I have a friend that has a 1986 bronco and when you push the brake pedal it goes to the floor. I checked the brake fluid and the reservoir for the back breaks was empty. I filled it with fluid. Then I bled all the brakes, front and back. The pedal got hard to push like it should be normaly. When we started the truck to go test the brakes it went to the floor again. So I double checked the brakes, I didnt get any more air out of the lines. I used a whole bottle of brake fluid and still the same results. The pedal is still hard when not running but falls to the floor when started

 

Sounds like a bad master cylinder. Is it still full after all the testing and bleeding and running the engine?

 

The master cylinder is still full of fluid. I does go low when I bleed the brakes but i never let it run out of fluid so it shouldnt get any air in it unless, like you said its the master cylinder. That was my second guess, Thanks for helping.

 

What happens to you stopping power when it go's to the floor? does it go to the floor the first time you push it after you start the engine or is it after you start moving? how are you bleeding the system?

 

It falls to the floor when you started the bronco. I think I have figuered it out, I hope anyway. One of the wheel cylinders is leaking but the out side of the drum and cover was dry. I didnt realize it until after I took the brake drum off and it looked as if someone filled it full of grease. So I think it is just a bad wheel cylinder. I am gonna replace it tomorrow.

The brake pad was completely gone it was just metal to metal. I think that is the cause of the wheel cylinder to blow out. I will keep posting. Thanks for help!!!!!!

 

ya that would do it. keep us posted

 

It may take me a couple of days to get back with you on this thread. I have to wait until my friend gets paid to finish the job. Thanks for the help.

 

Well I put the brake wheel cylinder on last night and new brakes and bled them out and the pedal still falls to the floor when you start the motor. Any Ideas.

 

OK so let me get this strait. truck off, brakes feel tight every time you push them. when you start the truck, you have your foot on the brake, it is firm and as soon as the motor starts the petal hits the floor?

 

Yes. When the truck is off (motor not running) the brakes fill nice an tight. When the truck is running (motor started) you push on the brakes and they hit the floor.

 

I have replaced every thing on the back brakes and still same results

 

At this point I would be looking at the master cylinder.
Again. when you are not bleeding the brakes....are you losing fluid every time you put your foot on the pedal and it goes to the floor. Perhaps do it a few times and recheck the fluid. If the fluid level hasn't moved....It is the seals in your master cylinder.

 

Could be the booster. It's vacuum operated. If the diaphram gives out, the pedal goes straight to the floor. Hear any hissing from under the dash?

 

Brake Performance Troubleshooting Hints

A brake pedal that pumps up with repeated use can be caused by air in the brake
hydraulic system, or if the vehicle is equipped with rear drum brakes, the brake adjusters may be seized or out of adjustment. A quick test for brake adjustment on vehicles with rear drum brakes is to pump the brake pedal several times and continue to apply pressure to the brake pedal. With pressure applied to the brake pedal, engage the parking brake. Release the breake pedal and quickly press the brake pedal again. If the brake pedal pumped up, the rear
brakes are in need of adjustment. Do not compensate for the rear brake adjustment by adjusting the parking brake. This will cause premature brake lining wear.
To test a vacuum brake booster, pump the brake pesal several times with the vehicle's
engine off. Apply pressure to the brake pedal and then start the engine. The brake
pedal should move downward about one inch(25 mm)

Having said that....if the pedal goes to the floor and the system is free of leaks, then replace the master cylinder. A bad brake booster will not cause the brake pedal to go to the floor.

 

Alrighty then.