Have someone hold the brake moderately a bit and see if you can turn the rear tire by hand (jacked up) sometimes youll only see one shoe move then when it bottoms out on the drum the other will have to move to follow.

 

That's a good point. The other piston needs something to push against. I will try that and report back. I still cannot figure out the soft pedal issue. I ordered a pressure bleeder. Is that what it take to get all of the air out???

 

Not saying this is whats happening to you but...
Ive had new master cylinders in the past that sat on a shelf for too long so the pistons arent sealing 100% in the cylinder so it feels spongy af. Ive used an AC vacuum pump into a mason jar in the past to bleed stubborn stuff. Sometimes leaving it alone over night will let whatever air escape from the master. Sometimes jabbing the pedal 100x like you're pissed works (much first hand experience lol).

 

Update...Still not working. Need ideas.

Its been a busy week so I am just now able to work on the YJ. I have carefully bled and re bled the brakes and I have decent pedal when the jeep is off. I can pump, press and hold pedal down and it seems to hold pressure. The issue is that when I start the Jeep, the pedal travel is very soft and I can push it to the floor. I am able to drive the car and it does have brakes, they are just very weak. A couple of other points of information-
*The parking brake works correctly.
*With the parking brake engaged, there is no difference in the brake pedal travel.
*The rear shoes are adjusted close (ie: there is a slight drag on the drums)
*I cannot locate a leak anywhere in the lines or cylinders.

Could it be the booster? It is the only component I have not replaced. It does somewhat of a farting sound when I push the pedal but my thought is that if the pedal pressure is reduced when the car is running - the booster is working.

Any help would be greatly appreciated!!!

 

Nothing wrong with the booster. The rear shoe adjustment is fine. Guess what this leaves????


AIR in the system.

 

Are you using a 1 man bleeder? The new rear bleeder screws are poorly made, probably loose and letting air in. You have to two man bleed. Push Pedal to floor, open bleeder, close bleeder. release pedal, check fluid level. Rinse repeat. I've seen guys use an entire container of brake fluid to get the brakes bled out.

Also if your bleeding on a angle this can cause air to get trapped in the master.

 

Silly question, but you bench bled the master first right?

Other than that, the best way to get it done is just like Master4th says.

 

seems to have covered all the bases but one, did you start at furthest point from the master--pass rear? if not you may have a section of line with air in it that got "bypassed" during the process.

 

Did you touch the front brakes? One time I took the front end completely apart instead of doing one side at a time. Installed the front calipers on the wrong side. This left the bleeder valve in the wrong location. Had a 93' XJ daily driver at the time. After two weeks of frustration I compared the YJ vs XJ bolt by bolt before discovering my mistake.

Different vehicle, had troubles with the new master cylinder. Eventually had my wife just barely (very softly) push the brake pedal. I gently tapped on the master cylinder (with cover off) with a screwdriver handle. Had lots of hidden air bubbles.

 

WCJEEP, Man I am soooo glad I am not the only person that has done that. I had the same issue as the OP, could not figure it out. Had to tow my YJ to my local shop to have it fixed. I felt so stupid!

 

Here is my standard answer for a soft brake pedal on a YJ:

Check the adjustment of your rear brakes. A soft pedal is an indication that an adjustment is needed. Try this:

1. Raise the rear of the jeep so you can turn the tire.
2. Adjust the star adjuster in the drum until you start feeling friction on the pads as you
turn the tire.
3. Have an assistant stomp on the brakes as you spin the tire. Stomping on the brakes centers the pads in the drum.
4. Tighten the star adjuster until you start feeling friction on the pads again and repeat the stomp procedure.
5. Tighten the star adjuster until you can't move the tire by hand anymore, then back off till you can just hear the pads rubbing but the wheel still turns. A little friction won't hurt them but you don't want to have to struggle to turn the wheel with both hands.

Also check your master cylinder to make sure that with the lid off, that both the front and rear chamber squirt fluid out of them when you push the brake pedal. Put a rag above the master cylinder with the lid off so you can see what happens and have someone push the pedal. If fluid squirts up out of the front and not the back you know where to start bleeding again. I actually cracked open the line at the master cylinder for the rear chamber to get all the air out.

One other issue you may have to address if the above doesn't work. You replaced the master cylinder only and not the booster. If you buy them as a unit they come pre-adjusted from the factory. But, if you just replace one or the other you may have to go in and adjust the push rod that comes from the booster into the back of the master cylinder. Hold the main part of the rod with a pair of channel locks and then turn the threaded portion out a couple of turns at a time to see if this firms up the pedal. Make small corrections because if you go to far then when the brakes heat up and the fluid expands it can lock your brakes up while your driving. I had to do this a couple of times until it was right and then carried tools with me so I could back it off if I had gone to far. Good luck on your project.

 

Got them working! The Jeep stops great now.

It turns out I had two problems..
1. I needed to adjust the rod between the booster and master cylinder. Once I reduced the free play, the pedal was much more firm.
2. I used a pressure bleeder to bleed the system again (no one to help me) and it took two big bottles of brake fluid to remove all of the air from the lines. I think the culprit was the passenger rear. It was surprising how small and occasional the bubbles were.

My advice for others is to follow the steps given is this thread but be ready to push a large quantity of fluid through the lines. Posters told me this but I was still surprised.

I am still concerned at how easy it is to push the brake pedal. My booster is definitely working.... a couple of follow up questions-

Is it possible to have too much booster assist?
Should the booster make an air swoosh noise when the brake pedal is pushed?

Thanks again for all of the advice and I hope this thread can help others.

 

With a new boosters, I've noticed some swoosh too, but never really had a performance issue. It's a brand-new booster. As long as the booster isn't leaking down and the pedal is returning to the top position, it's probably fine. The old one probably didn't have as good of a vacuum any more, and probably had reduced flow on the back side.

Since the booster is giving you more assist, it will be of course be easier to push. There are general guidelines manufacturers have regarding "pedal firmness" when they design brake systems. You can technically have "too much" assist with a bigger booster, or at least more than you intend. Just drive it easy and get a feel for the new pedal.

 

The air noise you hear is perfectly normal....
The booster is powered by vacuum... that is the sound of air being pulled in

 

Question, when some threads state that the front brake bleeders need to be up we are talking on the top correct? rebuilding my 1988 and the brake system is all new (except proportioning valve) and bench bled, bled the system twice and still soft pedal. Also thought I got a bad reman booster and replaced that again and no luck. Any ideas?

 

On the back of the caliper you can see the a round area of the caliper. That is the brake fluid reservoir for the caliper. Make sure the bleeder screw is on the top side of the reservoir. This is so the air can get out of the reservoir.

Hope this helps!