I have two Pilot Custom 91s (which use similar/same nibs.) F and EF.
Both pens behaved exactly as you described... With zero pressure they would hardly make a line at all, but with a little pressure they would open up and write just fine. That told me the feed was OK, it had to be an issue with the nib.
Upon close inspection the tines seemed to be incredibly tight. Pressure on the paper spreads the tines and allows the ink to flow.
I'm no nibmeister and there are probably safer ways to do this -- but I gently removed the nib & feed (they are friction fit and pull straight out) and used an index card between the tines to gently spread them apart.
The amount I spread them was nearly imperceptible. A very subtle change... I was incredibly careful, of course.
But what I ended up with is suddenly the pens write wonderfully! Nice, appropriately wet (but not super wet) writers. Smooth. Similar to (but feeling better than) my Metropolitans and Prera.
It was a subtle enough change that the EF reverted to its overly tight tines after a day or so, but a second flossing made it work well again and it's been great ever since.
I believe most people use "brass shims" for this. I was warned the card might leave fibers in the tines but it didn't.
PS. Yes, I thoroughly flushed the pens before doing any of this to rule that out as an issue.
Edited by JunkyardSam, 28 June 2016 - 18:07.