brake trouble need help

i have an 87 jeep cherokee. today my breaks were so bad the pedel was verry spongy. i have all new pads and rotors. i just bled the system and the front breaks work great but back wont work. i checked fore leaks and found nothing. what could cause this??? i was thinking mabey master or pressure differential valve. any input would be great thanks....

Double check the brake lines for leaks from each rear wheel back to the MC, and make sure the fluid level in the reservoir doesn't go down after pumping the brakes a bunch. If everything is spongy still, it's a blown #2 master cylinder seal. Replace the MC and change the brake fluid. Shouldn't be too bad.

Spongy brake pedal is usually caused by air getting into the system somehow, or gas bubbles from trapped water boiling off. First thing to do is bleed your brakes - if you don't know how long it's been since that was done, change your brake fluid entirely.

Use a vacuum bleeder kit. Start at the master cylinder - suction most of the fluid out (leave a bit on the bottom to cover the ports,) and wipe out the master cylinder with a CLEAN rag - no lint! Refill.

Working inwards (RR/LR/RF/LF,) vacuum bleed each wheel in turn. Make sure the master cylinder doesn't get sucked dry, and run the vacuum until you get clean fluid at the cylinder port (that's why the hoses in the bleeder kits are clear.) You'll want to use a six-point box-end wrench on the bleeder screws - they're usually 1/4" or 5/16" (I think the XJ ones are 5/16" up front, and 1/4" out back - but don't hold me to it.) Have a 3/8" handy, just in case I'm way wrong...

Was the pedal spongy before you worked on the fronts? And were the rears working before?

First make sure the rears are adjusted properly. If not you will get the feeling of them not working.

Also check your brake tubing under the jeep that feeds the rears to make sure its not been hit by road debri and pinched somewhere. Make sure there are no leaks anywhere.

Re bleed your brake system as 5-90 said..

If all this does not work I would say possibly the master is starting to bypass internally..

Hello,
My brakes are very weak, when i hit them going 20 they will most likely lock up, even if i drive it around (it sits a little) and I know its either air in system or master cylinder.

It goes to the floor almost and the last little bit locks them up. So its like none or full brakes.

What do you guys think air in system or MC?

FWIW, 1999-2004 grand cherokee booster and master swap is an awesome upgrade.

did it on my 89 with 31's and the braking is great, only cost $100.

just recommending this because if the master is bad, there is no reason to buy a new master when for a couple extra bucks and time you can do the swap and get great braking.

well a reman. fecon master cylinder is 33 bucks.

Im gonna check how much a ZH reman. master is

Try manually adjusting the rear brakes and reseting the valve in the brake line.