air pack - brake failure m275 reo LD-465 multifuel

did anybody do a rebuild of the hydrovac brake system used on the reo m35-m275 ? the master cilinder is ok, the vent hole is open, pedal freeplay is ok, pedal return spring is present, filled with silicone brake fluid...still when building up pressure all brakes lock on, when shutting down you hear inside the hydrovac clearly a noise that sounds like when you hold an air hose under water and open it, so i think the problem must be in there, i open one gladhand on the rear so there 's almost no air and than i'm able to move it back and forward but with or without air pressure the brake pedal is super hard (it does brake a bit when pressing the pedal )
I have the older model air pack (the long one) with the brake light switch screwed in on the front, since there's air leaking into the air pack somehow i assume it passes trough the valve in the side of the air pack about halfway (between the cilinder and front end of the pack)

the buzzer is also connected again , and it works just fine, i'm not sure for what you should wear hearing protection in that truck, must be for that little buzzer and it made sense why it was disconnected

thanks

Yes, rebuild kits are out there. Should be a write up or two in the deuce sticky area.