Check to be sure there's a little free play between the brake pedal pushrod and the main master cylinder piston. If it's not able to depressurize after a stop, there will be just enough residual pressure to keep the hydrovac applied. We fixed a brake-locking problem on John's pumper fire truck that way at Homecoming one year.
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!