There should be a tag on the Hydrovac unit with the Bendix part number, what's the number?
My parts books show that the 55-59 Series 4, 5 & 6 (exc. HD Brake Booster) used the same Hydrovac.
Mike B

My mechanic has pulled off the Hydrovac and bypassed it for now. I brought it home yesterday. The only number I can find on it is on a brass tag attached to one of the bolts. That number is 378802.
On the end cover there is one of the tabs broken off where the long J-bolts go through that hold the end cover on. So I doubt this particular unit can be rebuilt, unless the rebuilder is able to repair that. So would probably need a source for a new one.
Also, I have not yet driven the truck since the Hydrovac has been removed, since it's still in the shop. So the jury is still out on whether to replace it at all. But my gut is telling me that I probably should.