I'm having problems with one of the plastic one way valves which head off up the left hand side of the engine bay (not sure where to, can't think off hand, but are by the screenwash bottle!). One (closest to TB) sounds like it's leaking, under very small amounts of rev.

I think I'm losing about 2 PSI boost at the moment, could this be the culprit?

What is this line for anyway?