Just had the same issue with the yellow line. No big deal if it is actually the o-ring... be VERY careful when you tighten the line back up to the head when you replace the o-ring as you can damage it by over tightening. It turned out that our issue was a mucked damper (these PH have an internal damper which is a PITA) and we had to replace the yellow head.
If you can't find the service manual, PM me and I can give you pretty detailed instructions on how to replace the o-ring. If you some how come across the service manual can you forward it on to me?
Chaz