Can't seem to get the brakes bled, damnit

#1

I need some help on this one please. 1997 Dodge 1500 4x4, 152k, rear-wheel-only ABS, rear drums. Came to me with a low, spongy brake pedal and no fluid in the reservoir for the rear brakes, adequate fluid for the front disks. Searching for a leak in the system but have been unable to find a leaking line and the cylinders are dry. despite repeated attempts using gravity and vacumn I have been unable to get a firm pedal back.

According to Hanes no special bleeding procedures are necessary with the rear ABS configuration. Do I need to bleed the master cylinder? Any suggestions welcome.

TIA

#2

Bench-bleed the master and see if that helps.

#3

Is there a load-sensing proportioning valve? If so, it probably also needs to be bled if you haven’t already.

#4

The CAB (Controller, Anti-lock Brakes) opens the dump valve to the [brake fluid] accumulator for a few milliseconds when rear wheel lockup is impending. What if the dump valve stays open? Much fluid could be dumped to the accumulator. What is the capacity of the accumulator?
Here is the description of the RWAL (Rear Wheel Anti-Lock) brake system: http://www.autozone.com/addVehicleId,2337401/initialAction,repairGuide/shopping/repairGuide.htm?pageId=0900c152800c0d77

#5

Check for DTCs (Diagnostic Trouble Codes) in the CAB (Controller, Anti-lock Brakes) by following the instructions here: http://www.autozone.com/addVehicleId,2337401/initialAction,repairGuide/shopping/repairGuide.htm?pageId=0900c152800c0d7e