96 GMC K2500 Brake Problem

I have a 96 GMC K 2500 Ext cab short bed, OD AT, 4wd w/ 5.7, all wheel ABS, 125k miles, Brakes are GM option code JB7. The brake system keeps developing air in the lines with no obvious leaks. The master cylinder has been replaced as well as all of the lines on the rear diff. The ABS unit has been disable because it is engaging for no apparent reason when coming to a stop ( I believe that I need to replace speed sensors but I am trying to isolate this problem first). The interim solution is to bleed the brakes at the passenger rear tire up to twice resulting in a temporary resolution. Disc brakes on front, drums rear. Any help would be greatly appreciated.