Description of the issue:

If you have literally anything in the offhand, you cannot sneak-right-click a fluid pump with the engineering hammer to toggle the none->input->output->none on the opposite side.

Versions & Modlist

IE 0.12-80
Forge 2645

This also affects belts as I've recently discovered, in fact I'd assume it affects all sneak+hammer operations. I'm not aware of any reasons for this to be the case on purpose, so an override on the hammer should be all that's needed.