Wierd Sticky Brakes

I have no issues with my brakes (2006 td4) they stop me everywhere but.....

If I have to break hard downhill so max load, they stick and rub, slowly easing back to normal. This has only happened twice. Normal driving or breaking at motorway speeds are fine.

Who has had this and easily fixed it?

sticky pistons? or is it that the pedal goes hard if the later then it will be the vacuum pipe has a hole in it usually the one that goes over the gear box

As above. The pistons are sticking in the caliper body as they heat up. Replace the calipers, before they stick on all the time.

I'm pretty sure Nodge and Teddy have it spot on there with a duff piston. I had very similar symptoms and I found that the near side piston was quite corroded.