Power brake problem

Hi... I have a '93 F150 supercab w/ 302, automatic, and 4x4.

Heres the scenario. The ambient temperature recently dropped around here as we are going into winter. Anyways I went to start the truck and found out the brakes werent working correctly. I have great pedal... but the brakes arent really engaging..... after running the truck for 10-15 min they started working correctly. Also I heard a slight hissing noise when i steped on them. But that noise went away when they started working again. It has done it three consecutive occasions now when it is cold.

Is it possible for the vaccuum booster to freeze? If not do you have any other ideas for what it might be?

Possibly vacuum hose to booster is brittle when cold.

i agree vacuum unless there's a small hole in brake booster....

GOOD LUCK....

I see your truck is 4wd, do you go mudding alot? I used to live in an area where it got pretty cold, and i not only liked mudding, but had to do it with the work i was doing. My truck was constantly in mud and water over the axles, and mud would cake up around the calipers and in the brake drums on the rear. When it got cold enuff for the mud to freeze, i (and alot of other people) had brake problems. Had to take it to the car was pretty regular, and pull the drums and clean the mud from around the shoes. This may not be your problem, but its a possibility if you go mudding. Also, brake fluid and water readily mix, and it will freeze if it gets enuff water in it. If youve had to add fluid, or have changed a master cyl....... make sure your fluid is clean and fresh. Ive seen people accidentally leave the top off a can and get rain in it, then add some and have problems. Most cold weather brake problems are moisture related in some way.

I figured it out thanks for the help everyone.... as it turns out the checkvalve fitting that connects the vaccuum booster to the vaccuum hose was worn out. I swapped it with the one out of my other truck and it works fine now.