Poor pedal feel and weak brakes.

[Myziric]

Hey all I have a 1990 XJ that I have been battling the brakes on. The first 2-3 inches of pedal are very soft with almost no resistance with enough braking power to drive around 15mph and come to a stop. the last inch of pedal gets very hard and if I push it about as hard as I can stop it with decent power from about 50mph but still worse than before all this. Originally my master cylinder was leaking and I wanted to refresh the brakes at the same time. I bought a new master and bench bled it and my brakes went to the floor, I checked everything noticed one of my brake calipers were leaking so I replaced those and made no difference.

At this point I have replaced 2 new master cylinders, the calipers, wheel cylinders, soft lines and pads. I have bench bled the master 3 times out of desperation and ran gallons of air free fluid through the system. Only things I haven't replaced is the booster(which I feel I am getting good assist from) the hard lines and the combination valve.

I've read countless threads and am at a loss at this point.

[RCP Phx]

Try bleeding the propositioning valve and adjust the rear brakes then you can evaluate the booster.

[Saudade]

Well, not sure how long your had your '90 but the brakes are lees than stellar on the Renix models. This is largely due to the single diaphragm brake booster. Many folks swap to the dual diaphragm booster from the later XJs. Not quite a drop in but relatively straightforward.

Also, what brand pads did you use? Did you "burn in" the new brakes properly?

[Myziric]

RCP Phx: I’ve bleed the proportioning valve and adjusted the rear brakes. I have checked to make sure the bleeders are right way up. I might take the proportioning valve apart to see if I can clean it out.

Saudade: I’ve had the jeep for over a decade now, the brakes were never great but never had this no resistance feel until now and worked better than this. I installed new rotors and drums with Bosch quiet cast semi-metallics all around and bed them in coming from 50-10mph 10 times. I’m thinking about going to a later model or what swap but want to get this sorted out before adding even more variables.

[Woodswanderer]

Engine off/Key off:

Pump brakes 4-5x, pedal feel should be firm, not spongy. Hold down pedal. Does the pedal slowly or suddenly sink to the floor? If so, master cyl is bleeding off internally.

pump the brakes 4-5x until the brake pedal stiffens up. Then, push down the pedal and start the car.
Check for a slight give underfoot followed by resistance. If the brake pedal stays stiff without giving slightly, the brake booster may be failing.

While the engine is running, pull your vac hose off the booster. Should hear it depressurize. If not, the check valve on the booster could be your issue.

Check the obvious culprits like vacuum lines, replace any that look suspect. silicone 3mm fuel line is cheap by the foot and wont rot/collapse, etc. replaces all small vac ports/elbows and plastic tubes. I *think* the larger ports/elbows are 6.5mm but havent confirmed.


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[Myziric]

The pedal feel is what I would call normal like how it used to be and most other cars I’ve driven, doesn’t really pump up. When I do the same and turn the car on the pedal instantly goes about an inch from the floor and gets hard.

The booster holds pressure for a at least a a couple minutes after turning the car off

[Myziric]

When the car is on I can pump or hit the brakes hard and the brakes gain another inch or so of pedal but then goes down to that last inch where I can still push hard to get the car to stop.

[Woodswanderer]

Originally Posted by Myziric

When the car is on I can pump or hit the brakes hard and the brakes gain another inch or so of pedal but then goes down to that last inch where I can still push hard to get the car to stop.

This sounds like there is still air in the master cylinder to me, or somewhere in the lines. Possibly a damaged steel line from replacing other parts?

I would suggest bench bleeding again and/or replacing with another known good master cylinder. While its apart, you may as well do the booster and upgrade to the later one that gives better feel (should be cheap from a junk yard)


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[Myziric]

I actually did replace a steel line for the master to the combination valve because the old one was cross threaded. I’m not getting any air from the lines or leaks around the area and I have good pressure coming from the bleeder so I assume I did it right. Another symptom I have is when I jack the rear of the car up and put it in first I can’t stall the car out with the brakes but almost can with the e brake. I think I should easily be able to stall the engine out with the brakes but never tested it before.


I’m thinking the swap is the way to go like you said at least once I figure it out I’ll be stoked on how they work.

[Woodswanderer]

Originally Posted by Myziric

I actually did replace a steel line for the master to the combination valve because the old one was cross threaded. I’m not getting any air from the lines or leaks around the area and I have good pressure coming from the bleeder so I assume I did it right. Another symptom I have is when I jack the rear of the car up and put it in first I can’t stall the car out with the brakes but almost can with the e brake. I think I should easily be able to stall the engine out with the brakes but never tested it before.


I’m thinking the swap is the way to go like you said at least once I figure it out I’ll be stoked on how they work.

Look at that line you replaced. Metal lines are very easy to collapse when bending them without a brake line bending tool.

Master cylinder could very well still be bad. It could be leaking internally past the seals, causing your issues.

Is it front brakes not grabbing or the rears?

Bench bleed the master cylinder again (can be done on car with a helper and rags to catch fluid). Re-bleed afterwards starting from the master cylinder to the right rear, left rear, right front, then finally left front.


If that doesnt work, planning for another master cyl and booster would be my next step


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[Woodswanderer]

One last thing to check when bleeding, make sure the wheel cylinders havent blown their seals. Pull your drums and look inside.


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[Anak]

FWIW, I had to replace the MC on my '96 a couple years ago. I went through at least 2, if not 3, MCs from Napa. Could not get a single one of them to properly bench bleed. In desperation I picked up a MC from O'Reilly's. It bench bled perfectly in short order.

Point being you can't assume the new parts you buy are any good, and you can get multiple bad ones in a row. There are many companies and employees who just could not care less anymore.

[techno1154]

Originally Posted by Myziric

I actually did replace a steel line for the master to the combination valve because the old one was cross threaded. I’m not getting any air from the lines or leaks around the area and I have good pressure coming from the bleeder so I assume I did it right. Another symptom I have is when I jack the rear of the car up and put it in first I can’t stall the car out with the brakes but almost can with the e brake. I think I should easily be able to stall the engine out with the brakes but never tested it before.


I’m thinking the swap is the way to go like you said at least once I figure it out I’ll be stoked on how they work.

That is quite normal for an XJ. Both XJ's I have owned did the same thing. Both were/are AW4. With the rear wheels off the ground, I have to really try to get the wheels from turning while in gear and the engine at idle speed.

My opinion, 1) you have a bad master cylinder or 2) check the rear brakes and adjust them until they are tight in the drum then back the adjuster off a little to allow the wheels to turn.

[Myziric]

Thank you for the input, I appreciate it. I’ve been leaning towards bad MC aswell just hard to keep throwing masters at it and not getting anywhere. I’ll try bench bleeding it once more(mrblaine block off method) and if that doesn’t work I’ll make a trip to pick-a-part and do the late model swap.

[Myziric]

[QUOTE=techno1154;246769240]That is quite normal for an XJ. Both XJ's I have owned did the same thing. Both were/are AW4. With the rear wheels off the ground, I have to really try to get the wheels from turning while in gear and the engine at idle speed.

I have an AX15 manual trans.