Cylinders
Dav,
Yes several of the better quality cylinders have quick-release fittings at the top so you can pump up then release the tube/pump and maintain the vacuum level.
There are tons of places to buy cylinders, but some of the ones I see mentioned most here are Boston Pumpworks, LA Pumpworks, NW Pumpworks, VacuTech, Newart etc.
I’m not sure about the hydro pumps, maybe someone else can answer that question and/or give you some more resources for quality cylinders that have quick-release valves etc.
Just do a search with Google (or your favorite search engine) using those names and you should be able to find the respective website for the cylinders.
LizardMan