Pontiac FIREBIRD 1972 Service Manual page 501

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1972 PONTIAC SERVICE MANUAL
SWITCH TERMINAL
FRONT MASTER CYLINDER REAR MASTER CYLINDER
INLET PORT
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INLET PORT
OUTLET PORTS T
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OUTLET PORT TO
FRONT BRAKES REAR BRAKES
SECTION SECTION SECTION
Fig. 5E-5 Cross Section of Three Function Combination Valve
function combination valve (which is used on all disc
brake equipped vehicles other than station wagons and B
Series cars)incorporates the rear brake propor-
tioning valve in the same housing. The two combination
valves are physically identical, except that the parts of the
proportioning section are omitted from the two function
valve. A cross section of each valve is shown in Figures
5E-4 and 5E-5. The combination valve is mounted on the
left front side rail of the frame on all but the F and X Series
vehicles, where it is mounted on a bracket immediately
below the master cylinder. A wiring harness connects the
electrical terminal of the distributor switch to the brake
warning light on the instrument panel.
OPERATION
The metering valve (hold-off) section prevents front brake
application until 1 10-1 50 p.s.i. pressure has built up in the
hydraulic system, so that the rear drum brake linings
contact the drum at the same time the front disc brakes
apply braking pressure to the discs. The metering valve
allows free flow of brake fluid when the brakes. are not
applied (below 3 p.s.i.). Upon brake application, flow re-
striction is maintained until hydraulic pressure reaches
the range of 440-750 p.s.i., at which time the valve permits
unrestricted flow of hydraulic pressure.
brake systems and, when activated, completes a circuit to
the brake warning lamp in the instrument panel. The
switch is designed to compare front and rear brake pres-
sure and is activated at 100 p.s.i. minimum to 300 p.s.i.
maximum pressure differential. When activated, the
switch latches in the "on" position and can be turned off
only when the failure has been repaired and an equal
pressure in each of the hydraulic systems of 450 p.s.i. is
applied. After the repair is completed, the switch can be
reset by applying moderate pressure on the brake pedal
with the engine running (to get power assist).
The Distributor Switch (or Failure Warning Switch) is
activated when there is a failure in either the front or rear
SERVICE
The Proportioning section (included in all combination
valves except those used on station wagons) proportions
pressure to the rear brakes after a pre-determined line
pressure has been reached. This prevents rear wheel lock
up on vehicles with light rear wheel loads or upon high
deceleration stops when a percentage of the weight over
the rear wheels is transferred forward. The proportioner
does not operate during normal brake stops. The valve is
also designed to provide a "by-pass" feature which assures
full pressure to the rear brake system in the event of a front
brake system failure.
The combination valve is a non-serviceable unit and, as
such, the complete assembly must be replaced should a
failure occur in any portion of the valve.
LINING INSPECTION
formed at least every 12000 miles. Need for future service
should be estimated at this time. Particular attention
Complete disc brake system inspections should be per-
should be paid to disc brake lining wear. If the lining