1999 GMC Sierra 1500 Brake Problem
 

I am posting this question for a friend.
The brakes fail randomly. They may work fine 10/15 times and then they fail completely, and then they work again.
The shop has replaced the ABS module, the pads, and rotors. The brakes have been bled several times, the fluid has been completely replaced.

The shop is a private shop, but the mechanic worked in a GM dealership for 11 years. He seems to know what he is doing, but he doesn't know why this is happening or how to fix it.

Thanks