Read the quotes in my previous post. My understanding is that the .040" opening only allows a very small amount of air to enter the chamber, then the leak percentage gauge shows what percentage of that volume of air is leaking. If you're not using a leak % gauge then it's probably not as useful, I think it just helps prevent the chamber from pressurizing too quickly and also stabilizes the reading on the psi gauge.
I bought my LDT off ebay. IIRC it was less than $40 shipped, I'm sure you could build one for half that amount. If you have other DIY-LDT links please post them up and I'll put them to the "build your own" chapter.. thx & gl
