Vehicle is a 92 F-350 4x4 w/ rear anti lock. Brakes were working fine other than a front caliper was sticking. Purchased two rebuilt calipers and after bleeding the them noticed that the pedel would go to the floor slowly when I kept pressure on the brakes. After making sure there was no leaks I assumed a master cylinder problem. Have had similar problems in the past when an old master cylinder starts to bypass internaly after a broken line or bleeding. Purchased a rebuilt master cylinder and re bled the front and rear brakes at the wheels and still have the same problem. Have good pedal and decent brakes as long as it isnt pushed hard or for very long. If you hold pressure against the brakes with engine running the pedal slowly goes down 2/3 of the way to the floor and braking power goes away. Bad master cylinder? Can the rear ABS cause this?
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Still learning, please take it easy on me
Master cylinder is the only culprit I know of when the pedal fades. (other than a leak somewhere else of course, which would be visable and the fluid would be disappearing) For the money, buy new master cylinders. Not rebuilt ones. The cost difference isn't worth the worry of a rebuilt safety part.
My money is on the master. I have had several vehicles with the same issue. Like others have said...grab a new, not rebuilt maser cyl. Don't foreget to bleed the crap out of it, make sure you get all the air out.
To answer your question about RABS causing sinking pedal, yes it is possible. As the other guys point out it may not be your problem as it was working before you did the brake job. It does sound like you still have air in the system. Did you bleed the RABS valve block on the inside of the left side frame rail? If not give that a try. However there are two solenoid valves inside the RABS valve block, one of which opens to let fluid into a chamber against a spring loaded piston if the rear wheels lock. If this valve is faulty it can allow fluid to flow into the chamber every time pedal is pressed and thus the pedal slowly sinks. There is a large hex plug on the front of the RABS valve block. If you unscrew this (against spring pressure all the way) and remove the spring and then replace the plug, you can see if the RABS is the problem. If it makes no difference simply reinstall the spring.
