Bleeding the Brake System

TM 10-3930-633-12

(3) Be sure the master cylinder reservoir is

parts. During brake inspection, check wheel

filled, and keep it full during this procedure.

cylinders for signs of leakage. Final connections

(4) Refer to figure 4-47 and bleed front and

to front wheel cylinders are made by means of

rear brake lines at their highest points first; "A"

flexible hoses. Check these hoses for kinks,

and "B" respectively. Loosen each one separately

chafing, and any sign of wear or deterioration.

and bleed as follows: Depress break pedal slowly

Tighten loose fittings and replace parts to

and hold, allowing fluid and air to escape.

eliminate leakage. Always bleed the brakes after

Tighten fitting, then release brake pedal. Repeat

such a repair.

procedure until fluid is free of air bubbles.

4-78. Bleeding the Brake System

(5) Install a bleeder hose on one of the wheel

a. Proper operation of the hydraulic brake

cylinder bleeder screws and submerge the

system requires a solid column of fluid without air

unattached end of the hose in a clean transparent

bubbles at all points in the pressure system.

jar containing several inches of brake fluid.

Under certain conditions it becomes necessary to

NOTE

bleed fluid from the system in order to expel air

During bleeding of the wheel cylinders the jar

bubbles which have become mixed with the fluid.

should be elevated to a position higher than the

b. The necessity of bleeding is indicated by a

bleeder screws making sure that the end of the

hose remains submerged in the fluid at all times.

soft, spongy pedal or at any time a brake line is

(6) Loosen bleeder screw and slowly push

removed (or broken).

(1) Raise all four wheels far enough to clear

brake pedal to the floorboard. Hold pedal in this

the floor and place heavy blocking under the

position until bleeder screw is retightened. Repeat

machine frame so it cannot accidentally become

this operation until all air bubbles disappear and

lowered. Deflate the tires and remove the wheels

clear fluid is being pumped into the jar.

from the hub assemblv.

(7) Install bleeder hose on the remaining

bleeder screw and proceed as in step (5). After all

bleeding has been completed, recheck fluid level

Deflate tires before removing front

in master cylinder. Fill to within -inch of the

wheels from vehicle.

top with SAE 70 R3 brake fluid, Specification

(2) Check the brake pedal free travel (see

VV-B-680. Replace cylinder cap.

paragraph 4-73). Clean dirt from around the filler

(8) Replace wheels. Inflate tires. Remove

cap of the master cylinder reservoir. Brake fluid

blocking and lower machine to floor.

should be within -inch of the top. With filler cap

off the master cylinder, depress and release brake

NOTE

pedal. A small displacement of fluid should be

Remember that the brake pedal should be

depressed slowly and held to the floorboard until

noticed in the cylinder reservoir. If this happens

the line connections or bleeder screws are securely

the brake pedal (upon being released) is returning

tightened. This prevents the possibility of air

the master cylinder piston to its normal position

being drawn into the system during the bleeding

to open a master cylinder port. This port must be

operation. bleeding operation. Check master

cylinder reservoir level periodically during manual

open. If fluid does not return to the reservoir

bleeding and fill to within -inch of the top as

(when releasing brake pedal), this indicates

required.

improper pedal free travel and a pedal adjustment

is required.

4-64