I have been looking into Hydrovac retrofits recently (for a Canadian military Chevrolet but a Hydrovac should work the same no matter what it is hung on). One of the common problems seems to be that people retrofit a Hydrovac (containing a check valve) onto a vehicle that has a check valve included in the master cylinder. The two check valves in series typically cause the brakes to lock on. The standard solution given is to either swap out the master for an equivalent master without the check valve or to remove the check valve from the existing master cylinder. Another thing to be aware of if you don't trust the previous owner's maintenance skills was that Bendix published guidelines for the bore and stroke that master cylinders should have for the various Hydrovacs depending on the weight of truck they were installed in (and how many wheel cylinders were involved).