Brake pedal on the floor.... ?????

hey guys got a weird one...
56 chevy with a power disc brake conversion on the front, drums out back.
everything on the car was normal and as i was driving, the brake pedal went straight to the floor. bam. just like that. luckily i managed to get it it back into the lot and it was to the floor. had just enough to hold the car so i could pull it into the garage but definitely couldn't drive it safely.

pulled a visual inspection and there are no leaks, ruptured hoses or brake hydraulics. master cylinder is full. pulled the master and its not leaking the cylinder in the booster is stroking... everything APPEARS normal... figgered it might be a bum master cylinder. swapped in a new master cylinder and bled it out. ran the car up and down the driveway and BAM happened again... hmmmm...

good chance we got a bad master right out of the box, because its acting like a bad master, and obviously something is going on here, but i've been looking at it for two days and could use a fresh perspective. any ideas?

 

could it be a problem between the pedal and the cylinder?
if there is no loss of fluid you may have broken something in your linkage.
(hard to see maybe right near the firewall?)

Stopping is an important part of the car driving experience.


Good luck

 

So you're starting out with good pedal and after a couple of applications it goes to the floor? Does the pedal stay down (sounds like it from your post)?

 

I had a Chevy van do that once, lost all pedal, no sign of a leak. Turned out the brake line that ran across the front cross-member had leaked and the fluid stayed in a channel and didn't drip on the floor at all.

 

Have you checked the vacuum lines?

 

Low vacuum would mean the pedal would simply be harder to depress.

 

Sounds to me like a very stubborn air bubble that won't bleed out.Did you bench bleed the master before you put it into the system?

 

i have disc brakes in front on my customline... when i turn hard to the left or all the way to the right... it does the same thing... pump em a few times and theyre back to normal??? you said yours stayed to the floor?

 

Try to bleed the brakes again, or just pump the brakes like crazy and hope what ever air bubble might be in there, gets out.

 

What method are you using when you bleed the brakes?

 

It's got to be a bad master, bypassing the fluid back into the reservoir.

 

You already replaced the master cylinder but just for kicks the next time it looses the pedal remove the cover and see if the fluid level is going down in one reservoir and overfilling the other reservoir. That is how you can check for an internal seal failure.

 

You need to make sure that the master cylinder is for the disc brake set up. If you run a drum brake master on a disc brake set up it will act very weird. Jegs sells a universal non power assist disc brake master...

 

Rear brakes out of adjustment.

 

On a dual circuit master, that would only let the pedal go 1/2 way to the floor.

 

You said you had a good pedal for a while before it suddenly went to the floor so that eliminates trapped air or brake shoe adjustment. Air doesn't materialize and shoes don't go out of adjustment that fast. both would be possibilities if you had to pump the pedal to get it up.

The only thing that makes any sense is a defective M/cyl. going by what you have said. I have received defective M/cyls from the parts stores before. It's rare but it has happened to me at my shop. Some times the internal seals let the fluid go past the piston and the pedal goes to the floor. I have seen it as an intermittent problem. Sometimes the seals seal and sometimes they don't.

Once you've got the air out and the shoes adjusted providing a consistent high and hard pedal, If the fluid level stays up and there are no leaks and suddenly the pedal goes to the floor then there is no other answer...you have a defective M/cyl. letting fluid get past the internal piston.

 

i had a 73 chev pick- up do that...it was the booster..happened in a friends driveway,all the way to the floor.

 

If you bench bled the M/C you may have pushed the piston too far as you were bleeding it, an ruptured the seal!
Good Luck