I found this in another forum, don't post much there any more but thought it was a basic problem worth recounting.
(I've got a very slow, nearly blocked 3way valve. Pressure comes back through the group head onto my coffee puck after extraction also.)
Followed in the next post from the same person approx 10 days later.
(So if anyone here gets a blocked valve. Undo the valve from the group head, pull out all the springs and other parts. Soak in Cafetto for a while put back together and bobs your uncle.)
I suspect had he been using Cafetto for its intended use, regular backflushing, the disassemble and and soaking procedure would not have been necessary in the first place.
Wonder if he thought to remove the basket and peek inside the portafilter, things can get pretty grotty in there, nothing a good soak in Cafetto won't cure.

PS, no smart ass comments about blocked valves please. ;D