When my son bought his 75 project car, the previous owner/seller, replaced the power brake booster with a new one, as well as, a new master cylinder. After three years of putting the car back together, we finally got it back on the road. We have bled the brakes four times and even tried a Motive power bleeder. All 4 calipers are new. The problem is the brake pedal is very soft almost all the way to the floor (no pressure) and then the brakes slam on. You have to be careful to not lock up the wheels. Took it to a mechanic and he ran 2.5 pints through it bleeding them and it still has the problem. Someone told me that the booster might be bad or installed incorrectly, but wouldn't that mean the brake pedal would be harder to press? Anyone got any idea what is happening?

 

I suspect the master cylinder might be bad...even though it's 'new' it might still be failing.

You can test the booster. With the engine off, press the brake pedal. does it get hard? It's supposed to get hard and stay hard.

Then, while pressing on the pedal, start the car. Does the pedal go soft and down? It's supposed to.

If it doesn't do either of those things you might have a booster problem but I suspect it's the master cylinder.

 

I pick up the car tomorrow and will try this.

 

not sure but you can also check to see the lenght of the push rod between the booster and the master cylinder. I think in the 75's you are suppose to have the longer push rod. I in 77+ they switched to the shorter push rod.

Something to check since it was already installed when you got the car. Maybe the guy did not install the right lenght rod.

 

Booster pushrod could need adjustment to address the pedal, but if they're that sensitive when they do come on also make sure it has the correct master (power 1-1/8" bore); if it has a manual master (1" bore) the brakes can be very sensitive. What kind of pads are on it?

The booster pushrod adjustment can be tricky and take time because as the car heats up the pushrod expands and if you've adjusted the screw too far out it will begin to drag the brakes. Get it hot, and adjust the pushrod so that it brings the pedal up, but still rolls freely in neutral.

 

Is the pushrod you're talking about between the MC and the booster? If so, I'm assuming you have to remove the MC. How long should the pushrod be, that is how much should be showing from the booster? Also, what do you mean by the bore and where do I measure it at?

 

From my past experience, think you have a bad booster I had the same problem where the pedal went almost to the floor and then slammed on to the point that the brakes would lock up .

Javier

 

I been driving GM cars for some 50 years now, and every damn one of the brake pedals feels like the sharks, only not so exaggerated, as most of the other cars are at best a disc/drum combo, they exhibit the problem to a lesser extent, but the sharks were the very first GM car to have 4 wh disc brakes....

it's in the booster, and in my switch to HBoost some 5? years ago I did a direct A-B comparo.....totally left the lines in the braking system alone, removed the m/cyl, and installed it on the HBoost unit, without disturbing ONE THING in the brake hydraulic hookup.....

car stopped on a dime, 9 cents change, nose into the steering wheel....and that was with air in the P/steering hydraulics on account of my having the lines open for the HB install.....

SO, after fighting these brakes for some TEN YEARS before doing the HB install.....the very first time the car had a decent pedal was when doing the HB......

and its remained fine since then.....

WHY GM brakes are all soft as sand, I dunno, must be a design goal for something to do with safety....but I like a short throw pedal with a hard as nails manual brakes feel to it...none of that squishey soft feel that makes you think there is a bananna on the brake pedal....

 

Any way to check and/or fix the booster?

 

I just replaced it.

Javier