Here's a link to Biohazardman's excellent writeup on the mod.
From my own experience, use 1/4" refrigerator ice maker tubing from the hardware store. I've done it on a 1377 and it really helps. A shallow 1/4" hole in the valve and the barrel provides a shoulder for the tubing to seal against. A flat bottomed, milled hole is much preferred to the angled bottom a drill bit leaves. It's delicate work, if you go too deep you can ruin the barrel or the valve. If you open up the port hole in the barrel, you'll need to de-burr the port from inside the barrel.