1995 F250 Brakes - Foot goes to the floor

My friend has a 1995 F250 which has what I think is a weird problem. Has about 250,000km. Recently the brake pedel has been going further to the floor so he replaced front pads and rear shoes and fluid is
fine. Still had to press further to the floor...he replace master cylinder (twice), rear brake pistons, bled the system twice and it has been getting worse. I sat in it and when the truck is not running I can pump up the brakes and they stay up. While the brakes are up I start the truck and my foot goes right to the floor and I can't pump up the brakes. Is there air somewhere? Some dealer say need to pressure bleed the ABS, some say normal old school bleeding is all that is required..help.
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On 16 Dec 2006 09:55:28 -0800, snipped-for-privacy@hotmail.com wrote:

Has he bled the RABS valve? It can be a bit of a problem to fully bleed the air. If he had a bad master cylinder or did not bench bleed it before installing, I would suspect air trapped in the RABS valve.
Lugnut
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I have also seen leaking RABS units.... the dump valve can leak allowing fluid to escape into the accumulator chamber....

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Jim Warman wrote:

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Another thing I read was the the RABS valve is normally open so that normal wheel bleeding gets rid of the air in the whole system....or does some air get trapped in the RABS unit anyway? I am assuming that the RABS unit is the block located on the driver's side frame rail under the hood...I am going by memory since the truck is at my friend's place.
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The dump valve on the RABS unit is normally closed.... When the system feels that rear wheel lockup is imminent, the dump valve opens reducing pressure to the rear brakes. The fluid that is "dumped" enters an accumulator - basically a chamber with a spring loaded piston - moving the piston back against the spring pressure. When the brakes are released, the spring pushes the fluid back into the braking system through another, spring loaded valve.
Yes, this is the unit located on the frame rail.... two brake lines and, IIRC, a 4 or 5 wire plug... Tiny little bleeder screw on the top, IIRC.
The only times I get to work on anything older than about 5 years is when an independent shop comes upside of a concern they can't handle - the memory grows vague, at times....

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On Sun, 17 Dec 2006 00:28:41 GMT, "Jim Warman"

I think I would try bleeding a lot first. He doesn't really want to know the cost of a Recon RABS valve.
Lugnut

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how do you bleed the RABS unit?
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On 16 Dec 2006 19:22:26 -0800, snipped-for-privacy@hotmail.com wrote:

It should have it's own bleeder valve. It should be bled before the rear cylinders. I usually come back to it after bleeding the rear just to be sure. You also need to make sure the master cylinder is purged well by bleeding at the line connections. Close the connections or valve while the fluid is still flowing - do not wait for it to stop since the pressure that keeps the air out goes to zilch when the fluid stops flowing while bleeding. The pedal should not be released until the connection is tight. I point this out because many people wait until well after the pedal is on the floor before closing the connection which may allow air back in. Brake fluid is cheaper than sheet metal when the brakes don't work.
Lugnut
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lugnut wrote:

Thanks, I will pass along the info to my buddy and see if he can get it working. That's why I drive a '70s chevy half ton....well also a '60s Merc. At least I can fix anything on those vehicles on the side of the highway.
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