piplowe

Hi, My brakes work perfectly day to day on short journeys, rarely above 50 mph but if I take it on a long motorway journey the pedal starts to travel to the floor after about 40 mins. The brakes still work but not ideal, If I come off the motorway and keep driving the brakes slowly return to normal. I'm assuming this could be the brake fluid getting hot due to friction in the brakes and the viscosity of the fluid changing. Any thoughts Thanks in advance.