steering pump provides the hydraulic pressure. The master cylinder is mounted on the end
of the power booster.
An accumulator is filled with brake fluid each time the brakes are applied. If the power
steering pump fails for some reason, the accumulator will provide around three
power-assisted stops. Brakes can also be operated with no power assist, but will require
much firmer pedal pressure.
Check accumulator reserve charge retention as follows:
1. With engine running at medium rpm, holding steering to stop or hold brake with 100
lb. minimum brake pedal force for 5 seconds (maximum) and turn engine off.
2. Several minutes later, depress brake pedal (engine off). You should be able to
make two power-assisted applications. If this cannot be done, repair or replace the
booster.
Brake Booster Noise
Normal hydro-boost brake units will produce certain noises. The following noises usually
occur during high pedal efforts and quick pedal release:
Above-normal pedal pressure may cause the booster to hiss. Loud hissing sounds at or
below normal pedal effort indicates a possible problem.
Clunk, chatter, or clicking noises may be heard when the brake pedal is quickly released
from hard pedal efforts.
If you notice booster noise (hydraulic system at normal operating temperature) during low
brake pedal effort or with no pedal effort at idle, the booster may be faulty.
1.7.1 Removal
1. With engine off, depress brake pedal several times to discharge accumulator.
2. Remove deck plate (located to right of operator’s seat).
a. Disconnect parking brake cable from parking brake lever.
b. Remove round head screws from both brake access plates and remove
access plates.
c. Remove deck plate fasteners and deck plate.
3. Disconnect brake pedal linkage from power booster by removing linkage locknuts
and .
AUGUST 31, 2008 CHAPTER 3 OVERHAUL
PAGE 12 1 GENERAL REPAIR
M140 Tow Tractor Manual No. 46054
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