Need help - Lost brakes - no fluid leaks found

88 xj 5" lift 33's everything else stock... nothing new, tried and true, brakes died out of the blue.

anyways, one day im driving down the freeway and go to slow down and the pedal flops to the ground. I grab the emergency handle and prepare for SHTF. Thankfully after a few pumps the brakes were working again.

I go to check the fluid, all good. I check all the lines, no visual fluid leaks or fluid stains.

When the brakes decided to work, it did not swerve to one side or lock up. However, compared to before where I could stop using my big toe, i now have to actually press the pedal down a few inches before she grabs.

Im thinking Vac leak somewhere? but where? Anyone have any ideas or had the same problems?

Got air in the system?

x2 check for air in the system, also check to see if your pads AND shoes are low. If you bleed it and the problem continues the next place to check is your master cylinder.

If you had a vacuum leak at the brake booster the brake pedal would actually get harder.

Is the pedal firm or spongy?

If spongy or soft bleed the brakes.

If you hold the brakes on while stopped at a light does the pedal stay firm or slowly sink to the floor.

If firm, how about if you quickly jump hard on the pedal like a panic stop. Does the pedal go to the floor but the brakes can be pumped back up slowly. (Can be checked with engine running and not moving)

If either or both of the last two symptoms are true I would suspect a bad master cylinder.

Your master cylinder is on its way out.

Rockslut said:

Your master cylinder is on its way out.

Click to expand...

x2

if there is truely no leak(check rear brake cylinders), then the seals in the master are shot. or possibly the rear shoes are way out of adjustment.

unless you opened up the hydraulics(other then the cap on the master) there should be no aiir in the system.

x5 on the master. Look into a 96+ master and booster.

bwinter1946 said:

Is the pedal firm or spongy?

If spongy or soft bleed the brakes.

If you hold the brakes on while stopped at a light does the pedal stay firm or slowly sink to the floor.

If firm, how about if you quickly jump hard on the pedal like a panic stop. Does the pedal go to the floor but the brakes can be pumped back up slowly. (Can be checked with engine running and not moving)

If either or both of the last two symptoms are true I would suspect a bad master cylinder.

Click to expand...

It used to be be firm, too firm (I actually began to like it that way) Now its spongy.

The only thing i can come up with is the other day when i went to check the fluid level. I dont know what everyone else is sporting but Mines is the metal can with metal lid and retainer clamp that goes over the top.

That sh!t is pretty straight forward, right? Its not like the cooling system that needs to be burped, right? meaning i dont have to have someone step on the pedal while i take it off and put the cover on, right? (/ranting sarc) Because i swear its only after that, this started. And yes, i did check the cover and clamp, she was on right.

Whether its something new or not I noticed the swoosh sound when i step on the brakes coming from the upper left side . I cant remember if that was part of my lovely XJ symphony or something new.

Any easy way to test the master without removing it or getting too messy? I love wrenching my cars, just now is not the time for me to be getting dirty.

Thanks in advance for all the help Guys, Much appreciated!

I can tell you almost for certain that the swoosh sound is your booster after springing a leak. My '88 sounded like static on the radio (don't have a radio) until I replaced it with a new one meant for a '96.
Your pedal was hard because the booster wasn't providing power assistance. Kind of like the power steering pump helping turn the wheels.

The Renix Jeeps had a single-diaphragm booster. They are junk and when they fail will rob your intake of air too= power loss. I would be willing to bet that your idle RPMs dropped significantly while braking.

Sounds like your master cylinder went soon after. Probably due to age/corrosion and not necessarily because the booster was shot.

Good news is a dual-diaphragm booster and new master cylinder meant for a non-ABS '96 XJ will VASTLY improve your brakes and almost directly bolt right in.

The only snag is the booster's pushrod. You need to use washers to make up 1/4" in the engine bay side of the 4 mounting studs. Then the rod is the right length, but the end needs to be modified. The hole needs to be drilled out (I think to 5/8"). Then you need to carefully grind a flatspot on the end to match the old booster (for the brake light switch).

You can avoid all of that by sending out your old booster and the new one to Precision Rebuilders (in Missourri I think) and have them switch out the rods.

Check out madxj for more details.

Other than that rod, you'll need the right flare nuts and adapters to fit the new master cylinder.

This is probably the best upgrade I've done to the '88. WAY better braking, no stupid hiss noise, and no idle fluctuation.

If the fluid level in the reservoir hasn't gone down, there are ZERO leaks, and your pedal went to the floor, then your MC bypassed the piston/seals--Pascal's law.

Loosing the booster will only make the pedal hard, it will not let it go to the floor.

Also, when checking for leaks, don't forget to check the MC from inside the cabin, fluid will run down and corrode things pretty quick. Also, pull the drums and lift the front and rear rubber cups on both wheel cylinders and check for wetness--there should be NONE.

joe_peters said:

If the fluid level in the reservoir hasn't gone down, there are ZERO leaks, and your pedal went to the floor, then your MC bypassed the piston/seals--Pascal's law.

Loosing the booster will only make the pedal hard, it will not let it go to the floor.

Also, when checking for leaks, don't forget to check the MC from inside the cabin, fluid will run down and corrode things pretty quick. Also, pull the drums and lift the front and rear rubber cups on both wheel cylinders and check for wetness--there should be NONE.

Click to expand...

I was able to go out this morning and try my best to troubleshoot things. I pulled the MC cover off again, still full. Double checked all lines, Zilch on fluids. All bleeders are tight and secure with minimum fluid residue from previous bleeds.

With Jeep off, I press on brakes and she gradually gets stiffer like any other system i worked on and BTW, that same stiffness is what i mentioned earlier of how it usually was in which i liked it. However, when i start it up, its a different story. The first few pumps are somewhat stiff, and after the 6th or so pump she gets spongy and the pedal needs to be pressed at least half of its stroke to start grabbing. and at times to the ground with no results until i pump it a few times first... does that all make sense?

Anyway, I also checked the mounting area under the dash for the booster, yes there is slight rust but no fluid. The rust has been there since i got the XJ a few years back.

Ok, so you can get the pedal to go to the floor, right? If so the MC is trashed, no ifs, ands, or buts about it.