Taking a stock cap and drilling a hole in it doesn't work, you're close though. I drilled a hole and inserted a valve stem in a stock junkyard cap for my setup. I thought I was so clever. Turns out the rubber gasket on the stock cap is vented, brake fluid was everywhere! I felt like a dejected Gilligan.
I removed the vented black gasket from the cap and used a Fluidmaster "tank to bowl" toilet gasket as a seal. http://www.fluidmaster.com/our-produ...l-empaque.html
. I picked it up at Home Depot for a couple bucks. It was a perfect fit. Take the gasket with you to match the diameter because they had a few to chose from.