If you decide to rebuild the shuttle valve be sure to get the correct kit. I glanced at the Muscle Car Research website and the kit for a Pantera shuttle valve was not apparent to me. Installing the wrong o-rings could result in a leaky valve, an inoperable shuttle switch, and brakes that don't work correctly.
Eugen, did you notice if the brake warning light on the dash illuminated when the front brakes failed?
I got curious and just now tested my shuttle valve switch for correct operation. I put a plug on the output of the master for the front brakes, turned the ignition to RUN (engine not started) and stepped firmly on the brake pedal. The warning light (aka I am going to crash light) illuminated and I heard a small clunk at same time. The clunk is the piston moving and actuating the switch. I removed the plug , reconnected the front brake line, and pressed on the pedal again. I heard another small clunk and the light turned off. The shuttle valve and warning light switch on this car is working correctly.
The photos below show the front brakes plugged off and the warning light illuminated.
