welcome poboy. i have been going down this same road. i have 59 Spartan that was a pumper. i took all of the fire apparatus off and now is a cab and chassis. i bought a rebuilt hydrovac from O'Reilly. i don't have the part number but posted it on here a couple of months ago. you may be able to find that thread. i would like to know where you found the master cylinder as i have not been able to although i have not looked lately.
Gene


1959 Chevy 8500 - Spartan 80
366 BBC