I was going to wait a few more days to post this update, but since I've driven 50-60 miles yesterday and today in all kinds of traffic/driving situations and I'm sitting here not doing anything, here goes.
I was able to bleed the ABS using Forscan Saturday. Wasn't happy with the way the brakes felt on a test drive, so I ran the ABS bleed 3 more times in succession. On a subsequent drive, the hammering was still there - at times even when I wasn't braking. I decided there must be something broken inside the rear drums. I had put off pulling the drums because I absolutely hate everything about working on drum brakes - getting the drums off, dealing with all the springs and various other pieces, and getting everything back together in the right order and in the right place. I've done a lot of repairs over the years, major, minor, and some I was totally clueless about, but I'd rather do any of them than replace drum brakes. I have yet to replace valve cover gaskets on a SOHC V6 (coming this summer) or the water pump on a 5.0, so one or both of those may prove to be worse, but until then working on drum brakes is the job I hate most.
FYI - the photos in the Haynes manual and the illustrations in the factory 2002 Sport Trac repair manual are of absolutely no help. The photos in the Haynes manual are of the earlier style, and the illustrations in the Ford repair manual are hard to read and not at all like what's on my truck - and no, I wasn't looking at the illustrations for the parking brake for the disc brake rear. Oddly, rear disc brakes are covered in the 2002 Sport Trac/Sport factory manual even though the '02 STs had rear drums.
I was correct, both about hating everything about the job, and that something was broken. Everything about the right rear fought me every step of the way - both coming apart and going back together. When I pulled the right drum off, I found what may have been the cause of the whole problem. The pin that holds the rear shoe to the backing plate was gone and the spring was rolling around inside the drum. Plus the pads, or what's left of them, had separated from the steel shoes. The front one was about 50% gone, and the rear one was about 40%, and what was left of both was able to slide around. That's almost certainly what the hammering was. If the separated pieces happened to be in somewhat the right place, the brakes worked reasonably well, but in the wrong place, they caused the hammering and low pedal/lack of brakes. That's my theory.
The left side was only half as maddening as the right. The shoes were intact, but beginning to separate, and were pretty slick. Also discovered the front portion of the backing plate is warped or bent. (The truck had been totaled and rebuilt before I bought it.) I don't think it affects the fit of the shoe, but there's a gap between the backing plate and the drum at the leading edge. Both drums were in surprisingly good condition considering everything.
After replacing the brake shoes and the right wheel cylinder, and bleeding the right rear to fill the new cylinder, the truck now stops with no hammering or other symptoms. The brakes begin to engage as soon as I press the pedal, but don't fully engage until the pedal's about halfway down. Obviously, the shoes need to be adjusted outward. If I brake hard, it stops well. There's also a bit of rotational grinding from the left rear. Probably just an imperfection in the shoe or drum that will soon go away. Another possibility is the backing plate has an inner lip that has rusted and was a little bent in a couple of spots. I "straightened" it and it could be rubbing against the drum. I'll check it out and adjust the shoes when time and decent weather come together. We can't get more than 3 decent weather days a week, and we've already had 2 this week - if 50 degrees with 20+ mph wind, and upper 40s and light wind count as decent weather in mid-April when it should be around 65-70. Anyway, I'm driving the truck!
