CO2 injector caps are
clever, but fairly simple, occasionally they do not hold pressure, what should I check? There are a couple of things to check which might cause the CO2
to escape straight out and not pressurise the barrel. As always the cap
must be screwed on tightly, and the thread should have a little
Vaseline or similar on it to help it make a good seal. The rubber seal on the inside of the cap should also be in place, it is usually white on the standard pressure barrels with a 2" lid and on the underside of the cap. With King Kegs they have a black rubber ring under the cap which makes the seal with the barrel neck, this must be in place or the lid will not seal.
There are two rubber 'bands' on the brass/stainless steel part of the injector, one on
the top (outside) of the cap, and one on the bottom (which will be
inside the barrel once fitted). The first on the top of the cap, they
vary in colour, but it should be visible under the thread, it wraps
around the brass and covers a small hole. When there is excess pressure
inside the barrel it forces this rubber band open and allows the air out
before re-sealing again. This should be in place and covering the hole.
The second rubber 'band' is on the underside of the cap, and fits around
the brass/steel 'stub' which projects out, right in the centre of the cap.
The rubber seal again wraps around the brass, and is designed so that
when you inject a cartridge of CO2 into the barrel it is forced open,
and then re-seals itself once the CO2 stops being injected. Very
occasionally this rubber seal doesn't seal flat back down, or in very
rare cases falls off. If it is not properly in position and a seal is
not made, then the CO2 would just escape straight back out.
The brass/stainless steel valves themselves are fitted to the lid with a brass/steel nut and rubber washer, the nut on the underside of the lid must be tight, and can be tightened with a spanner to make sure it is nice and tight, otherwise gas could escape through this joint. The rubber seal compresses to make a good seal.
The taps can occasionally leak, especially after shipping, it is important to check that the taps are screwed in tightly (hand tight is NOT enough), give it a nip up with a spanner or pair of grips, and you will see the black washer begin to tighten. This is a good sign it is tight and has made a good seal. If a tap is leaking on the junction between the barrel and the tap the brew can run down the underside of the tap and drip off the end of the tap, making it look like it is the tap which is leaking, but sometimes it is coming from the joint and is fixed by nipping the tap up clockwise.
Occasionally the taps can drip from the end where the beer is dispensed, this is usually as a result of turning the lever too far, past 90 degrees. The tap should be at 90 degrees to the barrel, so by simply turning it back slightly until it is at a right angle to the barrel, the valve will close and the drip will stop.
To overhaul barrels a spares kit is available with the common washers and seals included, because as the seals are rubber they can perish over time, this spares kit can give your barrel a new lease of life and ensure it remains airtight for future batches; https://www.home-brew-online.com/spare-barrel-valve-multi-washer-pack-youngs/
Above all, check everything is tight and all the seals are in place, the barrels can hold a lot of pressure so all joints need to be tight, otherwise the gas will force it's way out. Hand tight is never enough, nip everything up with a spanner or pair of grips