it seems that every time I go out on my bike my rear brakes feel like the need bleeding. I have bled them twice in the last few months and after a little ride today, they, again, needed a bit of p8umping to get them going a bit.

WHY?

The front is fine and was bled the same way as the rear and I can bleed brakes (I think..)

Has a seal gone ?

Shimano XT M765's a few years old now, new to me in 2008 but not new then.

bit of an ironic thread.
have a read of this.
http://www.singletrackworld.com/forum/topic/shimano-xt-vs-avid-juicy-5