Brake Drum Refinishing

TM 10-3930-633-34

9-7. Brake Drum Refinishing

rubber cups will be enlarged and very soft. These

cannot be used and should be discarded.

a. Minor scores on a brake drum can be

(6) Cylinder walls must be smooth and not

removed with fine emery cloth, provided the

pitted or scratched and be free from burrs.

emery is thoroughly cleaned off the drum after the

Remove light pits, scratches and burrs with a

operation.

hone. If cylinder does not clean up with light

b. A badly scored, rough, or out-of-round drum

honing, cylinder must be replaced.

should be ground or turned on a drum lathe. Do

(7) occasionally grease retainers become

not remove any more material from the drum than

worn, allowing lubricant from wheel bearings to

is necessary to provide a smooth surface for the

leak through in the brake drum. When grease

brake shoe contact. In no case should the drums

comes into contact with end closures, they

be turned to more than 0.060 inch oversize.

become soft and enlarged, preventing them from

9-8. Vacuum Power Unit Decryption

protecting the cylinder from foreign matter. If

a. General. The vacuum power unit (fig. 9-4) is

this occurs, replace defective parts.

a self-contained vacuum-hydraulic unit for power

(8) Inspect anchor pins for rust, or scoring.

braking that uses engine manifold vacuum and

Clean all dirt and rust from anchor pins and coat

atmospheric pressure for its power. It permits the

pins lightly with "Lubriplate 110" or equivalent.

use of a low brake pedal, and provides reduced

(9) Check return springs, washers and

pedal effort, compared to the conventional (non-

retaining clips for wear or corrosion and replace as

power) hydraulic braking system. The dual-

required.

system hydraulic section of the Master-Vat is

(10) Check brake shoes for wear and grease or

similar to the manually operated dual-system

oil soaking. Replace shoes which are worn to

master cylinder except that the hydraulic pushrod

within 1/32 of the rivet heads or shoe surface.

is a part of the power section. The separate

d. Reassembly.

hydraulic systems provided by this master

(1) Reassemble front brakes in accordance

cylinder prevent a hydraulic failure in either

with figure 9-2.

system from affecting the other system. If a

(2) Reassemble rear brakes in accordance

hydraulic failure does occur, the driver is aware of

with figure 9-3.

it, either immediately when it happens, or on the

(3) Always install wheel cylinder spring,

next brake application, because of the increased

pie ton and cups, and cylinder boots, and mount

pedal travel required to get braking action. In

the wheel cylinder to the backing plate before

addition, vehicle deceleration with only the front

installing shoe and lining assemblies.

or rear brakes functioning will, usually, be less

(4) After reassembly of brakes, and in-

effective, requiring either greater pedal effort for

stallation of brake drums, bleed and adjust

the same stopping distance or greater distance.

brakes as outlined in TM 10-3930-633-12.

9-5