PRINCIPLES OF OPERATION

Released Position (Fig. 11 -41)

With no pressure applied to the brake pedal, the air valve and operating rod are held in the released position by the air valve return spring. This closes the atmospheric port and opens the vacuum port to the rear of the vacuum piston. With vacuum on both sides of the vacuum piston, the vacuum piston return spring holds the vacuum piston in the released position.

 

Applying Position (Fig. 11 -42)

As the brakes are applied, the operating rod and air valve move forward in the vacuum piston to close the vacuum port. Further movement in the applied direction allows the air valve to unseat the floating valve and open the atmospheric port. With vacuum at the forward side and atmospheric pressure at the rear of the vacuum piston, a force is developed which moves the vacuum piston, push-rod and the hydraulic piston in the apply direction.

As fluid pressure increases in the master cylinder, a reaction force is transmitted through the push-rod to the reaction disc to apply a pressure on the air valve. This reaction force moves the air valve slightly rearward in relation to the vacuum piston to close off the atmospheric port. The reaction force is in proportion to the fluid pressure in the hydraulic system and balances the force exerted on the operating rod, providing the drive with brake "feel".

In the fully applied position, maximum atmospheric pressure is allowed to enter at the rear of the vacuum piston. Any additional increase in hydraulic pressure beyond this point must be supplied by physical effort of the driver.

 

Holding Position (Fig. 11 -43)

During brake application, the reaction against the air valve works against pedal pressure to close the atmospheric port. With the vacuum and atmospheric ports closed, the brake is in the hold position. The brake remains in this position until pressure is either increased or decreased on the brake pedal.

DISASSEMBLY OF BENDIX POWER BRAKE (Fig. 11-44)

1. Deplete vacuum supply, then clean the outside of the power brake unit. Remove the filler cap, then empty brake fluid from master cylinder reservoir.
2. Clamp the master cylinder in a vise with the operating rod up. Remove the nylon bellows retainer and bellows from the rear housing.
3. Scribe a line across the front and rear housings and the master cylinder to facilitate reassembly.
4. Brush locking tangs of front and rear housings liberally with seal lubricant. Rotate rear housing so that cut-outs in rear housing line up with tangs of the front housing. Tap rear housing lightly with a plastic hammer to assist in removal. (Fig. 11-45)
5. After transferring the rear housing, piston return spring and push-rod to the bench, remove the return spring and push-rod from the rear housing.
6. Remove the master cylinder to front housing attaching nuts and separate the master cylinder from the front housing.
7. Remove the front housing seal.
8. Remove the vacuum check valve from the front housing if valve is defective.
9. Pry off the filter retainer and remove the felt and foam rubber filters. Use care not to chip the plastic housing when removing the filter retainer.
10. Remove the vacuum piston from the rear housing.
11. Remove the vacuum diaphragm from the vacuum piston.
12. While holding the vacuum piston and operating rod parallel to the bench and with the air valve retainer facing down, depress the operating rod slightly to release the air valve retainer.
13. Remove the air valve assembly from the vacuum piston. Remove the reaction disc from the vacuum piston with a blunt tool. Do not disassemble the air valve assembly.
14. Remove the rear housing seal. (Fig. 11-46)

CLEANING AND INSPECTION

1. Thoroughly wash all metal parts in cleaner. Use ONLY alcohol or brake flushing fluid on the plastic or rubber parts. Blow out all passages and air dry. Place parts on clean paper.
2. Inspect front and rear housings for scoring, pitting, dents, nicks or loose mounting studs. Small imperfections may be smoothed out with fine crocus cloth. Replace housing if damaged.
3. Inspect air valve for scratches, nicks, or breakage. Check seat for smoothness and flatness. Valve should have a free sliding fit when inserted in the vacuum piston bore. Check floating valve for distortion of metal parts and deterioration or abrasions of rubber parts. Replace complete air valve, floating valve and operating rod assembly if any parts are damaged.
4. Check vacuum piston for cracks or rough or uneven floating valve seat. Be sure all openings and passages are clean.
5. Replace air filter element if dirty.

ASSEMBLY OF BENDIX POWER BRAKE (Figs. 11 -47 & 11 -48)

For assembly of master cylinder, refer to MASTER CYLINDER - ASSEMBLY.

1. Coat a new rear housing seal with lubricant and install with Tool J-8761. (Fig. 11-49)
2. Coat the floating valve with lubricant and install in the vacuum piston. Depress the operating rod and insert the air valve retainer.
3. Install the vacuum diaphragm on vacuum piston. Inner lip of diaphragm must fit inside flange on vacuum piston.
4. Install the reaction disc into the vacuum piston with tip of the disc toward the air valve.
5. Install the foam rubber filter, then the felt filter over the operating rod. Install the filter retainer.
6. Coat a new front housing seal with lubricant and install into the front housing. The metal side of the seal faces toward the inside of the front housing. Install a new vacuum check valve if the old one was removed.
7. Install the master cylinder on the front housing. Torque nuts 15 to 20 ft. lbs.
8. Position the master cylinder in a vise with the front housing up. Install the vacuum piston return spring in the front housing with small ID of the spring over the hub in the front housing.
9. Insert the vacuum piston into the rear housing. Coat the large OD of the push rod with lubricant and install piston into vacuum piston.
10. Position the rear housing over the front housing. Compress the return spring. While maintaining pressure on the rear housing, rotate the rear housing clockwise to lock the housings together.
11. Check push-rod adjustment as outlined under PUSH-ROD - ADJUSTMENT.

PUSH-ROD ADJUSTMENT (Fig. 11 -50)

The push-rod incorporates a self locking adjusting screw to provide a means of maintaining correct relationship between the vacuum piston and the master cylinder piston. The relationship between the pistons is important because the compensating port must be open when the vacuum piston is in the released position.

Under normal service conditions, the push-rod does not require any attention, provided the adjustment has not been changed and the push-rod remains in the original vacuum unit.

When a new push-rod is used or the push-rod is transferred to another unit, the push-rod adjustment must be checked as follows:

1. With the vacuum unit assembled, position Gauge J-7723-01 over the push-rod with the legs of the gauge resting on the front housing. The push-rod adjusting screw should just touch the gauge.
2. If necessary to adjust, rotate the adjusting screw until the adjusting screw just touches the gauge.