Brake problem

[Dimbers]

Just had some xp8s fitted and changed the fluid to rbf600. Took the car out to bed them in and all was going well, got some good heat into them before taking it easy to cool them down. After a few mins the pedal went soft and nearly down to the floor, if I applied pressure again it would form back up. This kept happening, the first press it was soft and then began to firm up. Pulled over and let them cool down and the pedal would nearly touch the floor. After a 10 mins I drove off and all was ok again.

There was no problem before I tried bedding them in, no problem when heavy braking but only when I tried to cool them down. No leaks under the car or around the nipples, fluid is still full. Any ideas?