Nissan Repair Questions? Ask a Mechanic for Answers ASAP
Does the truck have ABS?
Never mind, I looked it up for 93 it looks like you had rear ABS only. On the passenger side frame rail you will find the ABS modulator. On it there is a bleeder valve. You need to bleed it down also as it sounds like you have air in there that is entering the rest of the system when the modulator self checks (when you turn the truck on). This causes the pedal to drop.
Okay. Unplug that modulator and see if the brakes stay up. If they do the modulator has failed.