1995 F250 Brakes - Foot goes to the floor

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

I have also seen leaking RABS units.... the dump valve can leak allowing fluid to escape into the accumulator chamber....

bleeding does?

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.

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

Lugnut

how do you bleed the RABS unit?

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

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.

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....