After taking measurement of my hydrovac and comparing the measurements of the 2 units above; the CARDONE 51-8028 is the unit that will replace the hydrovac in my 55 Chevy 2 ton.
As mentioned above by 2ManyTrucks. Model 3740000 hydrovac brake booster is also a number used to identify remans etc.. for this same unit.
I am very lucky to have found a re manufactured unit at thewrenchmonkey.ca. The price is higher than rockauto pricing although they have it in stock and ready to ship.
Thanks again guys
