Tom Gardner wrote:

I bought a bunch of 1-1/2" bore x 3/4" stroke single-acting pull-off air
cylinders to use for clamping fixtures. They are perfect with the
spring force to hold the part but the air input is on the front end.
The back cap can be drilled and tapped 1/8" NPT and a fitting installed.
This will make them push-type. BUT...there is a vent hole in the
stainless tube at the back of the cylinder that must be plugged. I'm
thinking of a hose clamp and a piece of rubber unless there's a better
way. If I could take the cylinder apart I could TIG the hole closed but
these are sealed, throw-away cylinders cylinders. They ARE worth
modifying because I bought them for $4 each and they retail for $60. I
need 16 of them modified.

If you can hold the piston and thus the seals away from the hole area I
bet you can TIG them and dump them in cold water before too much heat
gets neat the seals.