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 cylinder?
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?
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.
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.
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 and his brake pressure drops to zero.
If the master cylinder is rusted in the bore to the point it will damage the seal if it travels past normal pedal stroke, then your far, far better off finding out about it while the car is on the shop floor, or immediately after when your paying close attention to the brakes.
MotorsForum website is not affiliated with any of the manufacturers or service providers discussed here.
All logos and trade names are the property of their respective owners.