The question should be imediately off the highway do they work fine. A) Pump them up and then they are normal until next time out on the Highway? Or B) does the problem linger for a while and then go away?
A) Air in the lines is heating up, expanding and pushing the fliud back into the MC.
B)The MC is heating up and the piston is leaking.
My bet is still on "B".
