It can affect most brakes, in that any air in the reservoir can make it's way into the brake line when they're hung up. Pumping it a bit while it's right way up should remedy it.
Hmmm interesting. They were a bit temperamental and I realised they were worse after hanging up so I bled them today to perfection and hung it back up.
Any reason that hanging a bike up on the wall by its front wheel would effect Shimano XT 4 pot brakes?