Can you take and post pictures of the rear of the Hydrovac canister body? From what I've read in the shop manuals, there seems there should be an oil filler plug on the back side of the main canister that is where the oil should be placed. The 1948-51 Chevy Truck Shop Manual (see below) in the rebuild reassembly instructions for the Hydrovac says to fill to the level of the filler plug. It does not indicate a capacity there.
In the Lubrication information portion of the shop manual (see second pic below), the portion covering the Hydrovac says "One ounce of Bendix Vacuum Cylinder Oil should be added every 10,000 mile intervals or each six month period...".
My guess since you just reassembled it after rebuild would be to fill to the level of bottom of the filler plug, then maintain based on mileage/time as noted in routine service.
On the type of oil to use, many recommendations I read about to replace "Bendix Vacuum Cylinder Oil" which is no longer available, is to use "Neatsfoot Oil". It's formulated to be used on leather products, so should work for your new components.
~ Dan 1951 Chevy 3 window 3100 Follow this story in the DITY Gallery "My Grandpa Carl's Truck and How it Became Mine" 1966 Chevelle (Wife's Hot Rod) | 2013 Chevy Silverado (Current daily driver) US Army MSG Retired (1977-1998) | Com Fac Maint Lead Tech Retired (1998-2021)
