Spongy brakes

90 4.0 xj with 96 brake booster/mc mod.


I changed the system over because because the ABS pump was constantly on.

after converting I bled the brakes manually twice then bled them with a vacuum pump twice. After that made one of those pressure bleeders that pump into the MC and bled a liter of fluid through it.


I'm fairly confident there's no air in the lines. What's next? did I get a bad MC?

dunno.. describe further please... if there's constant 'friction' sound coming from the MC then it could be bad... are you losing fluid?

no sound not losing fluid. pumping barely helps. goes to the floor can stop the vehicle but can't lock the tires up.

Did you bench bleed the master cylinder before you installed it?

No I bled through the top of the MC using a home made one of these
http://www.amazon.com/Motive-Gear-0..._3?ie=UTF8&s=automotive&qid=1283610084&sr=8-3

Should I take it off and bench bleed?

No a bench bleed just means you have bleed any air from the master before installing. Sounds like you have all the air out. I would say you have a bad Booster. Would say you have brakes just not "power" brakes.... Same problem on my old 79 dodge... and my 90 Cherokee... changed booster and fixed both. Good luck

DownhomeMoto said:

No a bench bleed just means you have bleed any air from the master before installing. Sounds like you have all the air out. I would say you have a bad Booster.

Click to expand...

Um, not exactly. He said he has spongy brakes, which probably means there is still air in the system. The master should have been bench bled before it was installed, to get any air inside the master out, which would not be bled out during a normal brake bleed like he has done.

One way to make sure the booster is good is if you pump the brakes with the vehicle off, and if the pedal gets hard and doesn't sink, your booster is fine. If you then turn the vehicle on, you'll feel the pedal go down since you now have the vacuum from the engine helping.

OP, yes, you should have bench bled it.

If I bleed through the top of the reservoir with pressure it wouldn't bleed the MC? I assumed it would.

Begster said:

Um, not exactly. He said he has spongy brakes, which probably means there is still air in the system. The master should have been bench bled before it was installed, to get any air inside the master out, which would not be bled out during a normal brake bleed like he has done.

One way to make sure the booster is good is if you pump the brakes with the vehicle off, and if the pedal gets hard and doesn't sink, your booster is fine. If you then turn the vehicle on, you'll feel the pedal go down since you now have the vacuum from the engine helping.

OP, yes, you should have bench bled it.

Click to expand...

.... Like I said before... you bench bleed a master cyclinder to get the air out before you install it. You can still get an air free brake system with out bench bleeding you MC.

The OP said he did bleed back through the MC. The biggest gain from a bench bleed is that you can insure your MC is level. If it's tilted up or down the you can get trapped air in the MC.

OP did you install new parts or used parts...

As far as the Booster pump test... yes that will tell you if you have an air tight booster. But I have still changed boosters that have passed the pump test and had a much improved brakeing feel. My 79 was this same way... Step on the brakes and you have almost no feel... you have to stick your foot to the floor just to get it to stop. Do the booster test and it would get hard in two to three pumps... start the truck and it would bleed down with vac. Changed the MC did a bench bleed and line bleed apon install and it didn't get better. Changed the Booster never unhooked or touched the lines and I was able to lock up all four of my 36-14.5 boggers....But thats just me...

Ok It pumps with the engine off and barely gets firm before the floorboard. starting the engine drops the pedal and I can't get it firm with the engine running.

Got the parts used off CL guy said it stopped fine, and those CL guys are always honest...

lol... I here ya...

Just to safe you the money... I would pull the MC and bench bleed it, make sure it is level when you do it. Then reinstall and bleed the system. It souldn't take much to push any air out. Then see what you got.

It almost sounds like a bad MC. How much fluid do you push out when you pump them to bleed them... it should shoot out...

I want to do the upgrade to my 90 as well but I think I would go with new parts.

How did you bleed the brakes?

Right rear, left rear, right front, left front?

I too would pull the MC and bench bleed it.

Then follow the pattern above.

Sometimes you have to crack the lines at the proportioning valve.

If you are getting weak output at the wheel cylinders/calipers that could indicate a MC where the pressure is bypassing the piston.

Good luck.

my vote is there's still LOTS of air in the system..

if it was a bad master - you'd hear it (I think). My master went south and made lots of noise.. though it could be there are a few ways of having a bad one....

FWIW i've installed masters on 4 other cars, never bench bled them, never had a problem with getting all the air out. Maybe jeeps are different but i doubt it. However out of process of elimination you could bench bleed it...I would if i ran into this problem...

However if you have a pressure bleeder, i'm sure you could just crack the drivers side brake line and pump the brake pedal (while pressure bleeding) and this would rid it from air, but thats just speculation.

Anytime dealing with an 3 year or so, or older master cylinder, a critical thing must be remembered.... Do NOT stroke the master cylinder beyond its normal operating range. So for example don't bleed the brakes by pumping the brakes to the floor, if you have to put a block of wood under the brake pedal to keep it from bottoming or going beyond normal travel do so. The problem lies in the fact that the rubber 'piston rings' will rub onto a part of the cylinder bore that has not been used in a long long time. This surface is rough...relatively speaking, and contaminated. When you rub those nice soft rubber seals against this cheese grater surface you just won yourself a free blown master cylinder. I have personally seen this, thankfully only on friends cars.

However on time i did pad/rotor change and brake bleed (not over traveling the pedal) and the pedal never got firm after literally a gallon of fluid and every trick in the book. Turned out the master cylinder went bad. So either i thought i didn't over travel the master and some how i did, or some how the back flow of old fluid from the compressing the caliper pistons blew out the master, either way very strange never did really figgure that one out.

You can also try tapping the master lightly with a wrench and the hard brake line that you are bleeding, this will free up the 'clung' bubbles that will stick to the walls of the tubing...think a soda in a glass. Also if doing a normal manual bleed use smooth easy strokes on the pedal, do not pump the shit out of the brake pedal all fast. And when you pour in the fluid don't be the cool guy and pour it from 2' above the master. This will churn air into the brake fluid.

I've had remanufactured masters be bad right out of the box, as well as after a month of driving. So just b/c the part is new don't assume its good.

Good luck you'll figgure it out

I went through this on my '96 three years ago. The THIRD, NEW MC worked fine! Also, I changed the rear hard line, and the hose from the body to the axle. Braking effectiveness improved. Just last week, I put new hoses on the front to the calipers. Better still! Soon, I may have real good brakes.

I've tried vacuum bleeders, but they seem to like to suck air around the threads of the bleeder nipple. The best is the power bleeder, the pump bottle that goes on the MC in place of it's cap. So far, the best results. Although, my wife now feels left out, as I think she looked forward to our 'special' time of me lying on back, telling her to "push, now let go"!

one time the parts guy gave me the wrong caliper for side I was installing made the bleeder on the bottom. will not get all air out with bleeder on bottom. just something to check.

Yeah I had them do that to me when I was doing my F250's brakes I caught, I was just pissed about the extra trip lol.

Couple things- before pulling the m/c back out, try cracking the lines out the side of the m/c loose to bleed, same procedure as down at the wheel. Can be messy.

Another option that works surprisingly well- do you have one of those adjustable hood props or a brake pedal holder like used on alignment racks? Pump the crap out of the pedal and then using whatever means available, hold the pedal down as far as you can overnight. It will be somewhat better the next day.