Is it both front wheels locking, if only one suspect bad hose at the
caliper. When this happens try cracking the line into the hydro boost if the
brakes free up that is the problem, did this happen before all the work was
I assume you know the theory of operation, the pb booster is the Hydro boost
unit geting pressure from the p/s pump. Something is allowing the booster to
apply pressure to the booster to apply the m/c. Loosen the lines at the mc
to the front calipers, see if the brakes free up, if so the could be there
is a misadjusted rod from the pedal to the booster and it gradually applies
the brakes. Bad brake hoses could cause this but both of them would be rare
and this did not happen till the parts were replaced, why were all the parts
replaced originally? What both hoses to the calipers or p/s pump to booster?
It may be time here to have a experienced mechanic to get involved in
tracking down where the apply pressure for the brakes is coming from. You
must attack this in logical order as outlined.
This has all been done by a place called my little auto shop. I bought the
1st boost. He got the 2nd. It still did not work. He called GMC they told him
he needed a diesel booster. I do not know if we did any harm with the first 2.
Does your brake pedal return all the way? Is the spring hooked up? There
MUST be a small amount of freeplay between the pedal pushrod and the master
cyl. or what you describe will happen.
"Jim U via CarKB.com" wrote:
The brakes work fine for about 10-12 miles then lock up.
When they cool down I can drive again till they heat up.
The shop working on the van is stumped???!!!
Some rear brake lines have a residual pressure valve.
If the front and rear lines were interchanged, front
brakes would not fully release.
Wendy & John.
Hi Roy and thanks to everyone that tried to help.
We found out for some reason if I ride with the hood of my van up ( don't ask
why I tried this) everything runs great.
I put the hood down and the breaks lock up. Something about heat???!!!
I took the van for a ride I put about 150 miles on it.
With the hood down the breaks lock up.
Hood propped up more cool air under the hood no problem ???!!!
Sounds like the master cylinder is bad. There are a couple ports in it
that allow fluid to return into the tank as it heats up and expands. If
they are plugged the brakes will come on as the fluid expands. Usually a
little pressure and the crud comes out. However in your case either it
is a large hunk of crud OR the piston is adjusted wrong in the bore. If
you replaced it I would pull it off and check the length of the push
rod. It may be just a bit too long Or maybe even the wrong master for
"Jim U via CarKB.com" < snipped-for-privacy@CarKB.com> wrote in message
I tooke the van to GMC it was the rod from the master. it was to long. Bill
replaces the master 3x's.
As you can tell I know nothing about breaks, however Bill said he did. GMC
fixed the problem.
Thanks to eveyone that helped.
Message posted via CarKB.com
Hydro Boost. Kind of strange that GM used two different brake systems
on the gas engine vans. My 95 has a conventional booster with the 4
wheel ABS unit under the van. Talk about a plumbers nightmare....
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