🔌 Pack of 4 Inverted Flare Steel Plugs for 3/8"-24 – Available at The Stop Shop Review

These plugs sit just right when tightened, they sit flush with the outside and make for a really clean look, they also solved the gap problem I was having.

Just bought these to seal connections while working on the car. Without Teflon tape, even gravity won't seal. I can't imagine they will ever seal actual working pressure.

I used Stop Shop steel plugs to close off the master cylinder and bleed air out of it. However, I had to purchase additional fittings to get it working.

The larger 9/16 x 18 plug I tried on the GM master cylinder leaked right away. The plug is tapered, but the center is not deep enough to properly fit into the cone of most master cylinders. As others have pointed out, the larger forks don't have a hex head. They are hexagonal heads. Luckily I had an old set of connectors that I used for this project. I would look elsewhere if you want something that actually works.