I feel your pain! I have a 75 1.8, so if you have a 1.8 you have l-jet which means vacuum leaks are bad. If you have an l-let you should get the cap with the two rubber gaskets along with the o-ring for tube and the gasket for the base to ensure you don’t have a vacuum leak. When everything else is perfect this is your problem area or so they say.
Also, the lower o-ring that you will notice when you disassemble your oil filler tube is not on the PET diagrams but Rich has them. Just tell him you want the 1.8 o-ring that James got.
Getting the parts is actually easy. Call Rich Bontempi at High Performance House. He is contact info is:
HPHPORSCHE@aol.com
http://www.highperformancehouse.com/650-364-6234
Rich knows about and has every part that you can possibly want usually both used and you would be surprised NOS as well.
Regards,
James