Brake Master Cylinder Push Rod Clearance

I suggest you open the bleed nipple at each brake that is binding and if the brake releases the problem is probably caused by a sticking booster, master cylinder or a maladjusted linkage or booster push-rod.
When everything is correctly adjusted/set a small clearance should exist between the booster push-rod and the fully returned master cylinder piston.

If each brake doesn't release the problem is clearly lack of maintenance of the brakes which will need dismantling and checking to locate the trouble.

I suggest you go back to basics and adjust the brake pedal linkage correctly and then ensure there is a small clearance between the fully returned master cylinder piston and the booster push-rod.

The latter check is usually performed with a service tool but if that isn't available some very careful and accurate measuring must be done. In the absence of the manufacturer figure a clearance of about 0.006/0.008 is about right.
Sometimes the push-rod is adjustable and sometimes shims must be added.

Possibly there is no clearance between the master cylinder piston and the brake booster push rod which could be preventing the piston returning fully so the fluid inlet port remains covered or partially covered.

When replacing the master cylinder or any part of the brake pedal linkage or brake booster it is important to ensure everything is correctly adjusted and a clearance exists between the booster and master cylinder piston. In the absence of the correct service tool some fancy measuring is needed; a clearance of about 8/10 thou inch is about right. Sometimes the booster pushrod is adjustable and sometimes shims are needed.

Adjust the brake pedal pushrod so there is a small clearance or free play in the brake pedal linkage equivalent to a couple of millimetres of free play on the pedal.

It is then extremely important to ensure there is a clearance of about 0.010" between the brake master cylinder piston and the booster pushrod. In the absence of the official service tool some careful measuring will be required. Some boosters have an adjustable pushrod and some need shims to be inserted between the booster and cylinder and some manufacturers provide no facility for adjustment...

going to have to check the whole system --was this reason booster was replaced --recheck installation is brake rod properly adjusted -vacuum line and valve working --is master cylinder performing properly no seals damaged or blown ---are the lines in good shape --the abs controller functioning --are the calipers performing correctly not sticking or jamming --are the rubber hoses good sometimes the hoses deteriorate internally and not let fluid return --if rear drums are they adjusted correctly

Remove the master, two bolts. Move it out of the way some what. Be careful of the brake lines. Under the dash on the fire wall, at the brake pedal, there are four nuts to remove. Also remove the linkage on the brake pedal. Pull out the booster and replace.

The subdued sound of a burst of air is a sign the vacuum servo or booster is functioning as it should. For most people most of the time a power booster and a vacuum booster are the same things - there are such things as various types of "power brakes" but they are in the world of specialised motoring and commercial and industrial vehicles.

I suggest you revisit each wheel brake and ensure they are all as the vehicle manufacturer intended and the front brakes are not dragging. If they are dragging when turned by hand while lifted on a jack something is clearly wrong. Try briefly releasing a fluid bleed nipple - if that improves matters the hydraulic system has residual pressure. If there is no improvement something about the brake isn't correct.

Once you have satisfied yourself everything is as it should be between the master cylinder and the ground you should ensure the brake pedal linkage is correctly adjusted and operating properly and finally before re-bleeding the hydraulics you should ensure a small clearance exists between the brake booster push rod and the master cylinder piston, about 8 to 10 thou inch is about right...

REMOVAL & INSTALLATION NOTE: Make sure that the booster rubber reaction disc is properly installed if the master cylinder push rod is removed or accidentally pulled out. A dislodged disc may cause excessive pedal travel and extreme operation sensitivity. The disc is black compared to the silver colored valve plunger that will be exposed after the push rod and front seal is removed. The booster unit is serviced as an assembly and must be replaced if the reaction disc cannot be properly installed and aligned, or if it cannot be located within the unit itself.

1. Disconnect the stop lamp switch wiring to prevent running the battery down.
2. Support the master cylinder from the underside with a prop.
3. Remove the master cylinder-to-booster retaining nuts.
4. Loosen the clamp that secures the manifold vacuum hose to the booster check valve, and remove the hose. Remove the booster check valve.
5. Pull the master cylinder off the booster and leave it supported by the prop, far enough away to allow removal of the booster assembly.
6. From inside the cab on vehicles equipped with push rod mounted stop lamp switch, remove the retaining pin and slide the stop lamp switch, push rod, spacers and bushing off the brake pedal arm.
7. From the engine compartment remove the bolts that attach the booster to the dash panel. To install:
   
8. Mount the booster assembly on the engine side of the dash panel by sliding the bracket mounting bolts and valve operating rod in through the holes in the dash panel. NOTE: Make certain that the booster push rod is positioned on the correct side of the master cylinder to install onto the push pin prior to tightening the booster assembly to the dash.
   
9. From inside the cab, install the booster mounting bracket-to-dash panel retaining nuts.
10. Position the master cylinder on the booster assembly, install the retaining nuts, and remove the prop from underneath the master cylinder.
11. Install the booster check valve. Connect the manifold vacuum hose to the booster check valve and secure with the clamp.
12. From inside the cab on vehicles equipped with push rod mounted stop lamp switch, install the bushing and position the switch on the end of the push rod. Then install the switch and rod on the pedal arm, along with spacers on each side, and secure with the retaining pin.
13. Connect the stop lamp switch wiring.
14. Start the engine and check brake operation.

Fig. 1: Check the booster pushrod adjustment using a gauge with the shown dimensions


Fig. 2: Exploded view of the typical power brake booster Fig. 3: Booster vacuum hose connections - 2.3L & 2.9L


Fig. 4: Booster vacuum hose connections for the 4.0L

a defective master has just one common symptom, a soft sinking pedal.
Instructions Things You'll Need:

• Nose pliers
• Ratchet and socket
• Ratchet extension
• Slip joint pliers
• Wrench
• New cotter pin

Removing the Old Brake Booster
1. Step 1 Park your car in a safe place with enough room to work around the front and driver side of the vehicle.
   
2. Step 2 Locate the booster push rod connected to the brake pedal.
   
3. Step 3 Remove the cotter pin securing the booster push rod pin to the brake pedal using a pair of nose pliers.
   
4. Step 4 Release the push rod pin from the brake pedal and slide the push rod off the brake pedal.
   
5. Step 5 Unscrew the four mounting nuts off the brake booster studs using a ratchet, ratchet extension and socket. You should be able to see the four booster studs extending through the firewall, in front of the brake pedal.
   
6. Step 6 Disconnect the vacuum hose from the brake booster, working from the engine compartment. Use a pair of slip joint pliers.
   
7. Step 7 Remove the two nuts holding the brake master cylinder to the brake booster. Use a wrench or ratchet and socket.
   
8. Step 8 Separate the brake master cylinder from the booster just enough to make room for booster removal.
   
9. Step 9 Pull the brake booster off the firewall and remove it from the vehicle.
   Installing the New Brake Booster
10. Step 1 Set the new booster in place by sliding the push rod and four mounting studs through the mounting holes on the firewall.
    
11. Step 2 Slide the brake master cylinder flange over the two mounting studs on the brake booster.
    
12. Step 3 Start the two brake master cylinder mounting nuts by hand to avoid damage to the threads.
    
13. Step 4 Tighten the two brake master cylinder mounting nuts using a wrench or ratchet and socket.
    
14. Step 5 Connect the vacuum hose to the brake booster.
    
15. Step 6 Screw the four mounting nuts to the brake booster working from inside the vehicle. Start the nuts by hand to avoid damaging the threads.
    
16. Step 7 Tighten the four mounting nuts using the ratchet, ratchet extension and socket.
    
17. Step 8 Position the brake booster push rod over the brake pedal and slide the push rod pin.
    
18. Step 9 Install a new cotter pin to secure the booster push rod to the brake pedal
    

get ready for some work.you need to remove the master cylinder nuts that hold master cylinder to brake booster.once you remove nuts you have 2 choices remove master cylinder to remove booster. to keep from bending brake lines.or you can leave master cylinder in pull it toward you easily careful dont bend lines .disconnect vacuum line at booster .go inside vechicle to floor on driver side loosen four nuts on floor to booster .disconnect power brake push rod from brake pedal .dont force pushrod to the sides when disconnecting it.remove four booster nuts then remove booster.when in stalling the new booster loosely install four nuts then connect the push rod to the brake pedal .install new clip tighten the booster unit .if you decide to remove master cylinder use inline wrenches on master cylinder to remove lines.also plug the master cylinder brake line holes to keep from losing all brake fluid.