The power brake booster reduces the effort required to push the brake pedal and apply the brakes. Power assist occurs due to a pressure differential within the brake booster.
When the engine is running and the brakes are not applied, there is engine vacuum on both sides of the booster diaphragm. When the brake pedal is pressed, the input rod assembly moves forward inside the valve body until the vacuum port closes. At this point there is still vacuum on each side of the diaphragm. As the input rod assembly continues to move forward, an atmospheric port is opened and atmospheric pressure enters the rear half of the brake booster. The engine vacuum on the front side of the diaphragm and atmospheric pressure on the back side of the diaphragm assist the input rod in pushing the diaphragm plate forward and the master cylinder pushrod against the master cylinder piston.
When the brake pedal is held in this position, the diaphragm momentarily continues to move forward, compressing the outer edges of the reaction disc. This causes the center of the disc to press the input rod back and close the vacuum port or atmospheric port, depending on whether the brake pedal is released or depressed further.
The vacuum hose from the engine contains an internal check valve that closes when the engine is shut off, trapping engine vacuum in the booster. This reserve vacuum allows several assisted brake applications with the engine off.
REMOVAL & INSTALLATION
- Remove the master cylinder. Refer to the procedure in this Section.
- Disconnect the hose connecting the intake manifold to the power brake booster.
- Working under the instrument panel, remove the spring clip in the brake pedal clevis pin. Remove the clevis pin and brake pedal pushrod from the brake pedal.
- Remove the 4 attaching nuts that hold the booster to the bulkhead and remove the power brake booster.
- Have an assistant position the power brake booster on the bulkhead so the 4 retaining studs protrude through the bulkhead into the passenger compartment.
- Working under the instrument panel, install the 4 booster retaining nuts.
- Apply lithium grease to the clevis pin and install it through the brake pedal pushrod and the brake pedal.
- Install the clevis pin spring clip in the clevis pin.
- Connect the hose connecting the power brake booster to the intake manifold. Because there is a check valve located in the center of the hose, make sure the arrow on the hose points toward the engine.
- Check the power brake booster pushrod length and adjust, if necessary. Refer to the procedure in this Section.
- Install the master cylinder.
POWER BRAKE BOOSTER PUSHROD ADJUSTMENT
Except 1992 Vehicles with Anti-lock Brakes
WITH ADJUSTMENT GAUGE
- Install adjustment gauge T87C-2500-A or equivalent on the end of the master cylinder.
- Measure the depth of the pushrod socket in the master cylinder.
- Apply 19.7 in. Hg of vacuum to the booster using a hand vacuum pump.
- Invert gauge T87C-2500-A or equivalent onto the booster.
- Check the clearance between the end of the gauge and the master cylinder pushrod. There should be no clearance between the gauge and the pushrod.
- If adjustment is necessary, loosen the pushrod locknut and adjust the clearance.
- If adjustment gauge T87C-2500-A is not available, a pencil can be used instead.
- Insert the pencil in the pushrod socket and mark it with a hacksaw blade.
- Measure the length of the pencil to the saw mark with a ruler.
- Using a ruler, measure how far the master cylinder pushrod protrudes out of the booster.
- Measure the master cylinder boss with a ruler. Subtract the length of the boss from the length of the pencil. The difference in length between the master cylinder pushrod and the corrected pencil length is equal to the pushrod clearance.
- Adjust the pushrod length to get the correct clearance. It should be 0.025 in. (1mm) shorter than the pushrod socket when using this method.
- Remove the master cylinder.
- Loosen the brass holding screw on master cylinder gauge T92C-2500-A or equivalent and retract the gauge rod. Attach the gauge to the booster and tighten the retaining nuts to 87-140 inch lbs. (10-16 Nm).
- Start the engine and let it idle for approximately 15 seconds.
- Push lightly on the end of the gauge rod until it just contacts the power brake pushrod. Tighten the brass holding screw to secure the gauge rod in place.
- Remove the master cylinder gauge from the booster and turn the engine OFF .
- Using a depth gauge, measure and record the height of the master cylinder gauge rod as shown in Fig. 9. This measurement will be called ""D1".
- Loosen the master cylinder gauge brass holding screw and place the gauge on the master cylinder, as shown in Fig.10.
- Push lightly on the end of the gauge rod, until it just bottoms in the master cylinder piston. Tighten the brass screw.
- Remove the master cylinder gauge from the master cylinder.
- Using a depth gauge, measure and record the height of the gauge rod, as in Step 6. This measurement will be called ""D2".
- Subtract D1 from D2. Adjust the power brake pushrod nut to lengthen or shorten the booster pushrod the amount equal to the difference between D1 and D2. If measurement D1 is larger than D2, the pushrod must be lengthened. If measurement D2 is larger than D1, the pushrod must be shortened.