This is an issue a friend sent me, let me know if anyone has any ideas:

GMA Rear brake system:

"...sometimes the pressure just goes away. Happens suddenly too ... no warning, no slight drop off, etc ... they're just gone all of a sudden. Sometimes, after use, they won't completely release the disc..."

He has had several people work on it, and the problem remains. Can anyone help?
This is on a late model custom bike.
Thanks!