Confusing brake issue - need help

[doodah man]

Since it got cold out (< 50 degrees) I've noticed a strange thing with my brakes. Sometimes, after letting the truck warm up for a few minutes, I have to press much harder on the brakes to get them to stop the truck. This typically only happens the first time I need to stop. And it doesn't happen every time I get in the truck. Maybe one in ten times.

The only thing I've done on the brakes recently is change the front pads while I was doing the wheel bearings. The stopping problem didn't turn up until a month or two later.

I don't know what's going on. I tried pressing the brake pedal as far as possible before starting the engine and didn't notice any change once the motor was running. Does this indicate a bad vacuum booster or low vacuum?

Any advice is greatly appreciated.

Craig.