I went through the same thing.

The only way I could get it to hold pressure was to cut up a latex glove and make an extra gasket to fit in the threads. That rubber gasket in there didn't make any difference if I took it out or if I left it in.

I think that the problem is that the threads are the same "angle", but they are not the same "size". I don't know the technical terms for it.

Knowing what I know now, I'd get the cheap one with the flexible plastic cap that screws over the brake fluid reservior (which I think stands a better chance of making a good seal), instead of the inflexible metal capped one that doesn't make a good seal.

- Patrick

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1982 240D. 198k, Marine Blue/Blue, 4 Speed, Crank Windows, No Sunroof, No Rust, No Oil Leaks
2001 TDI. 197k, Lagoon Blue/Black, 5 speed, Chip, G60/VR6