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
On 16 Dec 2006 09:55:28 -0800, email@example.com
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.
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
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....
On 16 Dec 2006 19:22:26 -0800, firstname.lastname@example.org
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.
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
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