.. it could well be that the grease or oil on the side
of the piston has
dried and has become sticky goo. just clean well with white spirit ans
apply a thin layer of thin oil. adjusting the valve hole should only be
caried out starting the carriage from the far end.
An alignment problem between the piston and the
cylinder could also be
the problem. are the ball bearings of the carriage free
Alignment is the first thing to check here. You can clean and check
airflow of the piston into the cup, but be careful about what grease (?)
you use. I'd just clean and make sure there is no debris in there only.
If alignment fixes the problem you probably don't need to do anything else.
Use a nut driver to loosen the nut screws and carefully move the printer
and/or belt return a few millimeters at a time until you get the result
you're looking for. Clean the return spring (with something underneath to
catch any debris) if it's really gunked up with old oil. Don't use pliers
to open the nut screws, the investment in a nut driver is very worth it.