The injector adapter in the center of the HF pic is the one for vw/mercedes idi. Hansautoparts sells the exact same gauge and adaptor. Pressure relief valve on the gauge barely works on mine , had to press it a bunch of times- may have got a bad one... but it got the job done
HF adapter does need collar around the part that actually threads into the engine. In addition to that collar I put the copper washer betwee the collar and the adapter body. So I have the heatshield to the collar and then the washer and then the adapter. HF kit I have has a bunch of ways you can get the flow from the adapter to the quick connect. I like to keep the compression from having to build in the hose so I try to keep the part with the valve in it as close to the adapter as possible. Like in the picture. But there are 90 degree take offs and longer parts if you need em. My Kit is the 93644 one, plenty of pieces in the red box. The compression release works fine so perhaps yours needs more use or a bit of silicone spray. Not using the collar probably only cut a few lbs off the reading on the gauge. Without the volume taken up by the metal the compressed air would have filled that volume instead. But I doubt you would see it. I like that kit and if it is off a few lbs in measurement so what. You are measuring all the different cylinders with the same equipment and comparing the values to one another any hoo. Just my contribution to the cause or confusion.
....Interestingly I use a standard injector bottom with nozzle holder and needle but leave out the needle thrust plate, and pack adaptor gaps with lead:o[/color]
Go to prothe, buy his guage and adapter for like $20. Done deal. I've had mine since 2008, works perfectly, and tests out against snap-on units too. A machined piece of metal, a hose and a gauge are not things that can be screwed up anymore... no matter what country they were manufactured in.