My boss has an 03 plate Mondeo diesel and he said he had an issue on a couple of occasions where he went to brake and the pedal was soft and spongy but as soon as the pedal was released and reapplied the pedal was ok

My first thoughts were that it was a master cylinder starting to let go.

Car was dropped at a local garage who have diagnosed the issue as being down to the CV joint allowing movement causing everything to get hoy and making the fluid overheat.

I might be a pessemist but this sounds like bull to me. I know on track when I've ended up with a spongy pedal it's taken quite a while for the pedal to return to normal again.

Thoughts and opinions please?