View Full Version : Low brake pedal

06-27-2016, 08:48 PM
I have 65 thunderbird.

My brake pedal seemed kind of low for the last year, but not super low.

I was at a cruise and my right front brake started dragging and ate into the rotor.

I replaced the rotor, the brake pads and rebuilt both calipers
I changed the booster about 2 years ago. I double checked the rod length and it is the correct length according to the manual.

When the engine is off , the brake pedal height feels and looks like it is at a normal height. When I turn the engine on the brake pedal goes within a finger width of the floor board. When I try turning the wheels all four do not turn when the brakes are on.

When the car is running. Pumping the brakes does not make any difference. The pedal still goes almost to the floor when you pump it . It does not make a difference when it is off either. I bleed the brakes using a vacuum pump and manually.
It does not seem like any air is coming out.

The rear brakes were a little low and out of adjustment and I adjusted them. It seemed to help. That is where I got the finger width from the floor. I replaced both rear brake cylinders about 2 years ago.

The brake proportioning valve seems ok since the brakes are not locking up when I hit the brakes.

Since the master cylinder looked very old I replaced it. I bleed it according to the manufactures instructions.

What I am I overlooking?

06-27-2016, 09:16 PM
From what you are describing it appears to be either a vacuum line or a booster issue. If you have a nice firm pedal with the engine off then I don't think it's air in the lines or a mechanical issue with the brakes. I would start by making sure there aren't any vacuum leaks anywhere. You may have a bad check valve so I would make sure that is ok. Even though you have a fairly new booster that doesn't mean it hasn't developed a leak.


06-27-2016, 09:57 PM
Thank you for the quick reply.

I check for vacuum leaks could not find any and all my accessory are working normally.

I am not sure where the check valve is. I will try to find it in the manual.

06-27-2016, 10:00 PM
The check valve is on the booster. That's where the hose connects to.


06-28-2016, 12:25 AM
I doubled (and more) checked everything. I talked to an older mechanic and he said they used have the same problem with older Cadillacs'. The solution they put them back together and drove slowly down the street and they pump themseves back up and worked fine. Since I knew my brakes were working, I decided to give a careful try.

My brakes are working fine and feel better than before. You never know what going to work.

Thanks for your help.