Originally Posted by Zubdman
I had this happen to me. I thought it was the back cap oring too. Check that. BUT it was the bottom 006 orings. Change them to 007. It may be tricky to get them to stay in there but there are threads in here that can help you out.
I have my money on this one too.
Since you hold the engine down with an elastic, the downward pressure you put on it seals the leak. And the leaking air escapes backward so it seems like its coming from the backcap