If a good backflush doesn't cure it, the problem will often be perished seals in the the group pistons. Alternately the cam followers on the end of the pistons may be worn.
If I see wear, I tend to replace the pistons as a set. If not, the seals are these ones. Beware though as many a cam follower snaps off the piston when trying to do the seals- so you want to have a spare just in case. Here's the schematic.
Coffeeparts sells the bits or alternately your local ECA service agent will probably have the parts and/or can do the job for you.