Hi,

Thank you for your question. I am unsure if the inlet check valve should close tight and completely or not.. I think not, and if it looks so by design, it is probably ok.

But I think you problem is with the oil stop valve. This valve is installed below the compressor element. Please see the picture above.. the oil stop valve is called "Vs" in the picture.

The red arrow is the supply of compressed air, created by the compressor element. There is a check valve, which prevents back flow of compressed air when the compressor is stopped.

The blue arrows are the supply of oil to the compressor element (for lubrication and most importantly cooling).

Now, during operation. The pressure forces open the oil stop valve (orange arrow in drawing). The oil stop valve is opened and oil is supplied to the compressor element.

When the compressor stops, the oil stop valve should close, so no more oil can flow to the element. If it stays open or is leaking, the oil will keep flowing to the element at shutdown, and will leave through the inlet valve and filter.

So, in short: check your oil stop valve. Remove it, open it and replace the o-rings and clean everything.

While you're at it, also check the output check-valve (CV1 in the picture). It could also be a problem, although less, because it connects to the separator vessel far above the oil level.

Be sure to drain the oil first (DP2 and DP3 in the picture) before opening and of the valves below the element.

I have also included a picture of the contents of the oil-stop-valve overhaul kit from Atlas Copco, to give you an idea what it looks like inside..

ok, good luck and let me know if you found the problem.