Photo of tap cap- the thumbscrew core depressor is on the right

Photo of disassembled tap cap

It's much simpler to make than the old design and allows the use of a really gas-tight fitting.  The old design used an metal air stem (it's normally used on tires) and I could never find an air fitting/chuck  to connect to it to my CO2 rig that didn't leak if left connected.  If you are only going to use a $9.95 tire inflator gadget/12 gramm co2 cartridges (details/source below),  it does not have a leakage problem and the air stem is a better choice.  The inflator uses the high pressure of the CO2 cartridge to force open the Schrader valve in the air stem.   The downnside is that the cartridges are a bit expensive.  The rig is real handy for traveling- especially if your CO2 source is otherwise a 20# CO2 cylinder!

Caveats
1.  Don't plan on storing brew in the mini-kegs for much longer than a month or two assuming you keep them cold. YMMV with delicate brews and warmer temps.   Long term storage is said to cause the brew to become oxidized since the PET plastic the bottles are made of is not a good O2 barrier. Yes, the CO2 pressure in the bottle after filling is much greater than atomspheric pressure, but, as counter-intutive as it seems, oxygen will still pass through the plastic.
2.  I've only used plastic bottle caps. Aluminum bottle caps should work tho'.

The only problem I've had with the depressors is the small o-ring in the end that attaches to the access fitting will somethimes fall out after a bit of use.  I just keep a male flare fitting handy and screw it into the end when it's detached from the access fitting- this retains the o-ring..

1. Use one of the "blocky" type street elbows- they afford more surface area for soldering in the beer tube.  A street elbow has both a male and female outlet while a regular elbow has two female outlets.  If you can't find one, use a regular elbow and solder a close nipple into a female outlet.   The downside is that the nut won't run up tight against the washer/cap/gasket.   To remedy this, either 1) chase the thread(s)  (i.e. a threading die turned "backwards") on the close nipple or 2) use additional washers to take up slack between the nut/washer and the cap.
2. Firtst drill a 7/32" hole completely through the elbow from the 1/8" MPT end then enlarge with a 1/4" drill BUT don't drill completely through the elbow!   This is shown in the drawing above.  A drill press/vise is good for doing this since the bit will tend to grab and hence increase the chance of drilling to deep.
3. The 7/32" OD brass tubing is avaiable in 1' lenghts at any decent hobby store and at some hardware stores.  Clean the joint area of the tubing before soldering and use lead-free solder just to be safe.
4. Saw off any tubing that's on the 1/8" MPT end of the access fitting and screw into the elbow using teflon tape.
5. If the nut doesn't run up far enough on the male end of the street elbow, either  1) chase the male threads (i.e. a threading die turned "backwards") on the exposed part of the close nipple or 2) use multiple  washers to take up slack between the nut/washer and the cap.  Don't try to enlarge the threads on the nut while holding it in a vise- it'll likely deform during the process.   Maybe holding it in a socket rather than a vise would prevent this....
6. The 3/16" ID vinyl dip tubing in the bottle is whacked off via  two 45 deg. cuts-  the end is shaped like ">".  A single-cut end may come to rest against the bottle and cause dispensing problems.
7. To prevent the 3/16" ID vinyl dispensing tubing outside the keg from getting detached from the cap (results in a beer geyser!), secure it with a couple of nylon tie wraps or or wrap wire over it.
8. Don't sweat finding a washer with the precise ID hole-  shoot for "too small" and enlarge the hole with a rat-tail saw.  A smaller OD washers work better than a larger one since the lesser surface area can compress the gasket easier for a good seal.
9. Cut your own gasket rather than trying to find/buy one the correct size.   Ace Hardware sells  in 6"x6" pieces of 1/16" red rubber gasketing material.

A solution of two parts white vinegar to one part hydrogen peroxide (common 3% solution) will remove tarnish and surface lead from brass  parts when they are soaked for 5-10 minutes at room temperature. The brass will turn a buttery yellow color as it is cleaned. If the solution starts to turn green and the brass darkens, then the parts have been soaking too long and the copper in the brass is beginning to dissolve, exposing more lead. The solution has become contaminated and the part should be re-cleaned in a fresh solution.

Hydro Test
After assembly, do a hydro test on the assemblied keg. Don't skip the "hydro" part and use just air or co2- it's a safety thing. If something ruptures during the test with water filling the bottle, less energy will be imparted to targets in the blast zone since water is practically imcompressible- OTHO, there's lots of energy in a compressed gas.  Fill the bottle with water and secure the top to the bottle. Pressurize with air or co2.    I use 30 psig but once took a plastic 3L bottle up to my water tap pressure (55 psig) without any problem. You'll note that the bottle expands a bit and there is a bit of air or co2 atop the water in the bottle. This violates my advice on hydro testing but it's only a little gas space and it's actually a good thing since air or co2 will escape through any leaks more readily than water would. Assess leakage by applying a soap solution to the tap cap.  Assessing leakage by filling with CO2 and noting if the pressure drops over time is not a really good idea- the water will absorb some co2 and lower the pressure.  Air may work tho...

There are two faster and easier ways than counter-pressure filling to get brew from a cornie keg into a mini-keg.  Both are ideal when the brew will be consumed without much delay:

FAST/EASY:  Attach the snout of the cobra tap on a sanitized 3 liter mini-keg to a co2 line using an "adapter" made from 3/8" ID vinyl tubing and a hose barb, latch the cobra tap open, open the co2 valve at the regulator and loosen the cap on the mini-keg to allow the co2 to purge the bottle via the "beer out" dip tube for 30 seconds or so,  tighten the cap on the mini-keg to pressurize the bottle, close the cobra tap and the co2  valve to trap the pressure, attach the snout of the cobra tap to the snout of the cornie keg cobra tap using another "adapter" of made from vinyl tubing, latch open both of the the taps,  barely loosen the cap on the mini-keg to allow the brew to flow into the mini-keg- control the flow by adjusting how much the cap is loosened and fine-tune by pressing down on the cap, when just about full (leave some head space if you use a tire inflator gizmo for dispensing!), tighten the cap on the mini-keg to stop the flow of brew, close both cobra taps (this is very important- don't ask how I know! :-)   and finally disconnect the taps.   This is MUCH faster and easier to do than sanitizing and connecting up the counter-pressure filler.  In fact, I can fill a mini-keg about as quickly as it took me to write up the procedure!

FASTEST/EASIST-  Where the brew will be consumed without much delay, simply fit a 1' or so length of 3/8" ID vinyl tubing to the snout of the cobra tap from the cornie keg, place a 3 liter bottle at a lower elevation that the cornie keg, reduce the co2 pressure a bit (including in keg by cracking open the pressure relief valve), open the cobra tap fully to fill while adusting the co2 pressure regulator to adjust the flow of brew into the bottle.

Dispensing with an co2 tire inflator without a pressure gauge is pretty easy with a bit of practice.  What I do is add the co2 only as I'm actually dispensing brew- i.e. when brew flows too slowly, give it a brief blast of co2.  repeat if/as needed.   You''ll know when you've over-pressureized by the brew foaming too much as it's being dispensed.

I use about 10' of 3/16" ID vinyl tubing for the dispensing line and 10-12 psig of co2 pressure for dispensing ales.

If you want a tap cap that allows the use of both an access valve and a tire air stem, use a 1/8" NPT  tee and  close nipple instead of the street elbow.  You'll have to either chase the threads (i.e. a threading die turned "backwards") on the exposed part of the close nipple or use multiple  washers to take up slack between the nut/washer and the cap.

Make a giant 6 liter keg using a 6 liter plastic bottle.   It accepts caps from 3 liter bottles so you can use your tap cap.  The only downside is they have a round bottom hence will not stand up-right unaided.   Details on this and related stuff:
 

Photo	Description	Model No, '02 Price & Source
	Reusable 6-liter (1.5gallon) blue bottle. The bottles are 14" from end to end, and 6.25" in diameter with a 38mm (Standard 3-liter bottle) finish. Depending on what is put in them and how they are cleaned to how long they will last. We estimate around 8-10 refills for the average user	KEG930  $6.95 (from Beer Beer & More Beer)
	CO2 Injector Only Does not include ball lock or adaptor hardware. Use to flush kegs or carboys prior to transfer or to flush the headspace in wine fermenters. Uses 12g cartridges	KEG954  $9.95 from Beer Beer & More Beer)
(no picture)	12 gram CO2 cartridges- pack of 12	KEG955A $7.80 from Beer Beer & More Beer)