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Brake booster and MC swap issue

jeeping4fun

NAXJA Forum User
Location
Corona, CA
Hey guys

I have a 87 MJ that I swapped a 96 booster and MC into. I just took it for a drive and when pushing the brake I get nothing until the last inch or so then it goes full brake. Like lock the tires up lol.

The booster and MC were new. I left the original praportioning valve. Bench bled the mc and bleed almost 64 oz of fluid through.

I also put new calipers and brakes pads/rotors on.

Any suggestions?

Also when I hit the brakes with it off I can hear a suction sound coming from booster/mc area.

Thanks
 
That suction sound may be a bad booster, I went thru 2 before finding a good one. Also thoroughly bleeding the brakes is essential. I had a to more than once.
 
Damn ok. Its brand new but who knows. I'll take it back and get another new one.

I just bled them again and still the same. I'll try it again in a couple days and see if it just has that much air. Crazy I've put like 80oz through it now.

Thanks
 
You may have an air bubble trapped in the proportioning valve. Just loosen the end, fluid will come out. Be careful, there's a spring behind it, so it can be a bear sometimes when your hand is covered in fluid. At this point you're half way to putting the proper proportioning valve in their. You don't need to replace the body, just the screw in piece and the spring out of a ZJ. I prefer not to wear out my brakes prematurely when the fix is so easy and inexpensive.
 
Did you get the pushrods(there are 2) adjusted properly?
 
I'll try to crack the valve. I did the rear disc brakes on my xj and upgraded the spring. Does the ZJ spring fit in the old style propotioning valve?

I did not adjust any pushrod. Where are these located?
 
I did not adjust any pushrod. Where are these located?

You've got one between the pedal and the booster, and one between the booster and the master cylinder.
 
I didnt adjust either. I did add a 1/4" spacer between the booster and fire wall so I didnt need to trim the fire wall. Also by doing that the length of the rod inside the cab would be the same. If I didnt the pedal all the way up would have a 1/4" of pressure into the booster.

I also didnt mess with the one from the booster to mc.

I am looking at it again now and the MC is the one making a noise when I let off the pedal. Sounds like its ducking air. I cant find a leak anywhere. Damn thing.
 
Well I think the sound is inside the MC. Waiting on my wife because I can push the pedal and run to the front in time.

Also I got those quick bleeders so i could bleed them myself. Tried to save myself from the peanut gallery. "How long will this take" "my leg is tired". Anyways gonna replace those and bleed it with two people. If I cant get the air out then I'll assume the MC is bad and replace it...again. it will be a couple weeks before I have time but I'll update once I find the problem.
 
I am looking at it again now and the MC is the one making a noise when I let off the pedal. Sounds like its sucking air. I cant find a leak anywhere. Damn thing.

Well I think the sound is inside the MC.

It's not to likely on the sound being the MC would/could be air. There is a gasket that can leak air between the MC and the booster.
 
So these are ZJ rear disc's installed on a D35?
 
No I have a dana 44 with drums. I just upgraded the MC and booster to a 96 XJ.

My other jeep XJ I did a rear disc conversion and upgraded the spring.

Since my MJ is drum in the rear that's why I was asking if I should do the spring? I never heard of doing it in any of the mc booster upgrade threads.
 
For me the pushrod was a bit short so(if I recall right) I unscrewed the one in there and replaced it with a longer bolt that I cut the head of. I went from braking 300ft early to almost slamming my face into the steering wheel. Can't believe I waited 2 years to check something so simple. Anyways, check your pushrod.
 
Yes it is lifted. I had to replace ebrake cables also. I lifted it 5.5", swapped the dana 44 and all brake lines. Hence the issue it tracking the problem. It's all been messed with in the last 2 months. You think the rear pads aren't adjusted correctly? Causing the peddle feel?
 
I apologize for bringing the spring into the conversation. I misread the initial post and for some reason thought you had discs in the rear. The advice that there may be a bubble trapped in the "proportioning" valve is still true. DO NOT change the spring or plunger unless you have disc brakes in the rear, in which case, you should replace both with ones from a ZJ.
 
Lol not a problem. I was thinking something was misunderstood.

I'll crack the proportioning valve and see if air comes out. Bleed the system again and also change the bleeder valves. If that doesnt work I just order another brand new MC. So then I'll swap that. If that doesnt work I'll set it on fire. Thanks for the help!
 
If that doesnt work I'll set it on fire.
I'll give you fiddy bucks for it! :laugh:

A buddy had similar problems after replacing the wheel cylinders on his lifted XJ. He brought it to me on a snowy day. Given the driving conditions, I could feel the rear brakes engaging at 50% pedal travel, the fronts started working at 75% pedal travel.

I forget how much lift it has, but it rotated the axle housing enough that the wheel cylinders do not sit level. He theorized the wheel cylinders had air trapped in them. We put jack stands under the axle, pulled the wheels and drums. The drums slid off with no effort! Now, we are looking at two potential problems:

We bled the hydraulic system by unbolting the wheel cylinders from the backing plate, and used a pry bar to make it level. I did observe a few air bubbles when we bled the driver side, did not see any on the passenger side.

When we tried to adjust the shoes, we found one adjuster was very difficult to turn, and the other was frozen. We cleaned and lubed the adjusters. We adjusted the shoes to the point the drums were difficult to push over the shoes (the drums have wear ridges).

Before any further adjustments, we have full braking at 30% pedal travel.

The point is, you can have more than one problem. The only thing you can do is go through your equipment in a methodical process, to make sure everything is working as it should.
 
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