Try the hoppes, also a bit of heat from a heat gun helps if it's varnish. If the piston is stuck, unscrewing the cylinder will score it unless you can take the back off and get the rod loose first.

If you find it's varnish, Davis Disel makes great bore brushes for removing it.

If it's rust, soak it in a bit of kerosene or penetrating oil, then try some heat.

Bill