Brake pedal too firm after new Master Cylinder installation

[Patrick H.]

Re: Brake pedal too firm after new Master Cylinder installation

Booster Dewey

I see he only has a phone # listed, no e-mail. I'll probably have to call him tomorrow.

Patrick

[Monte M.]

Re: Brake pedal too firm after new Master Cylinder installation

Pat,
I figured I would wait a while to throw my two cents in. It MIGHT be a bit of air in the master cylinder. Air does not compress much is what makes me think this.

How to re-bleed the master cylinder is; Loose the main brake lines enough to be able to bleed the air out. Take the cover off the master cylinder. Cut a straw about 4" to 6".

Do the front portion of the master first. Put the straw over the little hole at the bottom of the reservoir. Loosen the brake line at the master. Blow in the straw as some gently pushes the pedal down. You will feel resistance until the piston in the reservoir reaches a certain place. When you hit that place, you will feel the fluid going in the reservoir. Lift the straw to allow fluid to fill the straw and repeat until no more air come out.

Do the same thing on the back reservoir. If this does not fix it, I will tell you how to check you Power Booster. You can remove the front cup with the seal in it, then spin the spring out of it. This will release the tension inside to be able to check everything. You can spin the spring back in and replace the seal cup without any issues. There is nothing to boosters really to go wrong except a seal or foreign matter blocking something.

EDIT; I meant to say "I will tell you how to check your "Power Booster"

Monte

[Monte M.]

Re: Brake pedal too firm after new Master Cylinder installation

Sorry,

When I read it, I did not realize you got it fixed. "Ding Ding" should have been the clue.

There is not much to fixing boosters. Explain better exactly what happened.

[Monte M.]

Re: Brake pedal too firm after new Master Cylinder installation

Patrick,
I am not sure it is your booster. Just to double check, try it again. When you shut the car off, pull the check valve a bit and see if that is where the air is bleeding out of. I do not mean that seal. I mean that reservoir of air. If it is that reservoir of air, there is nothing wrong with your booster. It is just building up an abundance of air and needs to bleed a bit of it off.

Just a thought.

[Patrick H.]

Re: Brake pedal too firm after new Master Cylinder installation

Monte,

There is now so much air being sucked through the booster that the car runs rough with the booster connected. Remove the hose and block it off, and it runs fine.
It makes me wonder if the booster was on its way out for a while, and installing the new MC just made it that much more obvious.

[Monte M.]

Re: Brake pedal too firm after new Master Cylinder installation

Patrick,
Now that you mention that, it changes a lot.
A quick way to double check it is to plug the filter under the dash.
Where the rod comes out and attaches to the pedal, there a six 1/4" holes in a rubber boot. If you plug those up and it run better, your booster is bad.
It sounds like it is dirty and not letting the power piston to close all the way.

What car is this on? 95% of rebuilding a booster is cleaning it up. Once in a great while there is something more wrong. How mechanical are you?
Most people worry about taking it apart and getting it back together because it is spring loaded. It is very simple to take the spring out before taking it apart so you do not have to deal with that.

Just a thought. I can talk you through it very easily.

[Edward J.]

Re: Brake pedal too firm after new Master Cylinder installation

Ding Ding! We have a winner.

I tried the test a couple of times. Pedal doesn't change when the car is started.
I then checked the vacuum line, and the engine is giving plenty of vacuum to the booster. Check valve is working.

Turned off car and I now hear a leaking vacuum hiss at the power brake booster - a noise that wasn't previously there.

Anyone know of a good brake booster rebuilder with quick turnaround?

Pat a lot of times when the master cylinder fails it will let the brake fluid drip by the seals directly into the booster with no wear to go, so it will distort the booster diaphram.

[Monte M.]

Re: Brake pedal too firm after new Master Cylinder installation

Patrick,

Just for S***s and giggles, before you pull everything apart, take the master cylinder off, they try it and see if the booster is still frozen. My guess is it will not be.

This is a great way to triple check it before going much further.

[Patrick H.]

Re: Brake pedal too firm after new Master Cylinder installation

Patrick,

Just for S***s and giggles, before you pull everything apart, take the master cylinder off, they try it and see if the booster is still frozen. My guess is it will not be.

This is a great way to triple check it before going much further.

A great way to check for what?
If it's frozen that's good or bad?

If I can move it then what?

[Monte M.]

Re: Brake pedal too firm after new Master Cylinder installation

If it is still frozen, then it is your booster for sure. If it is not, that tells you it is not your booster.

[Patrick H.]

Re: Brake pedal too firm after new Master Cylinder installation

If it is still frozen, then it is your booster for sure. If it is not, that tells you it is not your booster.

Not necessarily. It could still be leaking like a sieve, ie. ruptured diaphragm, correct?

[Monte M.]

Re: Brake pedal too firm after new Master Cylinder installation

You guys checked everything. I was just trying to throw out a few other options. My intent was not to question what you guys were doing. I was just a few ways to double check to make sure you were headed down the right path.

I think I ended up making it more confusing, and I am sorry for that.

Best of luck,

Monte

[Monte M.]

Re: Brake pedal too firm after new Master Cylinder installation

It seems like boosters are one thing people do not rebuild for themselves for some reason,

They are actually one of the simplest parts on the car.

How they really work is a lot less complicated than most people think.

What happens is the engine vacuum pulls vacuum on the front side of the vacuum canister. What prevents it from going forward is the back side is a closed system and will not let air in to allow the piston to move forward.
When you hit the brake pedal, it open a small hole allowing air in the back half of the canister allowing the piston to move. When you let off the brake pedal, the small hole closes and the front starts pulling on the diaphragm again.

The big seal you have seen does not actually move. The big piston inside just kind of floats in the can. The check valve just prevents the air from going back out of the front portion of the canister if the engine is shut off.

It is similar to sucking on a straw with water in it. If your finger is on the other end, you cannot suck the water out. That is the situation the brake canister is in.
Then when you remove your finger, the water rushes forward or into your mouth. Stepping on the brake pedal is the same thing as removing your finger from the end of the straw.
The boosters are very simple inside. If you can rebuild a carburetor, you can do a booster with your eyes closed.

I put this out there just for the information of it.

I hope it helps someone some day.

Monte