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Old 05-25-2009, 02:39 PM
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Master cylinder problem?

On my Ford truck I have a problem that I thought to be the master cylinder because the brakes will hold just fine and seem to have normal pedal travel at first. The problem is that after the pedal has been held for more than a few seconds it will start to "creep down" until it reaches the floor and then the brakes will release, once this happens I can release the pedal and hit it again and the brakes will again seem normal for a few seconds then the cycle starts all over again. I have seen this happen several times in the past and I think it is a fairly common type of failure for a master cylinder but here is the catch, I replaced it with a rebuilt one from Auto Zone (I know, ) and nothing changed. Thinking it was just bad I returned it and bought a new one but when I installed that again there was no change. There are no leaks and no fluid loss so it is not just losing pressure and I don't know what else could cause this problem besides a bad master cylinder, any thoughts on this?

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Old 05-25-2009, 02:49 PM
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Did you bleed the new master cylinder before hooking it back up. Are the wheel cylinders showing any signs of leaks sometimes you have to tear them down to see if they are leaky . Cole
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Old 05-25-2009, 02:49 PM
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Check your wheel cylinders. (you said you already did) Pull the dust boots off and see if they are wet. If those are good I'd suspect a bad MC from autozone. Three in a row with the same problem is quite a coincidence, but I don't rule anything out these days. You are right though, a slowly sinking pedal is a classic symptom of a bad MC.
If the truck has an inline booster, (like the old midland ross units that went on big trucks),I would guess you could have a bypass there as well.

What year truck and what is the system comprised of?


Later, mikey
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Old 05-25-2009, 03:01 PM
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Did you bleed the brakes fully? And in the right order? depending on whether the brake system is a cross based system or a front-rear system the corners come in a different sequence. If it is a front-rear system then you need to start in the back on the passenger side, then go to the driver rear, then the passenger front then lastly the driver front. That way you chase all the air away from the master cylinder and get the brake system completely bled to prevent the pedal from sinking to the floor and feeling spongy. And as i stated before sometimes you have to tear down to check the wheel cylinders for leaks as Mikey said pull back the rubber and see if its wet or not. I believe you might just have air in your system though. If not another bad m/c or wheel cylinders seeping. Cole
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Old 05-25-2009, 03:30 PM
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Wow, that was fast!

Ok I guess I could have given a bit more info, this is on an old service truck from my shop and had been sitting for a while. It is a 1988 F250 super duty diesel, 8 lug wheels, etc. I replaced the front brake pads and rotors but the calipers were Ok, I inspected the rear brake shoes/drums/cylinders and everything was fine. I knew this thing was "creeping down" like that before I did anything to the rest of the system and that is why I checked everything for leaks and bought the master cylinder when I picked up the pads and rotors. I did everything in this order, inspected all four wheels/brakes/cylinders/calipers, replaced front pads and rotors, bench bled and installed the new master cylinder and then bled all the brakes in order starting with the right rear. This really does not seem to be an air problem because the brakes do not feel "spongy" they actually have a full pedal on the first pump and seem completely normal for about 5 or 6 seconds then the pedal will gradually creep down to the floor in about 5 or 6 seconds and the brakes will release. When I release the pedal and pump it again the brakes will again feel normal on the first pump and pumping it multiple times makes no difference, it will still creep down when held for more than a few seconds. This has me puzzled and before I return another master cylinder I thought I would seek some advice on what to look for but I can't think of anything else that could cause this problem.

BTW, the booster is a regular vacuum type just larger than found on a normal size pick-up.
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Old 05-25-2009, 03:44 PM
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The check/combination valves could be faulty or clogged with debris, causing them to not close & seal properly.do you hear any hissing noises and does the engine change at idle when pushing the brakes down. I do admit you have got my attention. you have done pretty much everything else sounds like. Imean steady pressure on the pedal and it is slowly sinking down is sign of system leak some where. Cole
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Old 05-25-2009, 03:49 PM
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If you look inside the master cylinder reservoirs of a bypassing MC while you have a helper stepping on the pedal and holding it down while it sinks you may be able to see a small geyser of fluid coming up through the 2 little ports or see a slowly rising fluid level, AFTER the brakes start building pressure. Those are not supposed to be there with a properly operating MC.
It helps to drain off most all of the fluid from the reservoirs, leave about 1/4-1/2" of fluid though.

I could not find pasteable text for that test, but here is a link to a book preview that describes the test.

http://books.google.com/books?id=H92...esult&resnum=1

Later, mikey
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Old 05-25-2009, 07:14 PM
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Got it!

It was the master cylinder after all, I went down and picked up the last one they had in stock and that one worked! I have spent the entire day chasing after a problem that should have not have even been there after changing the original cylinder but I wanted to make SURE of the problem before I went back again. The guys at the parts counter were very friendly and helpful about this and even offered an apology for all my trouble, they never even hinted that they thought maybe I was wrong about the cylinder being bad but even I was beginning to wonder about it. No matter now, it works and thanks to everyone for the advice that helped me to make up my mind about changing that darn thing again before tearing down the entire brake system looking for a problem that would not have been there.

Thanks again!
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Old 05-26-2009, 12:58 PM
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creeping pedal syndrome such as you had is always either master cylinder failure, or an external leak in the system.


bene here had this one many times over the years in so many different vehicles.

have fun
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