Generally seals on rigs are only designed to seal with liquids with water being the test medium for the best case.
As soon as you start drilling with air or aerated fluids valves and seals will leak. Most of the time redressing seals and making sure that there is no pitting or corrosion on the surfaces will work.
Make sure that you inspect the inside of the rotary hose as high pressure air can absorb into the rubber and cause chunking. This applies to all elastomers exposed to the air. (hoses, Non Return Valves, Motors, LWD tools)