ABS Problem

I have a 1993 Nissan Maxima SE, 3.0 liter double OHC engine, equipped with ABS, and 160,000 miles on the odometer that I've owned since new. After storing the car for about a year, I discovered that the brake pedal drops to the floor when the engine starts. I installed new fluid and bled the brake lines, replaced the master cylinder, replaced the brake booster, and installed another actuator & pump assembly acquired from a salvage yard. The problem still exists. I've discovered that if I remove the two ABS fuses, there is a partial brake pedal that slowly disappears after having driven for some distance or if I disconnect the vacuum to the booster there is a high pedal. But, without power assist, it is very hard to brake.

Logically, it seems that the ABS pump is not working. Does this mean the ABS sending unit is bad or are there other fuses or, possibly, relays that I'm not aware of? How can I check for trouble codes and what tool do I need?

I'm hesitant to drive to a Nissan dealer for two reasons: 1) it's dangerous to drive with little or no brakes; and, 2) without some idea as to the problem and solution, I won't know if their diagnosis is correct. Frankly, if there are fittings available to bypass the actuator assembly, I'd just as soon do away with the ABS function altogether.