I have an '87 F-250 Supercab 4X4 7.5 LTR carbureted pu. Problem is the brakes are very weak, spongy, fade, and the RABS light is on constant. It was on intermittently, at first, but steady now. Has been that way for quite a while. It has aftermarket trailer brakes control. Was on it when I bought it and works fine. I don't think that is a problem. The master cylinder was bad, rusty and leaking, so I replaced it. Yes I bled the master cylinder first. You can get pedal if you pump them. I was able to locate the RABS module behind the ashtray. I unplugged the connector. It's clean. The light is out with connnector unplugged (yes, duh, it would be!) I got a printout of the RABS diagnostic connector location but have not been able to locate it. Shows it just to the left of the steering colum on the wiring harness near the firewall. I looked all over and can't see it. If I knew exactly where it is and located it I could pull codes. Also I need to know where the RABS control unit is underneath. I assume it's on the left frame rail somewhere below the cab. I did a quick look and can't find it either. I'm sure it's time to do the brakes anyway because the fronts are rattling. So I need to tear into it to fix that anyway. And, my intuition tells me the brakes need bled as well. By the way I was a ASE master tech, mostly on MOPARS, some Ford, all dealership, until 1999. 
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