power brake booster check valve

I'm testing my check valve because of a high hard brake pedal on my 1968 Dodge Dart GTS 340. I understand that if I blow through the check valve from the booster side that if air goes through to the hose side that the check valve is bad. That's what's happening with mine and I'll assume that's correct so I'm looking for a new check valve. Anybody know where I can get one that'll work?