Well if I was going to offer an educated guess at this point, I would suspect the master cylinder. The braking system is fairly simple actually and if pressure is being released it has to be something like air, fluid compromise or delivery malfunction.
I did not think of your ABS, it is possible the ABS is sending signal to indicate the wheels are locked up and thus releasing the brake pressure from the system? Normally ABS stop working and no signal is transmitted and thus the brake pressure is not release, in this case, perhaps the opposite is happening.
Last edited by Raider4444; 06-01-2016 at 02:15 PM.
Reason: Add information