So I just noticed this issue this weekend. I've had the truck about 2 weeks. I've racked my brain and can't pinpoint the issue, so on to the problem and hopefully a solution: when the engine is running and in park, I can pump the brake pedal and it'll stiffen up as if the engine is off. If I rev the engine and keep the pedal pressed, I can feel the pedal slowly start to soften. When I'm driving and doing normal stuff, all is well. It's only at idle, even in drive, but I have to pump the brakes a few times to stiffen them up. Then I drive or rev the engine for a few moments, and it's back to a normal pedal. Vacuum line? Booster? Thanks in advance!