I posted this on jeepforum.com but haven't gotten a response. I know its a holiday but I figured our shade tree mechanics might have some insight.

This for an 84 CJ& with the 258 inline 6 with power brakes.


I lost all resistance in my brake pedal and stopping power. I figured air had gotten in somehow. The brake fluid level was fine.

I replaced the wheel cylinders and bled the system. Still no pressure at the pedal.

I removed the master cylinder and the the area the plunger from the booster would make contact was stuck at the bottom of the master cylinder. So that explains the lack of pedal pressure.

I get a new (not a reman) master cylinder from Napa and the plunger is extremely hard to push in. So bad that bench bleeding with that method is near impossible. It is so difficult that I opt to mount then bench bleed via reverse bleeding.

I struggle to get the master cylinder mounted because the mounting bolts on the booster are not square. They spread slightly as they extend from the booster. Plus the resistance when trying to compress the plunger on the master cylinder required a ton of force.

Finally after enough brute force the master is mounted and bench bled via reverse bleeding.

I test the brake pedal and it will not move. There is only about a quarter inch of travel in the pedal and that is only taking up the slack of the linkage.

So where do I go now? Is it a faulty master cylinder or do replace the booster or do I replace both? If both should I start with the master cylinder or the booster before checking the other? I would really like to get this fixed before family beach week and any insight would are greatly appreciated.

If you can’t move the plunger in the new master cylinder, I’d start with a replacement. How old is the brake fluid? Could be some fluid contamination that killed the first one.

The master cylinder mounting studs should not be spread apart, nor should the booster pushrod offer any resistance to mounting up your master cylinder. First compare (using a pick, pencil or similar object as a makeshift depth gauge) the depths of the hole in the piston of both the old and new master cylinders to be sure the new one isn't too shallow, but I suspect your power booster has taken a shit. The booster may have also accounted, in its initial failure stage, for your long pedal travel.

Do you have a person to help? If so try this test.

Turn on engine for the booster.
Go back to the passerger side rear brake cylinder have someone press on brake pedal and than you crack open the brake cylinder bleeder valve to let some fluild out. Close bleeder valve than have your helper let up on the brake pedal. Than see what you got.

I had a 75 Chevy Blazer that had that same problem. Got in it and drove to the corner and it would not stop, felt like there was a brick underneath the brake pedal. Called my buddy who was a mechanic and he recommended replacing the power brake booster and the plastic check valve that had hose attached to it that went to the intake manifold. Replaced both and problem solved.

A "new" part sometimes means, "never ever worked".

if you had the problem with it off the vehicle on the bench I'd take it back and ask to see a different one, see if it's easy or hard to move.

Google the PN on the box and see if it's correct for your application..

Sometimes I read too fast and skip over things.

quote:

I get a new (not a reman) master cylinder from Napa and the plunger is extremely hard to push in.

So you're saying that you're bench bleeding, with the master cylinder off the car, in a vise and pushing on the back side of the pistons with the ports open, the pistons won't move? That's not right. You should be able to push them through a full stroke (somewhere between 1-2 inches) with maybe 10 lbs. of force.

Yes, it sounds like the vacuum booster is bad.

I think the piston from the booster is sticking too far out thus bottoming out the travel in the mast cylinder. I'm trying to figure out how to adjust it now.

Twice I've had master cylinders\brake boosters replaced where the new part was bad from the start. It happens.

quote:

Originally posted by Apphunter:
I think the piston from the booster is sticking too far out thus bottoming out the travel in the mast cylinder. I'm trying to figure out how to adjust it now.

I recently went through just the opposite.

There's at least one video for sure on Youtube because I watched it. Need a wrench and a pliers, or two pliers (like I used). Hold the rod, turn the end/nut/cap. When you get down to fine tuning it, as little as a quarter turn can change a lot. Took a good deal of fiddling to get it right, but was so worth it. (I had no peddle)

You want about .020 lash between the rod out of the front of the booster the the dimple on the Master Cylinder piston's back side.

Use playdoh, claybar clay or similar clearance checking media.

Set the MC on the booster and finger tighten the mounting nuts, then remove it and check the clay.

Then put it together, and connect all your lines.

If there is a preload from the booster rod pushing on the piston and unseating it, it will cause a bypass and you will never get a proper fill stroke or proper bleed.

If you have the incorrect Master, you may have problem with the pin to far in or not far enough.

The MC for a manual brake has a deep dimple (almost a tube/hole) and a Power Brake system has a dimple about 1/8 - 3/16 deep.

quote:

The MC for a manual brake has a deep dimple (almost a tube/hole) and a Power Brake system has a dimple about 1/8 - 3/16 deep.

Thanks for the information. That tells me someone has been there monkeying around. There is definitely a brake booster on the fire wall and the master cylinder that was on it has a deep dimple you described but the replacement I recieved has shallow dimple. Sounds like I need a new brake booster too.

quote:

The MC for a manual brake has a deep dimple (almost a tube/hole) and a Power Brake system has a dimple about 1/8 - 3/16 deep.

That has not always been my experience, not universally.* But it you have a shallow hole and a deep hole stroke (hey now!) it would certainly explain your problem.

*(Perhaps at one time, but I wonder if manufacturers (or re-manufactureres) even bother trying to keep them straight or make both anymore? I know they ask, but that doesn't always mean that's what you get. That's probably what happened with my most recent experience. Both the one I took off and the one I replaced it with had the very deep "hole", so much so that the rod barely moved the MC piston. Poor brakes and almost to peddle. But when what you take off matches what comes new in the box, you assume everything is as it should be. Oddly it was like that for at least the last two previous owners. Though I can't say much, I drove it that way for a year before trying to fix it. But that means it was on the road that way for a MINIMUM of 12 years.)

This message has been edited. Last edited by: cas,