brake sometimes sinks to floor, sometimes drags

[mhead]

Hi All,

I've a 1992 XJ 4.0. This vehicle had a D35 rear axle and a anti-lok brake system. Replaced rear axle with D44 and removed the anti-lok system. So now I have a standare Jeep hydraulic system: 1992 master cylinder plumbed to a proportioning valve (from a junk yard standard system - not the valve used with the ABS) and then plumbed to the wheels.

The system has worked well for several years.

About a year ago I noticed that the pedal OCCASIONALLY seemed to be fading under constant slight foot pressure. Ok...replaced the master cylinder. But the problem continued. Just sometimes I'd notice pedal fade.

Recently the pedal faded to the point where the brake light came on. And just a couple days ago I was in stop/go traffic and noticed the front brakes dragging.

Things I know:

Brakes properly blead
master cylinder never ran out of fluid
No leakage of brake fluid at any point in the system
MC reserviors maintain their fluid level for months and months
Both front brakes dragged.
Fading problem observed in two master cylinders

I don't think the rear brakes dragged but I'm not sure

This is so wierd that maybe I've got two problems.

Can anyone think of any way that the pedal can seem to fade EXCEPT air in the brake lines, fluid leakage, or a bad MC?

How can pedal fade cause the brake idiot light to come on? Is there only the switch on the proportioning valve or is there a limit switch on the pedal somwhere?

Anyone know of sketches showing cut-away views of the master cylinder and/or proportioning valve?

I'm looking for unusual suggestions..

Is there a 1-way valve somewhere that could limit the return flow and cause the front brakes to drag?

Thanks!
Mike

 

[Kevinma255]

I had a similar problem and the problem was the distribution valve just off the master. If you are sure the valve is ok you could check the rubber lines. They sometimes disintegrate inside. This will cause fade due to weak internal walls and or drag from not letting the fluid return.. If not I would blame the booster or master even if it appears to be ok.

 

[kastein]

x2 on the rubber lines - they are like ten bucks and will probably solve at least one of the problems, they tend to get old and saggy so they can swell when you hit the brake pedal.

 

[steve796]

X3 on the rubber lines, but I will probably have to lean towars the booster. Vaccum Boosters are not the greatest pieces of crap.

 

[mhead]

I had a similar problem and the problem was the distribution valve just off the master. If you are sure the valve is ok you could check the rubber lines. They sometimes disintegrate inside. This will cause fade due to weak internal walls and or drag from not letting the fluid return.. If not I would blame the booster or master even if it appears to be ok.

Sorry, I don't know what you mean by 'rubber lines'... Are these the lines spanning from body to each of the front wheel calipers?

Anyway, I have slight pressure pedal fade from time to time. Seems to me if the caliper lines were elastic I would have a rubbery pedal feel which I don't. Pedal is always firm.

What were the symptoms you had with your proportioning valve?

Thanks!

Mike

 

[joe_peters]

If the pedal is falling away under steady pressure you have a leak. Leaks can be external, i.e., loss of fluid; or leaks can be internal, i.e., the seals around the MC piston are bad, and there is no loss of fluid. Replacing the brake lines is a good idea, but substandard brake lines will give you a spongy pedal. My bet is the MC needs replacing, and you should make sure you are getting a NEW one, of quality manufacture.

 

[ehall]

lift the piston cover and check the rear wheel cylinders

 

[mhead]

If the pedal is falling away under steady pressure you have a leak. Leaks can be external, i.e., loss of fluid; or leaks can be internal, i.e., the seals around the MC piston are bad, and there is no loss of fluid. Replacing the brake lines is a good idea, but substandard brake lines will give you a spongy pedal. My bet is the MC needs replacing, and you should make sure you are getting a NEW one, of quality manufacture.

My thoughts exactly. Pedal fade MUST be fluid loss somewhere thinks I. But I'm on my 2nd master cylinder with the same problem. So I'm hunting around for some other explanation.

I can't see how a proportioning valve could cause pedal fade, but maybe there's some way.

Do you know how the brake idiot light could be triggered at full pedal depression?

Mike

 

[joe_peters]

You may have multiple faults going on. Pedal fade w/o fluid loss my money is on the MC. Was the current MC and the one before that NEW quality units or rebuilds? Big difference. The drag problem can be numerous items, but if dragging at all four wheels then the MC return port would be the most likely culprit. The brake light in the instrument cluster is triggered by low pressure, which would also result from a bad MC by passing fluid--it would set up a scenario just like an external leak.

 

[mhead]

MC was a Pep Boys rebuild in both cases. I'm really sure I have no fluid leaks since the MC never needs fluid and since I've inspected the wheels and since I only sometimes notice the pedal fade.

I guess the brake light could come on if one of the MC pistons bottomed out before the other causing a pressure imbalance.

Yes, maybe two problems. Are there any 1-way valves anywhere in the MC or in the proportioning valve?