Brake Booster issue?

A missing check valve isn't your problem since the check valve is stop air flowing the other way (into booster), but you do need one so the booster can store vacuum in case the engine dies and you need to stop. First, insure your pedal is coming up all the way. It shouldn't bottom out on the brake lamp sense switch. Amazing how many people don't understand that, and the switch is adjustable. If the brake pedal were pushing in a bit, then it would leak air since that is what the air-valve does. If the pedal is all the way out, then sounds like the booster is bad - either a bad air-valve or a torn diaphragm.