I have some issues with a 1994 F350 4X4 with a 7.3 diesel engine. I recently got involved with a brake problem that several other people have been unable to fix. The brake pedal sinks to the floor when you push down with moderate pressure. It will stop the vehicle but setting at a stop light with your foot on the pedal you can feel it slowly sink down. I have bled the system starting from the master cylinder to the right rear wheel then left rear then RABS unit then right front and ending up at the left front. Before I got the truck the master cylinder has been replaced twice with the same results. If you remove the vac. line to the booster you can not feel the pedal sink. It is very firm which is understandable but if you drive the vehicle once you overcome the lack of power assist it will stop the truck and not sink down. Then someone else suggested that the RABS unit may be bad causing the problem. It does feel as if the rear brakes are not doing their fair share of work during the braking process so this does make me wonder about the unit itself. Neither light on the dash are lit (brake or ABS) even if you apply enough pressure to make the pedal sink to the floor and the pedal will not get firm if you pump the pedal up so I do not believe there is any air in the system I performed a search of several sites without much luck so I turn to you for any help you may be able to provide. Thank you for your time. Joe

 

Have you checked all the springs on the Shoes? I would assume that if they are worn this could happen. Just an Uneducated guess.

Chris

 

Yes I replaced the front and rear brakes and put in all new hold down hardware and return springs. I did not replace the wheel cylinders though because they were not leaking and the pistons retracted with no problem.

 

Make sure your rear shoes are adjusted up pretty close. I got in a hurry when putting new brakes on my F250 and figured the self adjusters would take care of it, but I barely had enough pedal to stop and it felt soft. Re-adjusted the shoes and all was fine.

 

Yesterday I readjusted the rear shoes very tight. Tighter than I usually put rear shoes. It made a improvement in the stopping but the pedal still sinks when steady pressure is applied to the pedal so I know there still is a problem.

 

Are you sure all the air is out? All it takes is one good bubble in the system to do this. You might want to invest in this positive pressure bleeder by Motive products. Its about $40 but it is well worth it. It is better than than having a pedal pusher because they can run the piston in an area where it would normally not travel. This can be corroded on an older truck and mess up the seals. I would bleed them again.

Don

 

You may have a piston simply bypassing. If there is no air in the system and no leaks it most likely is bypassing. Have you checked/changed the proportioning valve?

 

What you are experiencing may be natural for a Ford. Some time ago there was a post on another site with someone who experienced the sinking brake pedal when stopped and someone replied that was natural for a Ford. I didn't believe it, so I tried it with my 88 F150 and sure enough it does it. Try this with someone elses Ford truck. The only other thing I can think of is one of your rear wheel cylinders has started leaking, but it would have to leak externally and you would be losing brake fluid. Keep us posted.

 

Thank you to all who replied. I found a TSB which deals with this problem. I also discussed it with a shop that has much experience with this problem. Even the NAPA catalog had a footnote about it. The problem seems to be corrected by installing a master cylinder for a F450 truck. I ordered one and will hopefully get to install it next weekend. The shop also told me that one other thing could be causing the problem but I should install the master cylinder first and that is the RABS unit mounted in the frame rail. They said that it is common for them to get dirt inside and cause a valve to allow flluid to bypass and cause the pedal to sink. So I will hopefully post the solution next weekend.
I would be interested in a positive bleeder that dstevenson mentioned. I live in central Illinois do you know where I might find one or who sells them? Thanks again to those of you who took the time to help me. Joe

 

I am pleased to report that I installed the larger master cylinder this weekend and it DID fix the problem.

 

Joe need to know did you have to buy the booster and master cylinder for the same year truck you had ???