X-eliminator 2012-02-27 07:27:44
I have a 94 Crown Vic with 4 wheel anti-lock disc brakes. While
sitting at a stop light with my foot on the brake I notice that the
brake pedal very slowly goes to the floor. Is this a defective master
This started to occur after I replaced a defective brake line behind
the left front wheel (going to the left rear wheel) that rusted and
started leaking brake fluid. Although the problem is not so severe
that I can’t drive the car, something is obviously defective. I have
bled all 4 wheels, there is no brake fluid leaking and I do not
suspect the brake booster. Anyone have any ideas as to the likely
cause of this problem?
John carr 2012-02-27 07:27:56
Master cylinder seal(s), When bleeding the brakes you push the pedal to the
floor and damaged the seal(s) inside the master cyl..
X-eliminator 2012-02-27 07:28:00
Point well taken. Actually I used a vacuum bleeder after the repair,
however I’m sure that the damage that you mentioned was done when the
rusted brake line popped and the pedal went all the way down to the
carpeted floor. Sadly the result is the same.
Thanks for the info !!
Bruce l. bergm 2012-02-27 07:28:02
To be more specific, when bleeding the brakes you took the brake
pedal (and the internal piston of the MC) all the way to the floor,
into an area of the MC main bore that isn’t normally ‘swept clean’ by
the seals in the MC – your normal pedal stroke stops well clear of the
floor. There was a little rust or debris at the bottom of the bore,
and when you pushed past it the rust messed up the cup seals inside
the MC just enough to cause an ‘internal leak’. Ergo the sinking
pedal but no signs of external leaking.
Moral of the story, if the car is always power-bled by the
mechanics, don’t do a manual bleed at 100K miles all of a sudden. You
are much more likely to cause this problem.
But if the car is manually bled whenever normal brake work is done
(15K or 20K mile front brake pads), and you always put in fresh brake
fluid and bleed it well to keep the moisture levels in the system down
(and slow or prevent the internal rust) this won’t happen nearly as
soon. It might finally wear out those cup seals at 200K miles.
Now note that if you are losing fluid there is another way the
Master Cylinder can fail – it can allow brake fluid to get sucked into
the power booster, and then into the engine through the manifold
vacuum line without dripping on the ground.
You need to take the vacuum hose off the brake booster and check to
make sure it isn’t wet with brake fluid on the inside.
—<< Bruce >>–
X-eliminator 2012-02-27 07:28:05
Thanks for tha additonal info !!!
Ted mittelstae 2012-02-27 07:28:07
I disagree strongly with this advice.
Basically what your saying is that you WANT to keep a master cylinder
in service that has rust or debris at the bottom of the bore.
So, on the one day that he’s doing a panic stop and he DOES depress
the brake past the “normal pedal stroke” then the seal fails all the sudden
his brake pressure drops to zero.
If the master cylinder is rusted in the bore to the point it will damage the
if it travels past normal pedal stroke, then
your far, far better off finding out about it while the car is on the shop
or immediately after when your paying close attention to the brakes.