I was told my master cylinder is leaking because I recently overfilled resivior and that a "release valve" was just pushing out extra fluid and that my seal on my cap was bad so I changed seal and the leak stopped for a few days then I noticed it was leaking again today. What should I do?
You should never have to add brake fluid to the reservoir. Many mechanics are unfairly accused by uninformed owners of doing an improper job when checking fluids during a routine oil change because they don't top off the fluid. It is normal and expected for the fluid level to go down as the front disc brake pads wear over time. When new pads are finally installed, the pistons have to be pressed back into the calipers to make room for them. That's when the fluid gets pushed back up into the reservoir. There won't be room for that fluid if someone filled the reservoir previously. It will spill out creating a mess on the floor and brake fluid will eat paint off the car body. Other than that, overfilling it will not cause a leak. If fluid leaks out because the level is high enough to reach the rubber bladder seal under the cap, air is able to leak in too. Brake fluid loves to suck moisture out of the air. That leads to corroded metal parts and it lowers the boiling point of the fluid low enough that it could cause a low and mushy pedal if the front brakes get hot enough to make the water in the fluid boil.
September, 16, 2011 AT 10:40 PM
Sorry for not stating this but leak is were master cylinder and booster join and between the two is were I was told there was a relief valve
September, 16, 2011 AT 11:20 PM
There's no such thing as a relief valve. When fluid leaks at the rear of the master cylinder, you simply have a leaking seal and the master cylinder must be replaced. You can buy rebuilding kits for them but they often cost more than the rebuilt master with a warranty.
Overfilling it did not cause that leak. A leak is the only reason you would have to add fluid, contrary to what I stated previously. What you have is a common, ordinary failure and nothing to be excited about as long as no petroleum product got into the fluid to contaminate it.
If you want to replace the master cylinder yourself, I can share a couple of hints to make the job much easier. You'll need to tell me if you have two or four steel lines coming out of it.
September, 16, 2011 AT 11:37 PM
I have two linesand thanksfor your help
September, 17, 2011 AT 12:58 AM
Here's a copy of the trick I've used many times so I don't have to bleed at the wheels. You still can bleed at the wheels if you want to get more of the old fluid out. It WILL have some moisture in it, and it's good to get rid of that.
Loosen the two steel lines just a little, unbolt the master cylinder, then use it as a handle to bend the ends of the steel lines up a little. That will prevent the fluid from running out of them. Continue unbolting the lines. Be careful to not get brake fluid on the fenders because it eats paint.
Bench bleed the new master cylinder with the two hoses and fittings that come with it, connect the two steel lines finger tight, then push the master cylinder down to bend the lines back to their original shape. Bolt the master cylinder in place, then have a helper slowly push the brake pedal down. He can go all the way to the floor with a new master cylinder but never go more than half way with an old one because the lip seals can be ripped on the crud and corrosion that build up in the lower halves of the bores. It should take him at least 15 seconds to get the pedal half way down.
You'll see air bubbles coming from the lines. Snug the nuts, THEN tell your helper to release the pedal quickly. Loosen the nuts again, one at a time, and do the same thing until no more bubbles come out. When the pedal is pressed slowly, brake fluid will go to the wheels but any air bubbles will float back up the steel lines. If the pedal is pressed quickly, those bubbles will work their way down the line and need to be bled at the wheels.
When the pedal is released quickly, the fluid rushing back into the reservoir will wash any air bubbles back with it. That way no bleeding is necessary at the wheels.
Be absolutely certain to not get any hint of petroleum product in the brake fluid. Engine oil, transmission fluid, and power steering fluid will destroy ALL rubber parts in the system that come in contact with brake fluid. That is a real expensive repair.