Hello it could also be that the bolt that holds the retaining ring on might be worn or screwed in too far into the piston causing it to travel too far slightly pulling the diaphragm out of place and making a little spray solution slip past into the oil. However if it is a lot of solution you might have a hole in the diaphragm. If this is the case stop using immediately or you could
fry your pump.
Hope this helps
Toni Davies @ Rittenhouse Sprayers