Power brakes operate just as non-power brake systems except in the actuation of the master cylinder pistons. A vacuum diaphragm is located on the front of the master cylinder and assists the driver in applying the brakes, reducing both the effort and travel he must put into moving the brake pedal.
The vacuum diaphragm housing is connected to the intake manifold by a vacuum hose. A check valve is placed at the point where the hose enters the diaphragm housing, so that during periods of low manifold vacuum brake assist vacuum will not be lost.
Depressing the brake pedal closes off the vacuum source and allows atmospheric pressure to enter on one side of the diaphragm. This causes the master cylinder pistons to move and apply the brakes. When the brake pedal is released, vacuum is applied to both sides of the diaphragm, and return springs return the diaphragm and master cylinder pistons to the released position. If the vacuum fails, the brake pedal rod will butt against the end of the master cylinder actuating rod, and direct mechanical application will occur as the pedal is depressed.
The hydraulic and mechanical problems that apply to conventional brake systems also apply to power brakes, and should be checked for if the tests below do not reveal the problem.Test for a system vacuum leak as described below:
- Operate the engine at idle without touching the brake pedal for at least one minute.
- Turn off the engine, and wait one minute.
- Test for the presence of assist vacuum by depressing the brake pedal and releasing it several times. Light application will produce less and less pedal travel, if vacuum was present. If there is no vacuum, air is leaking into the system somewhere.
Test for system operation as follows:
- Pump the brake pedal (with engine off) until the supply vacuum is entirely gone.
- Put a light, steady pressure on the pedal.
- Start the engine, and operate it at idle. If the system is operating, the brake pedal should fall toward the floor if constant pressure is maintained on the pedal.
Power brake systems may be tested for hydraulic leaks just as ordinary systems are tested.
See Figures 1, 2 and 3
The method of brake adjustment varies depending on whether the vehicle is equipped with cam adjustment brakes or star wheel adjustment brakes with self adjusters. When the brake linings become worn, effective brake pedal travel is reduced. Adjusting the brake shoes will restore the necessary travel.
Before adjusting the brakes, check the spring nuts, brake dust shield to axle flange bolts, and wheel bearing adjustments. Any looseness in these parts will cause erratic brake operation. Also on 1971 models, make sure that the brake pedal has the correct amount of free travel without moving the master cylinder piston (free play). There should be about 1 / 2 in. of free play at the master cylinder eye bolt. Turn the eye bolt to adjust free play. On models from 1972 on, the pedal free travel is determined by the pedal pushrod length and is not adjustable. If pedal free travel is less than 1 / 16 in. replace the pushrod.
Release the parking brakes and centralize the brake shoes in the drums by depressing the brake pedal hard and then releasing it. It is best to have all four wheels off the ground when the brakes are adjusted so that you can go back to each wheel to double check your adjustments.Initial Brake Shoe Adjustment
If the brake assemblies have been disassembled, an initial adjustment must be made before the drum is installed. It may also be necessary to back off the adjustment to remove the drums.
When the brake parts have been installed in their correct position, adjust the adjusting screw assemblies to a point where approximately 3 / 8 in. of threads are exposed between the star wheel and the star wheel nut.
- Raise and support the vehicle on jackstands.
- Remove the access slot cover and using a brake adjusting tool or screwdriver, rotate the star wheel until the wheel is locked and can't be turned in the clockwise direction.
- Back off the star wheel until the wheel rotates freely. To back off the star wheel on the brake, insert an ice pick or thin screwdriver in the adjusting screw slot to hold the automatic adjusting lever away from the star wheel. Do not attempt to back off on the adjusting screw without holding the adjusting lever away from the star wheel as the adjuster will be damaged.
BRAKE PEDAL FREE-PLAY
Pedal freeplay is measured at the top of the pedal pad.1971 Models
Freeplay is controlled by the length of the master cylinder pushrod. Shorten or lengthen the pushrod to give a freeplay of 1 / 2 in.1972-86 Models
Proper free play should be 1 / 16 - 1 / 4 in. Free play is not adjustable. If free play is not correct, the problem is the result of worn or damaged parts.