I have a 1946 big bolt. When I got the truck the wheel cylinders were leaking bad with lot of brake fluid on the floor after sitting over night. No brake peddle to work with period.
I replaced all the wheel cylinders, and brake linning. I also rebuilt the master cylinder.
This was all done as soon as I got the truck some five years ago. I now have brakes enough to through a person through the wind shild and I did not touch the hydrovac. maybe I just got lucky but all is in working order.
The truck I have is a fire truck which has been housed most of it's entire life. maybe that adds to the life of the hydrovac or not I do not no.