Reddit mentions of Anderson Metals 56101-04 Brass Pipe Fitting, Barstock Tee, 1/4" x 1/4" x 1/4" NPT Female Pipe

I'm glad to see more people are mentioning this, it's (IMO) a much better way to leak test without the risk of actual leaks. Some Amazon links for my current setup:

You'll need a pressure gauge, https://www.amazon.com/dp/B0087UCI8C/ has a relatively low maximum for high sensitivity (easy to see if there's a drop)

You'll need a valve you can hook a hand pump on to (like what you'd use to pump up a bike tire -- I don't recommend an electric pump because of the low pressure you need). https://www.amazon.com/dp/B001PLI4BA/ is in the same threading as the gauge.

You'll need a tee to connect these things to -- https://www.amazon.com/dp/B000BQUTBS/

Finally, connecting the tee to your rig. I take a scrap of tubing, put a fitting on one end, and slip the other over: https://www.amazon.com/dp/B000CFNKBO/ (check for your tube's inside diameter!).

Assembly is pretty straightforward, wrap some pipe tape around each component's threads and screw into the tee. Before connecting to your rig, you may want to cap the open end and pressurize it a bit to make sure you're not starting with any leaks. Connect in to your fill or drain port as appropriate, and slowly pressurize the system. You don't need a ton, only 8-10 psi is plenty, and is more than the water will be operating at. Mark the gauge position, and come back in a few hours. If the needle has moved, you've got a leak -- you can swab glycerin or dawn soap around your seals to find where. If not, you know that air at higher than normal operating pressure isn't leaking out, and that's a nice tolerance to know you have!

I ferment in 10 gallon cornies and I just built myself a spunding valve to use as a test. Wasn't that hard the main bit is the valve itself, which I got off Amazon.

http://www.amazon.com/gp/product/B007GDY3CU/ref=oh_aui_detailpage_o08_s00?ie=UTF8&psc=1

http://www.amazon.com/gp/product/B002FYI58S/ref=oh_aui_detailpage_o08_s00?ie=UTF8&psc=1

http://www.amazon.com/gp/product/B000BQUTBS/ref=oh_aui_detailpage_o08_s01?ie=UTF8&psc=1

And then some fiddly bits to go from the Tee to the QD for the keg.

I'll go in order from the CO2 tank down the line:

• 10lb Aluminum tank, bought at local welding/gas supply
  
  
• This 2-stage CO2 Regulator
  
  
• Replaced the output fitting with one of these
  
  
• Then I have a smaller regulator reduce the line pressure down to a stable 15 psi - I find that the more regs, the better for stable pressure.
  
  
• That leads to two solenoids, one for each tank. You can buy the cheapish milwakee one, or anything that uses 1/4" npt fittings if you can find a better deal.
  
  
• After each solenoid I have a T-junction to a gas pressure sensor reading back to my controller, and the other outlet to the final regulator. You can use the one above, or one of these
  
  
• No need for a needle valve at this point. Go to your bubble counter and then up to your tank.
  
  
• On my big tank I am using a reactor instead of the normal ceramic diffuser I have on the small one.