Hydrovac is a brake power booster that is operated by a remote master cylinder. They work well as long as the brake system is in good condition. They do not usually survive if the brake system is allowed to sit empty for any period of time.
Many, many trucks have used Hydrovac. C50, C60, C70 trucks in '70s and '80s used them. They can be rebuilt but IMO if you don't like working with small parts its better to buy a reman. Both Midland and Bendix made Hydrovac boosters. There are also different styles of Hydrovac.
Hydrovacs can become badly corroded internally. This makes rebuilding challenging. If you are just looking for a replacement, maybe consider getting a unit that's already rebuilt and submitting your as a core.
https://www.powerbrake-drivetrain.com/hydrovacs/
This unit is listed for a '62 C50 and may work for your truck also?
https://shopbumpertobumper.com/en/p/.../c50/1962/6261