Hey all, first post here. I just recently purchased a 1992 F350 DRW 7.3 IDI 4x2. Discs on the front, Drums on the rear. Got in the truck a couple weeks ago and pedal went straight to the floor upon startup. There was brake fluid running down the RR wheel.

When taking the drum apart, it looked like the RR wheel cylinder had busted. I replaced the RR wheel cylinder along with springs for both the RR and LR brakes.

I'll try to establish my steps and bleeding techniques clearly:
1st attempt (New RR Wheel Cylinder, New Springs on LR and RR Brake Drums): Friend pumps the brakes and holds. I crack the bleeder. He calls floor and then I close it. I was using just a socket on the bleeder valve (no clear tube). When bleeding the LR, there was constantly air. Thought it was odd, but decided to replace the LR wheel cylinder.
2nd attempt (New LR Wheel Cylinder): Friend pumps the brakes and holds. I crack the bleeder. He calls floor and I close it. Using a socket on the bleeder valve (no clear tube). Same result. Air every time out of the LR only. We used over 2 quarts of brake fluid trying to bleed just this wheel and it never got better. We also tried bleeding it with no brake pumping prior to cracking the valve. Same result. Read some stuff online and decided to replace the Master Cylinder
3rd attempt (New MC): I bench bled the MC before installation. Friend pumps the brakes and holds. I crack the bleeder. He calls floor and I close it. Using a wrench on the bleeder valve and a clear plastic tube leading into a container of brake fluid. The ensures no air is being sucked back into the wheel cylinder. I can see the bubbles in the tube. Again, out of the LR only. They are very very small and hundreds of them. You would only see them with good lighting. Read some more stuff online and decided to pull the spring from the accumulator in the RABS and bleed at the MC
4th attempt (Spring pulled from RABS accumulator): Friend pumps and holds. I crack the rear brake line at the Master Cylinder. Air does come out for the first few and then ceases. I return to the RR. No air as usual. Return to the LR and there are still tiny bubbles every time we crack it. We tried with engine on, engine off, brakes pumped, and brakes not pumped. Same result.

I think it's less important to say this, but the pedal is firm with engine off and then slowly sinks to the floor with the engine on.
I do not see any visible leaks anywhere. I've gone through 6 quarts of brake fluid.

As you can tell, I'm a bit lost at this point. These are my next steps:
1. Bypass RABS just to rule it out
2. Plug the LR feed at the T-fitting in the rear to see if I can get a firm pedal. Again, trying to rule something out.

I understand it could be a defective new MC. I've read posts about that online. But after I installed the MC in vehicle with the plugs installed I pressed the pedal and it was very firm. No sink at all. So I think that rules out the internal seals.

I have bled in sequence several times (RR, LR, RABS, FR, FL) with no luck.
I also have tightened the self-adjusting screws after bleeding. But there is still air coming out of the LR. That is definitely when I need to fix.

Has anyone experienced this problem and found a solution? I've scoured the forums for days, but haven't found anything the solved this problem specifically.
Any help is appreciated. Thank you

 

I have been involved with my same set up and have done a lot of research. Usually those very tiny bubbles are caused by a poor hose fitting on the bleeder. Is the same tool you use to loosen the bleeder with bubbles the same size as the others? If no this could easily happen. Try putting the hose on and put a tie wrap around it to better secures it. Then use a wrench when you loosen the bleeder. You can not do this process using a socket.

Process is You say DOWN. He puts pedal down. He says DOWN . He keeps pedal down and you loosen bleeder a turn. When fluid stops you tighten bleeder and tell him UP. If you don't tighten the bleeder first before he releases the pedal you will suck air back in. Repeat until NOOOO bubbles. Your sequence is the correct order of wheels. After each wheel is done have assistant top off Master cylinder with FRESH fluid. Your bleeder hose must be submerged in fluid in a bottle with a small hole in it for the hose and a smaller one to let air escape. Master cap should be loose when doing this. See if that gets those bubbles out. Sandy

 

Check the connections on the MC, wheel cylinders,etc and make sure they are tight and do like Sandy suggested, it may take a few times bleeding to get it all out. I got frustrated at never having a 2nd person around to pump the brakes so I installed speed bleeders on all four corners and ran a rubber tube into a jar that had enough fluid to cover the end of the hose. Then I loosened bleeder,pumped 4 times,check MC fluid,pump,check,pump,check til i completely flushed the system on all 4 corners. worked like a charm. For the RABS unit I just gravity bled it into the jar.