Before I sent the booster in, I connected the booster only in my hand to a different vacume source from my daily driver. When I pushed on the rod with vacumme, I could push it easy. When they rebuilt the one I sent them, they said it wasn't bad, but we decieded that it has 50 year old rubber parts in it, so it was good to do. They sent a new check valve with it and I checked it by sucking and blowing on it. The hard pedal was there before I had the master cylinder replaced and booster rebuilt.