Brake boosters do fail occasionally. My dual diaphragm S14 booster failed several months ago. It started running lean and adding fuel to the trims whenever I stepped on the brake pedal. Eventually it failed to the point where I lost all power brake assist and the engine would stall out when I hit the brake pedal.
You should definitely install the check valve in the booster line. It's intended to keep a small safety supply of vacuum in the booster in the event of an engine stallout. It will also keep positive boost pressure from entering the booster.
Here's what happened to my booster. The rubber diaphragm tore and created a huge air leak.
