Brake Pipe Cap (metric M10)

(Update 29 Jun 25 - Please note…
Of the few bits of feedback on this model, most indicate that the central pin can break off and jam in the end of the brake pipe. It's then difficult and messy to remove. Therefore, please ensure that you print this in a filament with good layer adhesion, like the nylon that I used, and then don't overtighten the cap - it only needs to seal just enough to prevent brake fluid leaking. Also, I've uploaded an extra set of 4 STLs with a short central pin (3mm long rather than 8mm). These files have "Short Pin" in their name. I haven't printed or tried these but my intention is to mitigate against the possibility of a broken pin.)

Original Post 10 Mar 22
I had to send off one of my car's brake calipers for renovation because of a broken seal and seized piston. Having detached the male brake pipe union from the caliper, I had nothing to plug the end of the pipe, which was leaking brake fluid. Rather than allow the whole braking system to drain and then have to refill it with fresh brake fluid and bleed the whole brake system, I wanted to retain the fluid and then only have to bleed the one brake caliper once refitted.

This is a cap for temporarily closing off the end of a car brake pipe during maintenance purely for the purpose of preventing the escape of brake fluid. It is NOT for permanent use and NOT for use when the vehicle is being driven. It will not withstand the hydraulic pressure of a brake system and the brakes would fail!

I printed in Taulman Nylon 230 because I wanted the cap to be slightly elastic / flexible, and, as far as I could determine, this filament is impervious to brake fluid. DO NOT USE TPU if your car has DOT4 or DOT5.1 (polyglycol) fluid. TPU is, apparently, impervious to DOT5 (silicone) brake fluid.

There are 4 STLs with 4 slightly different sizes of cap. I wasn't sure how tight the fit would be so I did drawings in 0.5mm diameter intervals - 9.5mm, 10.0mm, 10.5mm, 11.0mm. I also wanted to print more than one at the same time to give more time for last layer cooling. I printed one of each and, surprisingly, the 10.0mm was the best fit.

As you can see from the cross-sectional drawing, the cap has a centre peg that is 2mm diameter at its tip and increases to 3.75mm diameter at its base. I measured the centre bore of a piece of brake pipe at 3mm and wanted the peg to act as a bung, which it does as the cap is tightened onto a pipe by screwing a male union into the cap. The bore and thread are nominally M10 fine (1.0mm thread pitch as per metric brake unions), with the 0.5mm diameter variants as described above. I added a hexagon to the base to allow it to be gripped by a spanner / wrench when slippery with brake fluid. The hex is nominally 20mm across the flats.

I printed at 0.2mm layer height with a 0.5mm nozzle, 100% infill, no supports.