Brake issue

[Dagoose]

I'm starting to pull my hair out over this issue. I have a 98 Tj, 4.o, manual.

I swapped my axles out a couple of months ago, bled my brakes and everything was working great.

2 weeks ago on a trail run my pedal felt spongy. I checked the fluid level and it wasnt low. I get home and the next day the pedal went to the floor. I thought it was just some air in the lines, so I bled them. That didn't work.

The pedal will go to the floor maybe once every 10 times, and it feels spongy. I changed the master cylinder, bench bled it, and bled the brakes. It still didn't fix it.

I checked all of the lines and none of them were loose. I can't see any signs of fluid leaking.

Any idea what I should check next?

[jpthing]

My guess would be that there is still some air trapped somewhere if the symptom has not changed despite swapping out the master cyl.

[cLAY]

Are the rear brakes drum or disc? Did the axles you swap in have the same size and type of brakes?

[TL-Iguana]

Disconnect the vacum on the brake booster and try pumping the pedal. It should be really firm. You can clamp off the vacum hose with a pair of vice grips or tape it to prevent a vacum leak and go for a test drive.. It may just be that the brake booster is in need of replacement as well.

[jpthing]

correction: by pulling fluid down with a pump.
They can be bled by forcing fluid up with a pump but I'm not sure on exactly how this is done. Are you sure you did the correct sequence i.e. furthest first? did you bleed with a friend or with the one-man type bleeder? I have had better luck the old fashioned way.

sometimes I have bled brakes to where i think they are done and then woops, another bunch of little bubbles.

I think you need to bleed. More.

Good luck.

[Dagoose]

The rear have disks.

I did the one man bleeding, it's always worked for me in the past. I'll try with a friend.

Yes, I went from furthest to closest.

I'll check the vacuum. I don't think it's the booster though.

[jpthing]

Are the rear brakes drum or disc? Did the axles you swap in have the same size and type of brakes?

same brakes?

[Dagoose]

No, the old axles had drums.

[cLAY]

Thats your problem. You went from drum to disc. There is a different proportioning valve for disc brakes then there is for disc. Discs take more volume than drum. Do some Google searches on TJ drum to disc swaps and you should find what you need to do.

BTW, trying to reverse bleed by forcing fluid UP from the brakes is a stupid idea. Where exactly do you think the air will come out? You'll wreck lots of stuff that way.

You are either thinking of the system where positive pressure is placed on the master cylinder reservoir to push fluid through the system or a vacuum system that pulls the fluid through the system.

[Dagoose]

Thanks, I'll look into that.

[w squared]

That pushing fluid up from the bottom thing works when you use belt dressing.

[jpthing]

yes

Thats your problem. You went from drum to disc. There is a different proportioning valve for disc brakes then there is for disc. Discs take more volume than drum. Do some Google searches on TJ drum to disc swaps and you should find what you need to do.

BTW, trying to reverse bleed by forcing fluid UP from the brakes is a stupid idea. Where exactly do you think the air will come out? You'll wreck lots of stuff that way.

You are either thinking of the system where positive pressure is placed on the master cylinder reservoir to push fluid through the system or a vacuum system that pulls the fluid through the system.

Like I said I'm not sure of the details but it was called "power bleed" and it was done with some sort of a pump from the caliper/ wheel cylinder. Thanks for the correction. I think the ZJ prop vave is popular for this application.

[cLAY]

Power bleed is when you apply positive pressure to the master cylinder reservoir then crack all the lines and let the pressure force new fluid thru the master cylinder and OUT the calipers.

FYI you can make a power bleed system really easy with a cheap weed sprayer and a spare master cylinder cap.