After dealing with brake issues for the past year, I finally decided to bypass the RABS unit. In the last year I've replaced the following:

Motorcraft master cylinder
2 Motorcraft zero loss boosters (the F5TZ-2005-CA one, 1st one went bad after 6 months, replaced under warranty)
Vacuum pump
3 (yes, 3) A1 Cardone remanufactured RABS units (all have failed withing a few weeks and another 1 bad out of box, all replaced under warranty. It's the ONLY one I can find anywhere)
All new brake pads, rotors, drums, shoes, and hardware
I even bought a used OEM RABS unit on ebay from another 94.5 7.3 but it had a different connector on it so I returned it

After all these headaches I had a shop build some lines and bypassed the RABS unit entirely (and at least got $230 back). Predictably, I can now lock up the rear wheels under moderate to hard braking. The RABS also acts as a proportioning valve from my understanding. Beats not being able to stop at all like before, but come winter time this won't be good.

I live at 7400 ft and travel up/down a hilly mountain highway daily from 4400 ft to 7400 ft. Before winter hits I'd like to put in an adjustable brake proportioning valve so that I can turn down the rear bias so I don't end up in a snowbank when easily locking up the rears on slick roads.

Does anyone have experience with this? How would it be installed on the truck?

 

I bypassed mine and the brakes do not lock up. I backed them off a bit though as I felt they were dragging.

 

I know the older 350's had a proportioning valve that was connected to the frame and rear axle. It would vary fluid with how loaded the rear end was. I guess if you got one of those you might be able to make it work.

Red