Loss of Brake Power Assist after Start

Mazda Protege5
The cold weather has caused my 2010 M6 to lose power assist on the brakes after starting the car. Everything is okay when backing out of the driveway, but then power assist seems to go away for a couple of minutes. It happened once over a year ago and twice yesterday. Obviously, a vacuum leak somewhere in the brake booster system. I'm thinking possibly the check valve is leaking intermittently. It looks like there might be one at the end of the vacuum hose that goes into brake booster. Of course, you have to buy the entire hose. So, before I buy the hose, I want to verify that there is a check valve on the hose.