in reply to my post f-350 brakes they are working great now. to catch up read old post. my conclusion is that the seal between the brake booster and the firewall is made out of a thin layer of foam and from time and heat it broke down. water from rain, snow, washing ect. running down the firewall while the engine was running sucked into the booster. while the weather was warm it was good. so when i replaced the first booster ( factory part) with a rebuilt unit it worked good and no more thought about it. it was spring time. next fall same problem. went to replace with another rebuilt and the clerk told me to check my vacuum line cause the booster had alot of water in it. so i cleaned the old gasket off and siliconed the dust cover to the booster and the booster to the firewall. now a month or so into the cold and everything is good. so with any luck problem solved. also, thanks tom for all your suggestions. this is one to remember, so easy but it will fool ya.
Thank you scooter-d for posting the fix. After finding water where it doesn't belong and why this problem should be solved. It's worth archiving as surely this can happen but isn't something most would be looking for,
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