got in my truck yesterday, temp in the lower 30's. cranked it up and as i was applying the break, the pressure cause a "hollow, echoing" sound each time i pressed the brake pedal. I was careful to make sure it didn't effect my braking ability, which it did not. So i drove off and after warmed up in cab about 10 miles later, the brake pressure did not make the sound. got in it this am and temp in lower 40s upper 30's, it makes the sound. Well, what do ya think out there????