Ford - Brakes - Gasoline - F150 :: 1989 - Both Rear Brakes Stuck Occasionally

I recently bought a beater '89 F-150 (I-6, MT) with the following problems:

1) Both rear brakes seem to stick on occasion--almost like there was a "check valve" on the rear brake line. When that happens, there is a "BRAKE" and "Rear ABS" light (although the light does occasionally come on without brakes sticking noticeably).

What I noticed in Haynes is that the "RABS I" system functions in the following way: a) Upon noticing rear axle spinning slower than front axle, it actuates a valve, isolating the rear brakes b) If the problem persists, it then dumps the pressure back into the main system.

So, I wonder: Is this an ABS system acting up? If I were to (temporarily) de-power the ABS setup, would it default to "no ABS," or would I need to remove the hydraulic valves from the brake lines? (Don't really want to if I don't have to.)

2) You can select either fuel tank, get that tank's level to indicate on the fuel gauge...yet it only "draws" from the front tank.

Again, Haynes says there is a low pressure pump in each tank, feeding a high-pressure pump in essentially a small "header" tank. The "dual-function reservior" has a wholly mechanical selector, and notes that "Tank switching versus gauge conflicts can occur under certain failure mode conditions."

So: I can understand that the mechanical selector can, in effect, "stick" on one tank. But what I don't understand is how it can still draw from the front tank, with the "rear" tank selected (and presumably, that tank's low-pressure pump trying to feed the header tank.)

Is it possible that the low pressure pump--while beneficial to prevent gravitation at the main pump--isn't strictly necessary to fuel the engine? Or do I have a problem with a low pressure pump that's "always on," regardless of the selected tank?