New brake booster and master cylinder problems (1 Viewer)

[mcruiser]

4/1979 FJ40

Hoping someone can help me figure this out. Long story short… with the engine off I can build up pressure in the brakes. When the engine is on the pedal blows right through the stroke building no pressure.

The brake booster went out last Tuesday. Brake pedal was hard to push, engine was stumbling. New master cylinder and booster came in from Rockauto on Saturday. I also bought a push rod adjustment tool from Amazon. I bench bled the master cylinder, backing off the screw on the side to get full stroke. This would leak a little while bench bleeding. Is that normal? I also set the clevis position by copying the old booster.

I got everything assembled on Saturday, then Sunday I set to bleeding all four corners starting with the left rear then RR, LF, then RF. My one-man method was opening the bleeder, pressing down the brake pedal then holding the pedal down with a 2x4 wedged between the pedal and seat then shutting off the bleeder. Then repeat. I also did some gravity bleeding.

After bleeding, I started the engine and discovered no pressure build up. There was also a slight knock sound coming from the booster (or so I think). The knock would happen when I pressed the brake, let off, then rev the engine a little. It wasn’t a constant knock. This led me to believe that possibly there was a gap between push rod and master cylinder so I took it apart again, did the dab of grease on the end of the push rod and ended up having to extend the push rod out about 1.5 turns to get contact.

Started the engine again and still no pressure. Re-bled all brakes with the same method but this time with engine running. More air came out.

The pedal still doesn’t build up pressure with the engine on. The truck idles fines and revs easily so no vacuum leak. The pedal goes easily thru the stroke and before the last inch or so feels like it hits a detent. Like a sticky point then bottoms out.

Any advice or comments would be greatly appreciated.

 

[Brew40]

I’m sure some pics will help the forum pros decipher anything obvious. I had a heck of a time getting my brakes completely void of air. And adjusting the pushrod/mc/booster even using the tool and comparing the old.
There are some pretty good posts, just use the search function. Don’t stress-
your almost there

 

[70fj40]

If you are not getting anymore air when you bleed, absolutely none. Then I would consider two things as a next step. I would bench bleed the master again to be sure the problem is not there. Then I would replace my brake lines. Any soft lines first then the rest.

 

[mcruiser]

I'll check the gap between the booster and master one more time then bench bleed the master. Am I correct in loosening this screw to get full travel in the master cylinder? BTW, the brakes were working fine before the booster went.

 

[mcruiser]

This one:

 

[peesalot]

I would go get a vacuum pump and see if the booster holds vac if it does then I would vise grip clamp the soft line from frame to axle at rear and see if you get pedal, if not do the front soft line, check pedal, if still no pedal its in the master.

 

[70fj40]

I believe you should not open that screw.

 

[mcruiser]

I misread another post. This is the step I forgot to do:

 

[1911]

That bolt holds the piston assembly from coming too far out when the pedal is not depressed; need to stay in; like the manual says, push the piston in all the way before you install it and leave it.