Nissan Repair: ABS problem on 99 quest?, long periods of time, rear cylinders

Question
Ok Von, here you go: I have a 99 quest. When I drive for long periods of time (10 minutes to 1 hour) without touching the brake, when I finally do hit the brakes, the pedal goes down. The longer I have driven w/o touching it, the more it goes down. Pump it once or twice and its fine. Once its pumped up, it doesn't fade.  Here's what I've done/eliminated: replaced the master cylinder twice, bleed the brakes numerous times, new rotors and drums,  new pads and shoes, new rear cylinders, had the brakes adjusted front to back twice, checked to make sure the auto adjusting units weren't on backwards. Now a local mechanic tells me he's certain its the ABS unit - says it has an internal leak or something.  He says the way he knows this is by driving the car up to speak, slamming on the brakes and getting the car to skid, - thus engaging the ABS unit. After that's done, the car has great brakes for a few days..then back to the same old thing. 1. Is it the ABS unit (modulator valve?)? 2. Can I get a reliable used one and if so where? I don't see them on the internet, 3. I've changed master cylinders to transfer cases myself..think I can do it and save some money?
Thanks, Eric_Lowdermilk@pba.edu

Answer
Eric,

I hate to say the mechanic is correct as the Quest is known for this problem and the ABS unit is quite expensive if you can find one rebuilt or new.  I would source one from a wrecking yard with the lowest mileage possible.  Yes, you can do it yourself and bleed the brakes.  Also, use Ford High Performance Brake Fluid to do the bleeding.  It is DOT3 so it will not screw up your brake system.  But, you will have a much firmer pedal.