64 TBird stiff brake pedal when off, super soft when on.

[64TBird64]

So I have just completed my latest task of replacing the brake booster and master cylinder, I have a dual bowl master on there currently. Bled all the brake lines today, just finished bleeding them about an hour ago. Brake pedal feels just like it’s supposed to when the car is off, but when started the pedal practically goes right to the floor when you tap it with your foot. The bowls are full, have checked for leaks and there are none, all bleeder screws are tight. Any body have an idea of what this might be?

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[BigClay]

Did you “bench bleed” the master cylinder before installing it?

 

[64TBird64]

Did you “bench bleed” the master cylinder before installing it?

Bled it during the bleeding of the lines, pumped it enough that each bowl was filled about 3 times. No air was coming from the lines.

 

[Wide glider]

Does The pedal come back up after you pump it three or four times if it does still air in the system

 

[64TBird64]

Does The pedal come back up after you pump it three or four times if it does still air in the system

The pedal does still come back up, that’s kind of what I was thinking but I’m not all that familiar with brake systems. Thank you!

 

[Wide glider]

Im not either, just an opinion. Good luck

 

[fordrodsteven]

With power brakes the pedal would be very solid (hard) with the car not running. With the car running the power booster would do it's job making the pedal easier to push. Once it's properly bled it should only go down a little and start actuating the brakes. It should only go approximately halfway to the floor at most. If the pedal continues on down further it is possible the rod length may be wrong. If you didn't change anything with the rod my guess would be that you have a defective master cylinder.

 

[labman2]

The rod from the brake booster to the master cylinder should be adjustable. The rod has a critical dimension that must be checked every time the booster or master cylinder is changed.

 

[64TBird64]

The thing is, the brake pedal isn’t even that hard when it’s off. It feels a little softer than it should with the car on and brake booster working, but when the car is on the pedal sinks right to the floor when you touch it. It does come back up, but it’s like stepping on marshmallows.

 

[Angry Bird]

The bellows in your brake booster may be the problem it might have to be replaced.

I'm no mechanic, I usually leave brakes to the professionals. Too important for me to to play with..... stopping is important!

 

[fordrodsteven]

If the booster is at fault the brake pedal would be hard but the pedal still would not go all the way down to the floor. It's either an improperly adjusted rod or a defective master cylinder. I'm 99% certain it's the master cylinder. I would change out the master cylinder.

 

[doug7740]

Please stop guessing already! Since you said that you are not all that familiar with brake systems, you should have this repair done by a professional. Your life and the lives of others are at risk. Brakes are too important for you to play with.

doug7740
1955 Thunderbird Blue

 

[597LSCJ]

Bled it during the bleeding of the lines, pumped it enough that each bowl was filled about 3 times. No air was coming from the lines.

Did you bleed the M/C on the car or on a bench in a vise prior to installing in the car? You can't get a full stroke when in the car even using the bleeder kit. If it pumps up there is still air in the system.

 

[64ZCODE]

Hi there 64TBird64, just wondering how this turned out? On my '64, with engine off, the brake pedal cannot be pushed down very far. After engine started, brake pedal will depress until about half way to the floor where the brakes engage. The shop manual says this is what is expected.

 

[64TBird64]

Hi there 64TBird64, just wondering how this turned out? On my '64, with engine off, the brake pedal cannot be pushed down very far. After engine started, brake pedal will depress until about half way to the floor where the brakes engage. The shop manual says this is what is expected.

Still have not gotten down to the problem, but have an idea. Think i May need an extension on the push rod between MC and brake booster.

 

[64ZCODE]

Hi 64TBird64. I'm wondering if there is sufficient fluid in the dual master cylinder reservoir? The single master in my '64 was leaking into the booster, so the level in the master reservoir kept dropping but no leakage of fluid from the wheels, under the car etc. It was all in the booster, which was easy to see by removing the vacuum fitting at the top of the booster and shining a light down in. Keep us posted on progress please.

 

[Tater Salad]

I don't know if you've upgraded disk brakes or changed wheel cylinders,but if you have, make sure all the bleeders are on the top. You'll never get the air out if they're not. Ask me how I know.

 

[Akflyer]

Post #3 in this thread indicates that you did not bench bleed the master cylinder. If you did not bench bleed the master cylinder, you need to take it back off and bench bleed it. As was pointed out earlier, you won't get all the air out of the master when it is on the car.

 

[64ZCODE]

I'm not familiar with the bench bleeding procedure. Why would bench bleeding remove air from the master when bleeding with master in car would not?

 

[OneAmongOthers]

I'm not familiar with the bench bleeding procedure. Why would bench bleeding remove air from the master when bleeding with master in car would not?

Because you will not get the full length stroke while on the car. Take it off and put it in a vise. First you fill it up then gravity bleed it. After both ports are leaking fluid you close them up and get something with a round tip. DO NOT USE A SCREW DRIVER! You can gouge the inside of the master cylinder piston. There are actual tools, but a punch will work fine. You then push the piston all the way in. Repeat this process until all the air is pushed out of the master cylinder. You can tell that the air is out because the piston gets stiffer to push on. Then you can reinstall it.

Bleeding is done by starting on the back passenger wheel. Have someone sit in the car and pump the petal until firm. Then you open the bleeder screw and watch until no more air bubbles come out. Get a bleeder kit. Your local parts store will usually always have it. The kit is usually some length of tube and a cup to catch the fluid. This makes it easier to see bubbles, and it doesn’t leave a mess.

After the passenger side go to the drivers rear wheel. Same thing... have the person pump the petal until firm then they hold it. You open the bleeder screw and the fluid and bubbles come out. The brake petal will go to the floor. Make absolutely sure the person in the car is working with you and that they DO NOT start pumping the petal until you close the bleeder. Otherwise you can suck air back into the system.

Next is the passenger side front wheel. Same deal... pump the brake petal up until firm then open the bleeder. Close then pump back up again until all the air is out. Then drivers side. Remember during all of this to keep the master cylinder topped off. You don’t want it emptying out and sucking air back into it.

For the power booster... to get a good adjustment in the op rod position I myself used some grease or something like white lithium grease so when you set the master cylinder on it you can see inside the master cylinder when the rod is at the right length, and that it’s just touching. You don’t want to have the rod length too long because then it’s just prematurely pushing on the master cylinder piston.

Hope this helps. Let me know if you need anything else.