I was braking for the exit from the interstate when It felt like the set was pushing the tractor. I was going over the possibilities of why it felt like I didn't have brakes on the set and for the next hundred miles was very careful about my speed. I knew that the problem was in the blue line but what was it? I had just bled the air from the tanks, so water accumulation was out. I felt that the situation was manageable so I rolled it back to the barn. I get back to the yard, dock my lead trailer, and unhook the airlines. I had the blue glad hand in my hand, so I turned it over and there was the problem. The rubber grommet looked like someone had cut the edge off of the grommet so air was leaking every time the brakes were applied. I guess I am getting old but it never occurred to me to check the grommet. Oh well.