent rear wheel lock-up on vehicles with light
rear wheel loads. Certain models having high rear wheel
loads do not have the proportioning feature.
The valve is designed to have a "By-Pass" feature
which assures full system pressure to the rear brakes in the
event of a front brake system failure. Full front pressure is
retained in the event of rear failure.
The pressure differential warning switch is designed to
constantly compare front and rear brake pressure from the
master cylinder and energize the warning light on the dash
in the event of a front or rear system failure. The valve and
switch are so designed that the switch will latch in the
"warning" position once a failure has occurred. The only
way the light can be turned off is to repair the failure and
apply a pedal force as required to develop up to 3100 kPa
(450 psi) line pressure.
DIAGNOSIS
IN SPEC TIO N A N D TESTING BRAKES
T esting Brake s
New linings must be protected from severe use for
several hundred miles.
Brakes should be tested on dry, clean, reasonably
smooth and level roadway. A true test of brake performance
cannot be made if roadway is wet, greasy or covered with
loose dirt so that all tires do not grip the road equally.
Testing will also be adversely affected if roadway is crowned
so as to throw weight of vehicle toward wheels on one side or
if roadway is so rough that wheels tend to bounce.
Test brakes at different vehicle speeds with both light
and heavy pedal pressure; however, avoid locking the wheels
and sliding the tires on roadway. Locked wheels and sliding
tires do not indicate brake efficiency since heavily braked
but turning wheels will stop vehicle in less distance than
locked wheels. More tire-to-road friction is present with a
heavily braked turning tire than with a sliding tire.
External C o n d itio n s th a t Affect B rake Perform ance
1. Tires—Tires having unequal contact and grip on road
will cause unequal braking. Tires must be equally
inflated and tread pattern of right and left tires must be
approximately equal.
2. Vehicle Loading—When vehicle has unequal loading,
the most heavily loaded wheels require more braking
power than others. A heavily loaded vehicle requires
more braking effort.
3. Front Wheel Bearings—A loose front wheel bearing
permits the drum to tilt and have spotty contact with
the brake shoe linings causing erratic action.
4. Front End Alignment—Misalignment of the front end,
particularly in regard to lim its on camber and
theoretical king pin inclination, will cause the brakes to
pull to one side.
HYDRO-BOOST
Prior to performing the Booster Function Tests, or the
A ccum ulator Leakdown Test, the following preliminary
checks must be made:
N O T IC E :
Power steering fluid and brake fluid cannot be
mixed. If brake seals contact steering fluid or steering
seals contact brake fluid, seal damage will result.
1. Check all power steering and brake lines and
connections for leaks and/or restrictions.
2. Check and fill brake master cylinder with BRAKE
FLUID.
3. Check and fill power steering pump reservoir with
POWER ST EER IN G FLUID. Be sure fluid is not
aerated (air mixed with fluid).
4. Check power steering pump belt for tension and/or
damage. Adjust if necessary.
5. Check engine idle speed and adjust if necessary.
6. Check steering pump pressure; Refer to Section 3B.