There is a floating piston in the combination valve that isolates one end of the vehicle in case of a pressure loss. Some people remove the electrical connector in the center of the valve and lock that piston into place with a screw device while bleeding. Anytime there's a pressure difference front/rear (including bleeding) that valve is prone to move.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!