PRINCIPLES OF OPERATION

Released Position (Fig. 11 -51 )

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 the 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 -52)

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 insert to apply a pressure on the air valve. This reaction force moves the air valve slightly forward 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 driver 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 -53)

During brake application, the reaction valve 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 (Figs. 11 -54 and 11 -55)

1. Deplete vacuum supply, then clean the outside of the power brake unit. Remove filler cap then empty brake fluid from the master cylinder reservoir.
2. Clamp the master cylinder in a vise with the operating rod up.
3. Loosen the locknut on the operating rod, then remove the clevis, locknut, plastic boot and the air filters.
4. Scribe a line across the front and rear housings and the master cylinder to facilitate reassembly.
5. Remove the front to rear housing retainer, noting its location.
6. Brush locking tangs of front and rear housings liberally with seal lubricant. Rotate rear housing to separate the housings. Tap rear housing lightly with a plastic hammer to assist in removal. (Fig. 11-56)
7. Transfer the rear housing to the bench.
8. Remove the vacuum piston return spring and the push-rod from the front housing.
9. Remove the master cylinder from the front housing.
10. Remove the vacuum check valve and "0" ring from the front housing.
11. Remove the air filter and front housing seal. (Fig. 11-57)
12. Remove the vacuum piston from the rear housing.
13. Remove the seal from the rear housing (Fig. 11-58)
14. Remove the diaphragm from the vacuum piston.
15. Loosen the three guide attaching screws and remove the guide. Remove the rubber reaction bumper from the guide. (Fig. 11-59)
16. Depress operating rod and remove the reaction plate. Remove the reaction insert and the rubber reaction ring from the reaction plate.
17. Remove the reaction plate "0" ring from the vacuum piston.
18. Depress operating rod and remove the retaining ring from the air valve. (Fig. 11-60)
19. Remove the air valve, floating valve and operating rod assembly from the vacuum piston.

CLEANING AND INSPECTION

1. Thoroughly wash all metal parts in cleaner. Use ONLY alcohol or brake flushing fluid on 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. Replace air filter elements if dirty.

ASSEMBLY (Figs. 11 -54 & 11 -55)

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

1. Coat a new rear housing seal with lubricant and install as shown in Fig. 11-61.
2. Coat the floating valve and the "0" ring on the air valve with lubricant and install the operating rod assembly into the vacuum piston. Install the retaining ring. (Fig. 11-60)
3. Install the reaction plate "O" ring on the Vacuum piston.
4. Position the reaction plate on the vacuum piston, aligning three of the holes with the threaded holes in the vacuum piston. (Fig. 11-62)
5. Install the rubber reaction ring and the reaction insert into the reaction plate.
6. Coat the reaction insert bumper with lubricant and install into the guide.
7. Position the guide on the vacuum piston. Retain the three screws. Torque screws 80 to 100 in. lbs.
8. Install the diaphragm on the vacuum piston making sure the diaphragm is seated in the groove of the vacuum piston.
9. Apply lubricant to the ID of the rear housing seal, then insert the vacuum piston into the rear housing.
10. Apply lubricant to a new front housing seal and install seal in the front housing, metal ridge of seal up. (Fig. 11-63)
11. Install a new air filter over the seal.
12. If the vacuum check valve was removed, install a new check valve and "0" ring into the front housing.
13. Position master cylinder on the front housing. Torque attaching nuts 15 to 20 ft. lbs.
14. Clamp master cylinder in a vise with the front housing up.
15. Insert the push-rod into the master cylinder piston with the adjusting screw toward the master cylinder,
16. Position the vacuum piston return spring over the hub of the front housing.
17. Apply lubricant to both sides of the outer edge of the diaphragm.
18. Position the rear housing over the front housing, noting the alignment marks. Depress rear housing and rotate until the housings are locked. Install the front 'to rear housing retainer.
19. Install the air filters over the operating rod, small diameter filter first.
20. Install the plastic boot over the operating rod. Compress the sides of the boot so that the projections on the boot enter the holes in the mounting bracket.
21. Install the locknut and the clevis on the operating rod.
22. Adjust push-rod as outlined under PUSH-ROD ADJUSTMENT.

PUSH-ROD ADJUSTMENT (Fig. 11-64)

Under normal service conditions, the push-rod does not require any attention provided the pushrod 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 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 should just touch the gauge.
2. If the push-rod is high, grind the end of the push-rod until it just touches the gauge.
3. If the push-rod is low, a new service pushrod must be installed and ground down until adjustment is correct.