I am experiencing a problem where the brakes lock up after about four
to five miles of travel. I was told that the problem could be
resolved by replacing the master cylinder. I did, but the problem
continues. The only way to get going, is to release the pressure via
the bleeding valves. This works sometimes for an eighth of a mile or
sometimes up to ten miles. I removed all the relays from the ABS unit
with no luck.
Is there someone who has experience this problem? How was it resolved?
It sounds plausible. Pressing the brake pedal opens a valve to let
engine vacuum pull the brakes on harder, so a leak there could cause
such a problem.
That said, all cases of brakes locking on that I've seen have been
caused by a stiff linkage on cars that have the controls on the opposite
side to the master cylinder (ie RHD in a car originally designed for
LHD). But in that case the pedal would stay down.
On Thu, 24 May 2007 19:04:34 +0100, firstname.lastname@example.org wrote:
When active in the trade I had a couple of cars in with similar problem of the
wheels (4) locking after a short drive. Releasing the pressure by the bleed
nipple as stated did get the vehicle mobile again but the problem in the first
was traced to a malfunction in the release valve of the brake servo not letting
air into the chamber when the brakes were released. Swapped out the servo and
no further trouble - DID NOT REPLACE THE MASTER CYLINDER.
Second vehicle with similar problems had a leaky master cyl and needed either a
new one or seals. We stuck a new one on and all was well for about 2 weeks then
another tow in for locked brakes so I swapped the servo and no problems with the
brake till the guy sold the car some 4 years later.
I agree there may be other causes but from experience it would be one of the
first places to look as a seized caliper would NOT lock all 4 wheels.
Brake master cylinder would not cause this.
What are the EXACT symptoms you experienced? 'Brakes lock up' is a pretty
Going by your current symptoms, it's nothing more that a seized caliper.
I had a similar problem once with my Audi. All four wheels would lock
after about 20 minutes of travel. It seems that a problem was that my
"mechanic" filled up brake reservoir with wrong type of brake fluid,
that caused problem with rubber seals, and return spring was not able
to return cylinder back. I changed master cylinder and problem went
Pretty strange behavior allright. I see that the Bentley manual does list
this condition under trouble shooting and suggests that it may be caused by
the master cylinder although I don't personally understand how that would
happen. If it was a seized caliper it would probably only happen to one
wheel unless the car has been sitting unused for a long period of time. Do
all the wheels get hot or just one?? It seems like you have ruled out the
antiskid controller and the master cylinder so the only thing left is the
vacuum assist. You might try removing and plugging the vacuum line the next
time it happens to see if that fixes it. Just remember that you won't have
the power assist with the vacuum disconnected.
Good Luck and please let us know what the problem turns out to be.
The master cylinder has a relief port in it - that relieves brake
pressure when you let up on the brake pedal. It basically opens the
brake circuit(s) to the reservoir. This same port is also how the system
is kept full as pads wear and the caliper pistons move out.
If this port is plugged up, or the pedal isn't returning fully to the
top position, it will cause the symptoms described.
I would check:
1. Is there anything restricting the movement of the pedal? ie - can it
return fully up? (I'm thinking something like the pedal bracket got
bent, or the brake light switch is binding.) It doesn't take much to
cause this condition.
2. Has the entire system had a fluid flush? If not - it should. It's
possible there is some crap floating around in the system and it's
managed to find it's way back to the relief port.
Other than that - we probably need better diagnostics. When the wheels
lock up - do ALL of them lock up? (Temperature of the wheel is usually a
clue..) If it's all - it's doing this at a point common to all wheel
circuits - which narrows it down to the master cylinder (again) or the
ABS control hydraulic unit.
This is commonly caused by a hose that is broken internally. The fluid
is allowed past but the "break" in the hose acts like a check valve
and doesn't allow the fluid to return and the caliper piston to
retract. This is not the case here as it would happen the first time
you applied the brakes. I also believe it is caused by a blockage in
the system. I've never seen a vacuum assist do this but I can't give a
definitive no on it either... As stated before, disconnect (and
plug/cap the vacuum line to the brake assist unit and see if the
problem goes away. Please let us know what fixes the problem.
On Fri, 25 May 2007 16:00:36 +0100, "Dave Plowman (News)"
On May 24, 10:49 am, email@example.com wrote:
5/25/2007 - Thanks for the many responses. I will try the suggestions
on Monday (Momorial Day)
To clarify - The symtom is like it has the hand brake connected to all
four wheels, and is applied to the maximum. All four wheels get very
hot if I try to push ahead before a complete stop. Whenever it is
totally locked, and I try to move, it just burns the clutch and the
car just squats.
I will post the results of the suggestions I have already received.
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