Pontiac FIREBIRD 1972 Service Manual page 478

SECTION 5D
DELCO-MORAINE POWER BRAKE
T A N D E M D I A P H R A G M
The following caution applies to one or more steps in the assembly proce-
dure of components in this portion of the manual as indicated at appropri-
ate locations by the terminology "See Caution on page 1 of this Section".
CA UTI 0 N: THIS FASTENER IS AN IMPOR TA NT A TTA CHING
PART IN THAT IT COULD AFFECT THE PERFORMANCE OF
VITAL COMPONENTS AND SYSTEMS, AND/OR COULD RE-
SUL T IN MAJOR REPAIR EXPENSE. IT MUST BE REPLACED
WITH ONE OF THE SAME PART NUMBER OR WITH AN
EQUIVALENT PART IF REPLACEMENT BECOMES NECES-
SAR Y. DO NOT USE A REPLACEMENTPAR T OFLESSER QUAL-
ITY OR SUBSTITUTE DESIGN TORQUE VALUES MUST BE
USED AS SPECIFIED DURING REASSEMBLY TO ASSURE
PROPER RETENTION OF THIS PAR 7:
CONTENTS OF THIS SECTION
............................
Trouble Diagnosis and Testing 5D-1 Power Piston Disassemble
................................
5D-8
..............................................
General Description 5D-2 Master Cylinder Disassemble
..........................
5D-12
................................................................
Operation 5D-4 Master Cylinder Assemble
..............................
5D-12
......................................
Checks and Adjustments 5D-6 Power Piston Assemble
..................................
5D- 12
........................................................
Major Repairs 5D-7 Power Cylinder Assemble
..............................
5D- 15
....................................
Power Unit Disassemble 5D-8 Gaging Procedure
...........................................
5D- 16
TROUBLE DIAGNOSIS AND TESTING
Start engine. With transmission in Park or Neutral, open
throttle to medium speed. Close throttle and turn off en-
gine. This evacuates the housing chambers. Wait no less
than 90 seconds, then try brake action. If not vacuum-
assisted for two or more applications, vacuum check valve
is faulty or there is a leak in the vacuum system.
With engine off, deplete all vacuum reserve in the system.
Depress the brake pedal and hold foot pressure on the
pedal. If the pedal gradually falls away under foot pres-
sure, the hydraulic system is leaking.
If the brake pedal travels to within one inch of the toe-
board, the brake shoes require adjustment or relining.
With vacuum reserve depleted, depress brake pedal, hold
light foot pressure on the pedal, and start the engine. If the
vacuum system is operating, the pedal will tend to fall
away under foot pressure, and less pressure will be re-
quired to hold pedal in applied position. If no action is felt,
the vacuum system is not functioning.
Road test brakes by making a brake application at about
20 mph to determine if the vehicle stops evenly and
quickly. If the pedal has a spongy feel when applying the
brakes, air may be present in the hydraulic system. Bleed
system as described in Section 5.
The same types of brake trouble are encountered with
power brakes as with standard hydraulic brakes and disc
brakes. Before checking the power brake system for the
source of trouble, refer to the trouble diagnosis of standard
hydraulic and disc brakes. After these possible causes have
been eliminated, check for the cause as outlined below: