Cap for the brake fluid reservoir for pressure bleeding the brakes/clutch. Standard Euro E20 thread. Compatible with many brands. Tested on a BMW and Volkswagen, but also fits many other manufacturers.

Modified original model to use a 1/4" NPT thread rather than the G1/8 thread to hook it up directly to a standard US air compressor fitting. Word of caution here, if you do this make sure your regulator is working properly. Brake fluid reservoirs don't like more than a few PSI. And make sure to keep the fluid topped off, don't want to blow air directly into the brakes lines either...

A rubber seal is required, 44.5mm x 35.5mm x 2mm (1 1/2" x 2mm). On the cars I tested I just borrowed the seal from the existing cap, but if you have an extra seal laying around that happens to fit all the better.

There are 2 STL files included, one with standard 1/4" NPT threads and one with the NPT threads oversized by 2.5% (about 0.3mm extra diameter). The oversized version is good for getting better thread engagement especially if you're planning on using PTFE tape, but I found that the normal version twisted good and tight directly to an air fitting backed up with a wrench sealed well enough. It doesn't need to be a perfect seal, it just needs to not blow brake fluid everywhere ;)