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post #1 of 13 (permalink) Old 08-22-2011, 07:11 AM Thread Starter
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Brakes-pedal fade-need help!

Hello All -

I have a 94 2500 4x4 with rear abs that I am doing a lot of work to and I have an issue with the braking system. I have read several threads with similar issues but haven't found the true answer yet. I recently replaced the rear wheel cylinders with the chevy one ton upgrade, replaced the brake line from the master cylinder (abs valve) to the rear, replaced all the rubber brake hoses, new calipers, new pads,etc. I bled (vacuum and conventional method) the hell out of the brakes with new fluid...all the fluid at each wheel is clean and no air bubbles coming out of the clear line. I have flushed about 2 quarts of new fluid through the system. The pedal is hard with the truck off but when I start the truck, the pedal SLOWLY goes to the floor. I bled everything again and the same problem exists. I thought that I had a bad master cylinder so I replaced it and still have the same problem. Lots of people on the forums have mentioned the abs/proportioning valve may be stuck open or need to be bled. The abs light goes off once the truck is started and running. Sometimes when the truck is running and the brake pedal is pressed down, the abs light briefly comes on and then goes out. I don't know if I still have air in the system or if the issue is related to something else. I bench bled the master cylinder very well and the rear brakes are adjusted with a little drag when spinning the wheels. Do I need to keep bleeding? Please help!!! Thanks

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post #2 of 13 (permalink) Old 08-22-2011, 08:34 AM
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DO you have 2 or 4 wheel ABS? that will help with the diagnosis.

some of the ABS valves are susceptible to a condition called a "dump leak" where the cartridges responsible for dumping pressure off at the wheels during an ABS even are leaking. When you have a dump leak the pressure will go to fill the low pressure accumulators (LPA's) instead of supplying pressure to the wheels. The resulting action is that the pedal is mooshy, or it drifts to the floor after a normal apply, or you get something that feels like faded brakes.

There is an easy test to check for dump leaks. Step 1 find your LPA inspection cover.

Remove the boot to expose the inspection port.

Insert a small probe or screwdriver until it bottoms out and mark its position.

Once you mark the position have somebody waffle the pedal to the floor in the cab and watch the probe you've inserted into the inspection port. If the probe moves at all you have a dump leak. See here where the probe pushed out.

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post #3 of 13 (permalink) Old 08-22-2011, 10:23 AM
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Greetings
I have the same issue; however, it seems to be related to the vacuum system. It is really bad in the winter and lessens during the summer. The reason that I suspect that it has something to do with the vacuum system is that I when have depressed the brakes, the heater control systems do not actuate until I let off on the break. Ant thought about this?
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post #4 of 13 (permalink) Old 08-22-2011, 10:27 AM
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leaky or inoperable vacuum will not cause pedal drift. I'd suggest you start your own thread on the topic.

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post #5 of 13 (permalink) Old 08-22-2011, 08:34 PM Thread Starter
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Thanks for the feedback. I have 2 wheel abs (rear). If I do have a "dump leak" what is the fix?
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post #6 of 13 (permalink) Old 08-22-2011, 09:24 PM
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Quote:
Originally Posted by Willymc View Post
Greetings
I have the same issue; however, it seems to be related to the vacuum system. It is really bad in the winter and lessens during the summer. The reason that I suspect that it has something to do with the vacuum system is that I when have depressed the brakes, the heater control systems do not actuate until I let off on the break. Ant thought about this?
The vacuum pump can't keep up with continuous braking at idle.
If the problem persists above idle you likely have a vacuum leak.

Bill
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post #7 of 13 (permalink) Old 08-22-2011, 10:23 PM
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Thanks for the feedback. I have 2 wheel abs (rear). If I do have a "dump leak" what is the fix?
you can try removing the cartridges, those are under the two steel coil casings and coils. They thread out, there are lip seals on the bottom typically lip seal deterioration is one cause of the leak. The other cause is a rusted seat from brake fluid not being flushed often enough. The brake fluid absorbs water the longer it sits and once it gets to the right level steel will rust in the calipers and cartridge valves. Once the seat rusts the stainless steel ball in the armature won't seal any more and voila, dump leak to the low pressure accumulator. You can disassemble the cartridges and attempt to clean the seat and make sure there isn't any rust present. If they are rusted and pitted, my best suggestion to you is to replace the valve all together.

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post #8 of 13 (permalink) Old 08-24-2011, 10:05 PM Thread Starter
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I tried the test with putting a screwdriver in the inspection port and it did NOT move so I am guessing that I don't have dump leak. What are the other possibilities for a slowly fading pedal? Is it possible that I still have air in the system....even if I have already flushed two quarts or so? It seems that this condition of a slowly fading pedal is not uncommon, but the fixes for it are wide spread. What is the proper procedure for adjusting the rear brakes? (if this could be a cause)
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post #9 of 13 (permalink) Old 08-25-2011, 12:31 PM
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My bet is you got a bad master, New parts are crap, always the first place I look.

Don't think the rears are your problem but here's instructions> http://dieseldatabase.com/adjusting-...-brakes/?id=42

Bill
'95 2500 4wd auto
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post #10 of 13 (permalink) Old 08-25-2011, 10:41 PM Thread Starter
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So, I don't have dump leak and tonight I flushed another quart of new fluid through the brake lines. I have nice clear fluid at all 4 corners with no air bubbles...the pedal still SLOWLY goes to the floor with the engine running. I give up!!! The only thing I can think of is that I have a bad reman master cylinder that just installed or I didn't bleed the system of air. I did not bleed the lines (crack the lines) at the master or proportioning valve...could this be the problem? PLEASE help! Do I install another master cylinder? Thanks
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post #11 of 13 (permalink) Old 08-26-2011, 09:34 AM
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I agree with the bad reman. if it isn't going into the accumulator, the only way the pedal can drift down is if it is blowing by the seals internal to the master cylinder.

Back in the common snail game. 03 SO 2wd 530,000 miles and counting.
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post #12 of 13 (permalink) Old 08-26-2011, 12:54 PM
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Never tried it on a Dodge but on some vehicles you can tell if the master's internal seals are leaking by draining the fluid reservoir down to where it's below the divider between the the front and rear circuits.
Internal leak pedal creep will cause the fluid to flow from one side of the reservoir divider to the other.

Bill
'95 2500 4wd auto
'95 3500 5 speed heavy hauler
Stock for one day

Last edited by illflem; 08-26-2011 at 12:58 PM.
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