I've done a lot of searching and haven't seen anyone else with this problem so any help would be appreciated.

About once a month when i first start it i get a hard brake pedal like there is no power brakes then they come back after a about a minute. Sounds like a brake booster or check valve? the weird thing is that a few days ago i was up at altitude (6000ft) and over night it was down in the 40's, i had the same problem but after running it for about 10 minutes it never came back. i turned the engine off and restarted it and within seconds i had power back to the brakes. what do you think? bad check valve or booster? or??

thanks