As JIm has suggested, check the discharge line from your air compressor to the vey first fitting it will attach to.
It is very possible that it is restricted - blocked up with carbon.
As the compressors age, they pass very small amounts of oil into the discharge line.
The discharge line gets hot - from the compressed air, causing the oil to carbonize, creating the restriction or blockage.
You'd be amazed at how blocked up or restricted these discharge lines can become.
This is something I always check when replacing or servicing an air compressor, slow pumping complaint,etc.
i also recommend to service your air dryer at this time.
Check for restriction or blockage from that first fitting to air dryer assy too.
Don't forget to drain the air tanks - might have some contaminants in there too.
Best of luck with your repair.