You have probably got sticky pistons. You just need to lube the piston and they will be fine.
Also your brake pad backings might be worn and could be wobbling on the piston
There's a place near keswick in the lake district called Bowderstone.
I don't think anyone that does trials knows about it. But it;s seriously awesome.
I cant find it on google earth/maps
Here are some pics:
An old slate quarry nearby: