Threading reed seats Edit
Drone reed seat really do not need to be threaded. Some pipes have a reed seat better suited to cane reeds than modern synthetic reeds and could usefully use opening up with a tapered reamer (8 degrees, apparently).
Leaky valves Edit
Clean it, both the wood and the flap that makes contact.
Make sure there is a perfect seal between the mouthpiece and the body of the blowpipe. Leaks from here are very common, especially if your mouthpiece is not original to the pipes. Teflon tape on threads can help, and if all else fails electrical tape around the joint will sort it.
Back to the valve - inspect the wood for any crack in the tenon (dry it out first and use strong light, sunlight is best).
If this doesn't help, strip the hemp off and repeat the steps. You can also use fine grit sandpaper to square off the end of the tenon.
Use a rubber band to hold the metal strip in place while you try to get the positioning right so that it seals properly. Recheck while rehemping occasionally, there is nothing more vexing than finishing it off and discovering it moved slightly and you have to redo it.
I am, I must admit, a big fan of the Moose style valves that you install in the blowstick stock. Less so of the little mac valve, though it works fine. Your style of flapper valve is OK, but as you've discovered, can be annoyingly fiddly.
The last option is to dispense with a valve entirely and use your tongue to seal the blowstick when you breath in. It's not that hard to learn and a better seal than any valve.