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. 

 

Only way the brakes would be overheating is if they are binding on - you would smell them after stopping after a long run and your fuel consumption would be impacted severely also. 

If the pedal is creeping down I would take this to be a leakback on the master cylinder. While on the m-way you will be using the brakes less than if you were pottering around during a short journey.  

 

Phillip.

As Tom said, hot brakes would only be caused if they were binding, and that's easy to diagnose: if one or more wheels feel unusually hot (more than body temperature) after a journey then the brakes are binding. Does the car pull to one side under braking? That's another symptom of a brake binding.

I suspect your problem is more likely to be defective master cylinder seals or water contaminated brake fluid.