power brake booster check valve

I'm a little confused, maybe didn't read every word. The check valve should let you blow from booster towards intake manifold, but not the other way. Your check was thus sounds good, but you need try the other way too. Anyway, as told, at idle you have vaccum so don't require the check valve. That is just to hold vacuum for the future - full throttle or if the engine stalls. You can buy a new generic check valve in the "Help" section at auto parts if it proves bad.

A hard pedal is good, so not sure what you are concerned about. I didn't read that you have test-driven the car and found it hard to stop. True, that when the booster comes on, the brake pedal usually drops a bit, but I think that is because there is a little resilience in the system. Perhaps yours is really good (new hoses, no air bubbles).

I also use DOT 5 (silicone, not the 5.1 glycol rust stuff). People who never used it claims it is "spongy", but your hard pedal suggests not (my experience too). For kicks, read the silly claims that it actually causes more rust because when water drips into your brake reservoir it won't be absorbed. Perhaps those fools let water drip into their power steering reservoirs too, with similar results.