Front Disc Brake
Dave Stribling
February 13, 2015
Contributers: Dave Stribling

Boosted Brake Pedal Issues

I just got done rebuilding the brakes on my ’69 Mustang R-code. It has power front disc brakes, and everything is new. I bled the brakes and I get plenty of pedal pressure, but when I start the car the brake pedal goes to the floor. I don’t have any leaks and they work fine until I start the car. The brake power booster is brand new and passes the test in the Ford shop manual.

Ron (last name withheld)
Ocala, FL

You didn’t state all of what you did, so I am going to give you a checklist of what to look for on your power disc brakes:

1. 80 percent of the time the issue is in the rear drum brakes. Not adjusting them out all the way and letting them self-adjust doesn’t work. Make sure you have them adjusted up to the drums; letting the self-adjusters do your job doesn’t work. Also, the rear brakes can be a pain to get all of the air out of the system, so make sure you really bleed them well.

2. It takes 1,000 psi to operate disc brakes, and on a non-power brake car, the diameter of the master cylinder bore is smaller to get the pressure needed. When you are pressing down on your brakes with the engine off and no vacuum booster, you’re getting 100-200 psi, and you aren’t getting enough “push” on the brakes. It feels OK; it will even actuate the brakes. Add in the booster and all of a sudden you are at 1,000 psi. If the drain down test works the booster is probably OK (remove the stored vacuum out of the booster by pressing the brakes several times until you get a firm pedal, then press on the brakes about halfway. If when you start the car the brake pedal moves about a quarter of an inch, it’s fine).

3. Make sure the pushrod coming out of the booster is the correct length. Too short and it can cause the pedal to go to the floor, too long and it will build residual pressure in the brakes and eventually the brakes will lock up.

4. Block off the outlets to your master cylinder with plugs or old brake lines crimped off, start the car and engage the brakes. You shouldn’t be able to push the pedal more than a little bit. This means that the master cylinder is good and not internally bypassing. You didn’t say whether or not you rebuilt the master cylinder or if it is a new one. The master cylinder bore frequently corrodes and pits, and just changing the seals isn’t enough to keep it from failing. I have originals bored out and sleeved.

5. Check and see if your brake light is staying on. When the distribution block has a pressure difference, it pushes the little piston inside it one way or the other, which turns on the light in your gauge panel. If you rebuilt it you may have a problem similar to rebuilding a master cylinder. When you bleed the brakes, are you pulling the fluid out of the correct bowl, or are they both going down? This might tell you if you have a crossover somewhere, usually in the distribution block.

6. I assume since bleeding the brakes went OK you don’t have a collapsed brake flex hose, but check them anyway.

7. Finally, check your front calipers and make sure they are on the right side. It is possible to mount them on the wrong side, and when you do, you will trap air bubbles in the caliper. The bleed screw needs to be at the top facing the firewall, as shown here. If the bleed screw is pointing straight up they are on the wrong side.

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