That might be a good idea for lots of people but I picked up extra cap at the junkyard and put an air hose fitting in it.

I used the male fitting that my air line coupler snaps onto.

I add about ten pounds of air pressure on top on the fluid in the reservoir.

Pop the hose off when it get low and unscrew the cap. Fill it and do more if needed.

The whole affair is in a drawer when done. Not sitting on a shelf collecting dust.

Expense can be borrowed off your own air lines or the brass elbow for the coupler nipple.

Phil