Brake problem...

Still working on '90 with brake problems.Ok so I got pads and shoes on,wheel cylinders,and brakes bled.I got good pedal pressure while bleeding until I start the motor,then my pedal goes straight to floor with pressure applied.I searched but didnt find anything like this.What does the motor have to do with pedal pressure?Any test for proportioning valve or the brake booster?No fluid leaks.Taking off master tonight to bench bleed tommorow.Any one got ideas to check out.One thing struck me as wrong was when I pushed pedal only the front chamber on MC squirted fluid out with any noticeable pressure.The rear didnt seem to squirt any fluid out.

This really sucks,its one thing after another with these cherokees.I've had this thing 6 months but in the garage 5 months and I still have to drop tanny to fix soft plug leak on back of motor. :banghead: :smsoap: :flamemad: :gonnablow :hang: :soapbox:

Sounds like you have a lot of fluid in the lines.

When the engine is off, it's only pressing the MC with the 25 pounds or so pressure of your foot on the pedal. Should feel firm.

When you start the engine, the engine vacuum actuates the booster. Lots more pressure on the lines, thereby compressing the air and allowing pedal to the floor.

Did you open the lines for some reason (new calipers or new wheel cylinders)? A simple pad and shoe replacement shouldn't need any bleeding. [see 'edit' below].

I'm very curious about the 'squirting' from the MC.
MC's should NEVER squirt.
When you depress the pedal, fluid is forced out thru the lines; when you release the pedal, a little fluid should flow back into each half of the reservoir, but not as a 'squirt'.

Have you done anything yet to the MC?
What was the original problem that led to the pad and shoe replacement? You might have had a bad MC all along.
[edit: When you did the wheel cylinders, did the fluid drop low and expose the return hole in the bottom of the reservoir? If so, lots of air got into the MC and lines.]

As to trouble-shooting the booster, you've already done it:
If the pedal depresses more upon engine start, then the booster is getting vacuum and the diaphragm inside is applying pressure. It either works or it doesn't, no in-between on this system.

edit: Oh I see now, you replaced wheel cylinders. Did you bench bleed these? If not, there's a ton of air in the system. Bleeding at the corners won't remove all the air very easily. And, it may travel all the way back to the proportioning valve and MC.
Did you plug the open brake line with a golf tee while the wheel cylinder was disconnected? If not, then fluid dribbled out and air entered.

Yeah, go ahead and bench bleed the MC, then do a complete brake fluid change (it was time anyhow ).

Sounds like the master is bleeding down internally. More pressure when the booster has vacumn.

Thanks RIX I got solid pressure and flow at 4 corners and about 2 quarts of fluid pumped through so far.Every master cylinder I have bled will force out fluid with pressure applied to pedal,similar to when compressing front calipers you are forcing fluid back into reservoir.What led me to do the brakes was the dumb ass who worked on this rig before I got it.Rear brakes were not even put on correctly,front caliper bolts were not the right ones ,too long,and to many other things to mention.I plan on using this thing in the mountains for extended periods of time and dont like to walk out 50 miles to nearest town so it has to be right .
possibly an air pocket thats not burping out ?stanger things have happened.

anymore suggestions? be home in a little while to bleed MC.

MACH90XJ said:

What led me to do the brakes was the dumb ass who worked on this rig before I got it.Rear brakes were not even put on correctly,front caliper bolts were not the right ones ,too long,and to many other things to mention.

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MACH90XJ said:

This really sucks,its one thing after another with these cherokees.

Click to expand...

Please don't blame "these Cherokees" because the previous owner of yours (or whoever worked on it for him/her) was an idiot. I have over 236,000 miles on my '88 and I've only done the brakes twice in the front, 3 times in the rear.

Before you screw around with the master cylinder, bleed it with the engine running. Start with the right rear wheel, then left rear, then right front, then left front.

Eagle I was thinking that but I never had to on any other make of vehicle i done before.Is this something related to jeeps only?

Too late on MC got it off already.Is there also something with bench bleeding MC I should know about jeeps?Originally MC front chamber was full of black crud, I dumped and cleaned out MC.Do you have correct jeep bench bleeding procedure?I use the adapters to run hoses back into masters chamber from out ports.

One more thing ,whats your opinion on combo valve?Doing alot of reading old post and combo valve might be another item contaminated by old fluid and crud.Vehicle has 160,000 miles on it and obviously has not been serviced properly.

Advice is much appreciated!

You can remove the prop valve disassemble it and clean it out.