Pumpin the Brakes!
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Pumpin the Brakes!
1978 F-700 Dump Truck.
The brakes got to where they had to be pumped. First time all the way to floor, then they pump up. We pulled the rear wheels and didn't find anything the looked wrong, the wheel cylinders all looked good.
I replaced the master cylinder and bleed all the lines but still have the same problem.
Any suggestions what it might be?
The brakes got to where they had to be pumped. First time all the way to floor, then they pump up. We pulled the rear wheels and didn't find anything the looked wrong, the wheel cylinders all looked good.
I replaced the master cylinder and bleed all the lines but still have the same problem.
Any suggestions what it might be?
Ford Parts Specialist
Joined: Nov 2006
Posts: 88,826
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From: Simi Valley, CA
You have brake fluid leaking...from somewhere.
It could be the front wheel cylinders, a front or rear rubber hose that has split open, a steel line that has split due to corrosion, or?
Or the brake differential proportioning valve is leaking.
This valve is mounted on the drivers side frame rail below and to the rear of the firewall mounted master cylinder.
These trucks have an additional brake fluid reservoir mounted on the brake booster.
It could be the front wheel cylinders, a front or rear rubber hose that has split open, a steel line that has split due to corrosion, or?
Or the brake differential proportioning valve is leaking.
This valve is mounted on the drivers side frame rail below and to the rear of the firewall mounted master cylinder.
These trucks have an additional brake fluid reservoir mounted on the brake booster.
Senior User
Joined: Mar 2006
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You say you replaced the MC. Did you buy a new one or a reman one? I went with a reman one a couple weeks ago and had nothing but trouble. Spent a few extra $$'s on a new one and had no trouble with it. Remanufactured parts are crap. I havent had luck with anything remand. Just my opinion. One other thing did you bench bleed the MC before you installed it? You gotta do that to get the air out it.
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Joined: Dec 2007
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You have air in the line.. My brakes were fine but my MS leaked and i knew it. I let it leak all the way down and got air in the system. I topped it back off and you have to pump em now. Im just too sorry to go bleed em.
But you prob have air in the line.. Just simply bleed the system before you go buying parts.
But you prob have air in the line.. Just simply bleed the system before you go buying parts.
Laughing Gas
Joined: Dec 2007
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From: Alvarado, TX
Jack up 1 side and tighten it until the shoes just start to drag when you spin the wheel.
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That is fine but there is a lot of extra things on the backing plate that is not usually on lighter trucks.
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Laughing Gas
Joined: Dec 2007
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From: Alvarado, TX
They're still primarily the same. There should be a hole in the plate to access the adjuster wheel just like a light truck. I happen to have a 1978 Chilton manual that covers light to heavy trucks. It show a few different brake setups that could be on your truck but they all have similar adjusters and adjustment procedures.
However, your problem sounds hydraulic rather that a matter of adjustment because of the fact that you can pump them up. Some of the systems have 2 cylinders per wheel. Make sure you didn't over look this and skip bleeding one after replacing the MC. Look for leaks and check the MC to make sure its not bypassing fluid internally. When this happens 1 side of the resivoir gets drained while the other fills up. You won't see a fluid leak untill it overflows out the cap.
However, your problem sounds hydraulic rather that a matter of adjustment because of the fact that you can pump them up. Some of the systems have 2 cylinders per wheel. Make sure you didn't over look this and skip bleeding one after replacing the MC. Look for leaks and check the MC to make sure its not bypassing fluid internally. When this happens 1 side of the resivoir gets drained while the other fills up. You won't see a fluid leak untill it overflows out the cap.
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I have bleed until I am saturated with brake fluid.
We adjust them till them just drag.
M/C seems to be working fine. Pumps fluid and we get a geyser back in to the reservior when we release the pedal.
Pulled all the wheels and the only visible leaks where the LF wheel cylinders. We replaced them both.
I am at my wits end here.
The pedal doesn't seems to go down if I pump it up and hold it. I would think that would indicate no leaks.
Has anyone had issues with the power booster set up on these trucks?
We adjust them till them just drag.
M/C seems to be working fine. Pumps fluid and we get a geyser back in to the reservior when we release the pedal.
Pulled all the wheels and the only visible leaks where the LF wheel cylinders. We replaced them both.
I am at my wits end here.
The pedal doesn't seems to go down if I pump it up and hold it. I would think that would indicate no leaks.
Has anyone had issues with the power booster set up on these trucks?
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Joined: Jun 2008
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From: Duncan B.C.
Is it possible they gave you the wrong master cylinder? Like a disc/drum one when you need a drum/drum? the different volumes required might account for the need to pump up ( the one side and not the other)
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