Okay, I need some help here. last week my wheel cylinder was leaking so bad I could hardly keep fluid in the mastercylinder. I picked up a wheel cylinder, bled the line through the bleeder valve, and it was back to normal.

So, I drove it for a day and then went to drive and the pedal went to the ground, kinda spongy, and nowhere's near the brakes i had. I have figured that my master cylinder has been getting tired for a little while, the pedal has been going down a little ways before catching pressure. So I put a new master cyl in, bench bled it, and bled the lines. Still spongy. Took it back at part's guy's request and picked up another new one. bench bled it, still spongy. I bled the lines forever, and it still will go to the ground without stopping.

I vaguely remember something about a valve or some other step to take when bleeding brakes on these trucks, and I cant remember what it was. Am I forgetting something major? I'm not loosing fluid, so I doubt it's a leak. Any suggestions appreciated. The rig's a 76 F100 with power brakes and disks up front. THANKS!

 

https://www.ford-trucks.com/forums/s...hreadid=133360

 

There's a big difference between "spongy" and "goes to the floor without stopping".

"Spongy means that when you depress the brake pedal, it stops before hitting the floor but it fells mushy.

Going to the floor means it hits the cab floor without ever developing enough resistance to stop you from pressing it further.

Air in the lines will produce varying degrees of sponginess. It would take a lot of air in the line to make it so spongey that you go to the floor. In the thread suggested above, you'll note that the discussion is around "really firm" versus "reasonably firm" depending on whether or not you pull the prop-valve pin to bleed. Not "firm" versus "limp".

A brake pedal that goes to the floor without producing good brake pressure means a serious problem. Either you have tons and tons of air in the lines - which doesn't sound like if you've bled all the lines extensively. Or you have a bad MC - also unlikely since it's been swapped for new twice. Or you have a hole in a brake line somewhere, or a leak in the system somewhere.

Check all fittings. Are any leaking? What about the main line to the rear - it's hidden behind the frame in several points and may have a pinhole. The other wheel cylinder - maybe leaking inside the drum out of sight?

 

Get some line locks, and clamp them on all 4 rubber hoses leading to the wheels. is the pedal still mushy? if so, the problem is in the master or lines somewhere. If its high and hard, the problem will be at one of the wheels.
Then just remove one line clamp at a time, and compare how the pedal feels. if one of them feels lower or spongier than the others, you have found the source of the problem.

you also didnt mention if the shoes were contaminated with brake fluid or not, and if so, they NEED to be repaced as well. the friction material can swell up some, and also cause a crummy pedal, and worse yet, unpredictable locking up.