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Brake Problem on 2012 Unlimited Rubicon after Changing Rear Pads and Rotors and Master Cylinder

1.7K views 7 replies 3 participants last post by  dragsterguy1  
#1 ·
I changed rear pads and rotors with my boys. Brakes worked but super spongy, went almost to floor and made 2-3 second hissing air release type noise.

Concluded I probably needed master cylinder (MC) after searching. Replaced it. Did not check to see that the shaft was same length as old one. Rookie at changing these.

Bench bled the MC as best I could after watching videos. Possibly subpar but did it twice and installed twice to make sure.

Both times i have NO BRAKES. Pedal now is a much shorter push and then it bottoms out and is much firmer. The hiss when holding down is now more of a constant hiss. Bled all 4 brakes multiple times going back passenger, back driver, front passenger, front drive side.

I scheduled a dealer appointment, but I'll have to tow it and would like to resolve myself.

After reading a bunch of threads the only thing I can think could be causing this is (1) aftermarket MC has longer shaft, (2) booster is jacked form having to press the caliper pistons in a bunch and too fast to get them to fit.

Any experienced folks have a potential solution or a way to test these? Thanks
 
#3 ·
Might consider changing your brake lines if they are original. I've read posts that some have had the interior of the brake lines degrade, collapse, and cause brake problems. However, I would be more inclined in your case to think either you got the wrong MC, a defective MC, or air in your system. My guess the hiss is pressure bleeding out of your system.
 
#4 ·
Thanks. I wondered about MC after reading posts about some aftermarket MCs not fitting working correctly. Sounds like air leaking, but does that mean system is not bled correctly? I though air in system meant soft pedal and brake going to floor. Mine is stiff and only goes about 1/3 or so to floor before it won't go any further.
 
#5 ·
I'm not an expert but I dont think you should hear any "hissing" when depressing the pedal. When bleeding the brakes at the calipers did you see fluid being pushed from the bleed when pushing the brakes? If not then I would expect either an obstruction not allowing pressure to build up in the brake lines or a very leaky seals in the MC. I must have misread your original post I thought you werent getting any braking until you were at the floor. If it is stopping more like 1/3 of the way, I'm assuming you arent seeing any brake fluid leaking. One thing you might try (dont know if it will work) is to remove the cap on the MC and have someone push the brakes will looking at the MC reservoir. I would expect to see some movement of the fluid if the MC is trying to push fluid into the brake lines or air bubbles if there is air in the lines. Good luck