No Brake Pedal

Just got done doing an axle swap in my son's 87' MJ. Installed a Dana 30 in the front, 8.8 in the rear. (Disk Brakes) We are using the stock calipers from both axles, all four calipers were dry. We also replaced the brake line on the 8.8. Heres where I ran into problems:

First we bled the brakes the old fashion way (2 people) could not get any pressure what so ever. Replaced the master cylinder; Bench bled and installed. We figured it would fix the problem. Unfortunatly we were blessed with no brakes again. We could get a little pressure with the engine off but when the engine was started, it was a different story.Brakes went to the floor.

Came to the conclusion that there was still air in the lines, so my son went to his Automotive class and got a vacuum bleeder, unfortunatly with it we could not tell when we were done bleeding since the air travels around the threads of the bleeder screw. Ended up where we started slight brakes when engine is off, but as soon as we start it, We have nothing.

Could we have a Brake booster problem? Or is there something we are overlooking.

Thanks

Les

Take a step back.

If you loosen a bleeder and step on the pedal, does the fluid come out under pressure?

If so, you have pressure and if everthing is tight, you should have brakes.

If you "pump" the pedal, do the brakes move? You may just need to "take-up" the calipers tight to the rotors.

If none of this works, and you do not have any leaks, I would suspect a bad master.

Rev

If you're going to vacumn bleed, remove the bleeder screws and clean them with a wire brush. Then smear the threads with grease or anti-seize and reinstall. That will prevent you from such air past the threads. You don't have the calipers up-side down right? The bleeders should be at the top. If not, you probably have them swapped side-side.

The MJ is different from the XJ and there is a special method for bleeding that needs to be followed. The MJ has a second brake line to the rear. Normally you only get pressure down the first line that goes to the adjustable prop valve above the rear axle and then to the rear brake lines. If the combo valve up front senses that the front brakes have lost pressure, it sends fluid down the second line which Tees in after the the rear prop valve (essentially bypassing it and giving you full braking). If you don't get this second line bled, it's a big pocket of air connected right to your rear brakes.

Now for that special procedure. Do a normal bleed all around first. If you are pumping the brake pedal or using a pressure bleeder on the master cylinder, then open a front bleeder slightly and leave it open. You should see the brake line come on if you have the ignition on, indicating the combo valve is in emergency bypass and sending fluid down the second line to the rear. Bleed the back brakes, close the front bleeder and rebleed the front if needed.

If you are vacumn bleeding, then you may close the front bleeder once you see the brake light come on. It should stay on the whole time. If not leave it open.

-Chris

Make sure the calipers are on the right side with the bleeder screw on top. A buddies truck had the same problem and it turned out that he had the calipers on wrong, but then again it sounds like you havent had the calipers off.

Something to check

TORX

Thanks guys ... yes, the calipers are on correctly. I even had the guys re-do the brake pads because they initially had them on upside down (capture tabs on the top). They were a bit aggravated having to swap pads. We have followed the proper sequence too. It's been very frustrating, so say the least. This has been more of a problem then both axle swaps!

We have replaced the Master Cylinder already figuring the old one was just not able to pressurize the additional pressure due to the rear discs. Also, in checking for trapped air (after "several" bleed attempts), we compressed the caliper pistons with the bleeders open slightly. There was no air pushed out of the calipers. The pads were pushed up against the rotors where they needed to be "relieved" to get off the rotors. The system just does not build pressure (or pedal).

We checked the line that returned from the load sensing valve just to see if there was a possible problem with back pressure (air) in that line. There was nothing. We will open a front bleeder when bleeding the rear and see if we can get the warning light to come on. I actually have not seen that light come on throughout this whole process. It may not even be connected as far as I know. We will check this out. We will also look for the "Y" in the line that was mentioned.

Also, if necessary, we will try to bleed using a jar of fluid with a tube running from the bleeder to the fluid. Instead of pumping the brakes before releasing the bleeder, we will slowly press the brake pedal with the bleeder open, close the bleeder, then release the pedal, and repeat. Maybe we are generating air due to being too aggressive when pumping the pedal.

Thanks everyone ... keep thinkin' for us. Hopefully with your help, we can figure this one out.

Les

LBEXJ said:

Instead of pumping the brakes before releasing the bleeder, we will slowly press the brake pedal with the bleeder open, close the bleeder, then release the pedal, and repeat.

Click to expand...

That's what I was going to suggest.

In order to properly follow up on this thread and provide an answer to those who may be searching for an answer to a similar problem, please follow this link:
http://www.naxja.org/forum/showpost.php?p=783989&postcount=10

Thanks ...

Les

Since it's an MJ, there is another bit of crap to deal with. The rear load sensor valve MUST go away with rear disc brakes! It has a 2nd line that goes back to the front and until I loosened mine at the front (under the prop valve) I couldn't get any pedal on mine with stock brakes either!

carnuck said:

Since it's an MJ, there is another bit of crap to deal with. The rear load sensor valve MUST go away with rear disc brakes! It has a 2nd line that goes back to the front and until I loosened mine at the front (under the prop valve) I couldn't get any pedal on mine with stock brakes either!

Click to expand...

As was posted in the follow up thread ... we did eliminate the rear Load Sensing Valve.

While researching this forum for answers to our problem, we read several threads about the MJ Brakes. Here is a very good thread about the Proportioning Valve and the rear Load Sensing Valve.
http://www.naxja.org/forum/showthread.php?t=68708&highlight=MJ+Proportioning

Les