Thanks for the post!

You should actually be getting SOME air escaping from the oil cap, it's also a breather to let the positive crankcase pressure bleed out.

You shouldn't, however, be getting air escaping from the bottom of the pressure switch, that's not normal. Most likely you just have debris in the line, in between the pump head and the tank. I would remove the #57 Check Valve from the tank, debris could be constricting air flow; this is common.

Here's a link to the breakdown and parts list for this unit:
www.mastertoolrepair.com/portable-oil-ba...153101-p-746866.html

Also, as long as your reed valves aren't sticking to the valve plate or aren't rusted, you should be fine (as far as the pump is concerned.

Typically this happens because the valve are sticking to the valve plate or there is debris in the line, so again just double check the transfer tube and check valve...most likely the check valve just needs to be cleaned as there could be gunk in there holding up air flow.

Hope this helps, feel free to post additional questions or your findings here!

- MTR