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Crazy Brake Problem

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lawman5050

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I have an '86 GMC Jimmy with a 10-bolt rearend. About three months ago I replaced the rear brake shoes. While I had the brakes apart, I went ahead and replaced all the hardware and both wheel cylinders because one was leaking slightly. Heres where it gets weird.

After putting everything back together and bleeding the brakes, the brake pedal went farther to the floor than normal and was mushy. I went ahead and replaced the master cylinder since it was old also, bled the master cylinder before installation, rebled all four wheels, adjusted rear brakes, and still had low/mushy pedal. I ran about three of the big bottles of brake fluid through the system bleeding it but it did not get any better. One thing I did notice was the the left rear bleeder would always spit out a little bit of air.

I was busy for the last three months and did not have time to work on the truck. Over the past week I started looking at it again and have not been able to figure it out. Over the three months that I drove the vehicle daily, no brake fluid had leaked out. I have inspected all lines without finding any leaks and made sure all fitting were tight. Replaced all rubber brake hoses, the hard line to the the rear left, tried another new master cylinder, and replaced the left rear wheel cylinder four times and the problem is still there. I can suck fluid through the left rear with a pump, close the bleeder, the brakes will be firm at first but after pumping the pedal four or five times it gets mushy again. bleed all wheels and only get air out of the left rear.

I took the truck to a local respectable garage who power bled and manualy bled the system with no luck. They also inspected the whole system like I had and could not find a leak.

I have run out of options to troubleshoot this problem. I don't understand how air could leak in without fluid leaking out but it seems like that is what is happening.

If anyone has any ideas, they would be greatly appreciated.

Thanks for any help or suggestions.
 
air can leak without fluid coming out. this happened to me at my proportioning valve. since it only gets air on the left you should replace the left hard line. its not hard it did mine without a pipebender.
 
Thanks for the replies.

Unfortunately, I have already replaced the hardline on the left side and also bled then proportioning valve. still no change with the problem. /forums/images/graemlins/mad.gif

THE EXCUSE - You said that air was leaking in at your proportioning valve. How did you diagnose this or did you just happen to replace the valve and it fixed the problem. Just curious because I have replaced everything going to the rear left wheel and still have the same problem. I don't know why I would only get air to the left if it entered at the proportioning valve but I'm getting desperate here. /forums/images/graemlins/1zhelp.gif
 
howd you bleed the prop valve??

also.. you say you replaced the rubber hoses as well? did you replace the copper washers at the caliper end??

just a couple thoughts to think about..
 
While searching on this site, I read a post that said sometimes an air bubble will get caught in the proportioning valve and won't come out with regular bleeding. To bleed it, you simply loosen the 2 lines going into the proportionong valve from the master cylinder and let them gravity bleed. While they are bleeding, you can tap on the valve with a wrench to help the air float to the top.

Also, when I replaced the rubber hoses, I did replace the copper washers where the hoses bolt into the calipers. As far as the brake hose feeding the rear brakes, there are no copper washers, only compression fittings.

Thanks for the ideas.
 
does the truck still stop pretty well? it may be the nature of the shoes.. especially if you did not machine the drums when you replaced the rear shoes, the difference in friction materials can cause a soft pedal..
 
The truck still brakes pretty well. I am going to go ahead and replace the drums with new ones because they have 150,000 miles on them. I know this can cause the pedal to go lower because of wear but I still don't understand why the left rear brake line keeps getting air into it.

Any other ideas?

Thanks in advance.
 
its a possibility that the flare on the end of the hardline at the left rear wheel is losing or has lost some integrity..maybe try re-flaring it?
 
The hard line at the left rear is brand new. Had the same problem with the old line. After replacing with a brand new one the problem did not change.

Thanks
 
[ QUOTE ]
I went ahead and replaced all the hardware

[/ QUOTE ]
I take it you replaced the adjusters too? Also, If I read everything correctly, you did NOT replace or turn your drums?

I had this problem on another rig once. It was a combination of a couple things for me. 1) My adjusters were not working properly, and 2) my BRAND NEW master cylinder was bad. That really p*ssed me off! /forums/images/graemlins/angryfire.gif I spent weeks trying to figure it all out before I decided to "exchange" my MC and give that a shot. For some reason, that cured it. Don't know exactly what was wrong with it, but it sure didn't work very well! /forums/images/graemlins/tongue.gif
 
Confedneck - I have also replaced the rubber hose so the fittings there are brand new as well. This also had no effect on the problem.

Shane74 - I didn't replace or turn the drums. The adjusters were replaced and appear to be working correctly. When I originaly replaced the rear brakes I put a new master cylinder on it. When I started working on the truck again last week I went ahead and replaced the master cylinder again thinking I might have gotten a bad one from the store. Also, I replaced it with a new one, not a rebuilt one. Still have the same problem.

I just went and got a hard line to replace the right rear. It doesn't seem like this will fix it but since the right and left rear brakes are connected it's worth a try. Once I replace this hard line, everything on the rear will be new.(shoes, hardware, adjusters, wheel cylinders, hardlines, and rubber line). The only thing left to replace would be the drums and the backing plates. I know if the drums are worn they can cause low pedal and also not as much friction but it still doesn't explain the air leaking into the left rear only.

Thanks for the help everybody. If you have anymore ideas, please post.
 
Cornfedneck - right rear, left rear, right front, left front. I have had the system power bled and the problem is still there.

Just finished replacing the hard line to the right rear. Girlfriend will be off work in about 30 minutes and is coming over to hold the pedal while I bleed. This probably won't fix the problem but it's cheap and easy so I figured I'd give it a try. I will let everybody know the outcome shortly.

Thanks for all of the help.
 
Just finished bleeding the brakes. Now I'm getting a small amount of air from both rear wheels. I got the normal large amount of air at first and now I can bleed all of the air out, pump the pedal 5 or 6 times until it gets mushy again, and then I have a small amount of air out of both rear wheels instead of just the left. The hard line to the right rear I just replaced didn't appear to be obstructed but maybe it had a slight obstruction and didn't flow as freely as the left one so before it was replaced the air always went to the left side. Anyways, now everything is new from the rear brake hose down except for the drums. The only thing that I cam think of now is possibly a problem with the proportioning valve leaking some air in. Still seems strange that no fluid would leak out but I can't think of anything else it could be. I looked over the hard lines running from the rear brake hose to the proportioning valve for the fourth time and still don't see a leak. The line appears to be in good shape.

Has anybody heard of a proportioning valve leaking air in but not leaking fluid out?

Anymore suggestions would be greatly appreciated.
 
/forums/images/graemlins/yikes.gifProportioning valve is $99.99 plus shipping fom LMC /forums/images/graemlins/angryfire.gif

The more I think about, this is the only part left that hasn't been replaced, so I went ahead and ordered it. Hopefully it will be here in about 3 days.

I'm still open to anymore ideas so post away.

Thanks
 
turning your drums couldnt hurt, but replacing isnt necessary unless its scarred up, the adjusters, if properly functioning, will make up for the loss of drum surface.
 
drums can be excessively worn.. maximum diameter is a measurement, where the adjusters cannot make up for the distance of travel anymore..
 
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