94 Brake Issues - JeepForum.com
 
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post #1 of 6 Old 05-22-2020, 12:39 PM Thread Starter
jeepjeepster
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Join Date: Dec 2004
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94 Brake Issues

Its always seems I have strange things happen, hardly ever is it simple.

Recently noticed my 94 seemed to be having a harder time getting up hills. It appeared that one of the rear wheel cylinders was sticking. IR gun confirmed one of the rear drums was hotter than the other. Ordered new wheel cylinders and installed them, couldnt get any brake fluid to the wheel cylinders. Loosened the hard line where it goes into the rubber line going from the frame to the axle, pushed the brake pedal and got plenty of fluid. Replaced the rubber line going from the frame to the axle this morning and even after putting grease on the threads of the bleeder screws, my vacuum bleeder was pulling air. Thought the bleeders must just be letting air in but gave the brake pedal some good pumps to help push any air out and called it good. Cleaned everything up and started the Jeep, brake pedal goes to the floor! What the heck, I know I didnt let the reservoir run out of fluid but I grabbed my red brick and bled all 4 corners while the abs pump ran, and I pumped the pedal a few times while it was all bleeding, got tons of fluid.

Brake pedal still slowly settles to the floor. If you pump it up it gets hard, then settles down. Sigh..... Idk why the master cylinder would coincidentally go bad like this. Even with whatever was going on with the drums, the brakes were fine just two days ago.

Something odd I read in the FSM, it says you need to bleed the combination valve? I've never heard of anyone doing this. Cant find anything on how to do that either.


1994 ZJ-I6 w/216,000 mi
1998 ZJ 5.9 w/65,155 mi Slate Black
2004 KJ w/90,300 mi
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post #2 of 6 Old 05-22-2020, 12:52 PM
zjosh93
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1993 ZJ 
 
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Location: Austin
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The only time I've had trouble bleeding the rear wheel cylinders has been when I had the parking brake on so the rear drum couldn't move to push the fluid out. You still get a little from the master cylinder but it's a long process. Drum brakes really take very little fluid to work so the master doesn't move much.

You sometimes can kill the master if the pedal goes all the way to the floor. It pushes the piston past its normal range of travel and drags the seal (probably already old and hard) through the sediment at the bottom of the master cylinder bore.
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post #3 of 6 Old 05-22-2020, 02:31 PM Thread Starter
jeepjeepster
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Ive read that before and maybe thats what happened, I tried to not push the pedal further than normal. Hopefully Ill have a new one in tomorrow.

1994 ZJ-I6 w/216,000 mi
1998 ZJ 5.9 w/65,155 mi Slate Black
2004 KJ w/90,300 mi
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post #4 of 6 Old 05-23-2020, 01:20 PM Thread Starter
jeepjeepster
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This thing is throwing me for a big loop. I suppose Im getting caught up in the fact that my vacuum bleeder is pulling air, so I've wasted most of my day along with a quart of brake fluid today....

I unhooked the hard line at the inlet of the rear axle flex line and put my vacuum bleeder on the hard line, nice clean fluid with no air. I unhooked the hard line at the rear left wheel cylinder and put the vacuum on it, nice clean fluid. Same thing on the rear right hard line, nice clean fluid then suddenly it started sucking air. I've done this several times and it always changes. I even unhooked the rear right hard line, plugged it to rule out a new wheel cylinder someway allowing air in but no fluid out, and now Im getting air at the rear left hard line. There are no leaks anywhere that I can find.

Also, the only time the pedal slowly goes to the floor is when the booster has vacuum on it. With the Jeep off, the pedal pumps up and does not settle to the floor. Idk what thats about. This makes me think the master cylinder is ok? I can push on the pedal with two feet as hard as I can and it doesnt settle.

Took the Jeep for a ride and as you can imagine the brakes are awful. Under hard braking, the brake light came on which tells me the pressure differential valve is moving, which indicates either a leak or air in the line. Im just so confused.

1994 ZJ-I6 w/216,000 mi
1998 ZJ 5.9 w/65,155 mi Slate Black
2004 KJ w/90,300 mi
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post #5 of 6 Old 05-23-2020, 04:23 PM
StormChaserTim
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1995 ZJ 
 
Join Date: Sep 2013
Location: Wichita
Posts: 184
Well, this still sounds like either a master, or an air bubble still in there.

Had a Ford in the shop recently doing the same thing. It turned out to be a pretty good size air pocket in the line going from the ABS modulator to the right front tire. We only found it by doing a brake flush on it.

You can pull the master away from the booster, and ensure that it isn't leaking out the rear seal into the booster. I've seen that way too many times to count. Also, keep in mind, this is an old car, with an old master cylinder. It's completely possible that taking the brake pedal to the floor has scarred the cup in the master cylinder, and it is bypassing. This would explain why it works with NO power assist, but sinks with power assist. It's likely just scarred enough to let go under extra pressure.

Tim

1995 Jeep Grand Cherokee Laredo, 4.0L 2WD, 355,000 miles.
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post #6 of 6 Old 05-23-2020, 05:21 PM Thread Starter
jeepjeepster
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Just pulled it and its dry, but that doesnt mean internal seals arent bad. Im just going to try to forget about it till the new master cylinder comes in, just dont want to start throwing parts at it. Seems that master cylinder is a week out from anywhere.... Geographical oddity.

After pondering on it, I guess if the master cylinder is only putting fluid in half of the combination valve, that could also cause the brake light to come on.

1994 ZJ-I6 w/216,000 mi
1998 ZJ 5.9 w/65,155 mi Slate Black
2004 KJ w/90,300 mi
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