Have a 2003 last week it started devolved an interesting problem with the breaks. While driving the break peddle will get very stiff and when I apply the brakes the truck will stop with the peddle going down about a tenth of an inch when stopped I can press down hard and the peddle will go straight to the floor and a small popping sound comes from the master cylinder. The time I start the truck it will go back to normal for a couple of hours. I checked to see if there were any vacuum leeks and I could not find any. To me it sounds like the master cylinder but I am not sure and I don’t plan to replace it unless a need to, given the price. Any ideas?