49TT - Your hydrovac should be a Bendix 374000 that is about 15" long overall. Same as the hydrovacs for all 1946-57 "Utility" trucks (except those with the H.D. brake booster). That's according to the1929-57 Chevrolet Master Parts book (excerpt below). Also attached below from the Master Parts book is an exploded diagram of the unit. Go check out the 1948-51 Chevrolet Truck Shop Manual, starting on
Page 5-29 for info on repair and replacement info.
If ypu do send off your hydrovac to "Then and Now Rebuilders", send them your master cylinder too. They can do both.
