The upstream master cylinder (the one operated by the brake pedal) must have some free travel before it begins pushing fluid to the hydrovac. John's fire truck had the same symptoms at the homecoming in 2017, and we fixed it by shortening the pushrod between the brake pedal and the firewall mounted MC. There's a bleeder port that must be uncovered each time the pedal is released, or it keeps a little pressure on the shuttle valve to the hydrovac. That causes the brakes to drag after a few applications.
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
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