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I'm working on a 63 Impala, and got a new master cylinder in. The 63 has a single brake line, but the new master has two ports. The manufacturer included a plug for the extra port if it's not needed, but when I put the plug in and tightened it down solidly, it still leaks brake fluid.

What's the right way to seal the threads? Will Teflon tape work or will the brake fluid dissolve it?

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Teflon tape will not dissolve and should seal the plug, unless there is a larger issue with the plug itself or it's not seating correctly. It could also be, you just don't have the plug tight enough. Secondarily, you did use the correct port on the master?

As for Telfon itself, because of its chemical makeup, it is one of the most non-reactive substances known to man. Wikipedia has a great article on it, but basically, for Polytetrafluoroethylene (PTFE - the chemical name of the Dupont trademark Teflon) the only chemicals known to affect it are reactive metals like alkali metals. There is nothing like it in brake fluid or the metals in the master or plug (I know this because if there were, our human skin would not stand up to it either). According to the Wiki article, Teflon maintains high strength, toughness and self-lubrication at low temperatures down to 5 K (−268.15 °C; −450.67 °F), and good flexibility at temperatures above 194 K (−79 °C; −110 °F). I'm not sure where people get the idea it will be dissolved by any fluid in an automobile or pretty much anything we have around the house.

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    Have you actually used teflon tape on brake lines before? We actually talked about it in the shop before and was specifically told to avoid it because 1) if you need teflon tape, something isn't seated properly and 2) it will 'dissolve.' That was enough for me to avoid ever using it on brake systems. Have you actually used teflon tape with brake systems before? – justinw May 21 '15 at 3:00
  • I think I would be more concerned about a shred of the tape getting into the brake system somehow. There should be no need for the tape, so I'm concerned that something must be wrong (assuming the plug is tight enough), it could be a poorly machined seat on the plug or the cylinder. Better to return it for a good one. – dlu Jul 30 '16 at 19:36
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Use the correct Master Cylinder, they sill make them for your model.

Its not really a 2 line (dual line MC), it is manufactured to fit different models, use the correct line out for your model so you do not have to bend stock the line.

If this is what you have tighten the plug more using a 6 point socket to stop the leak.

enter image description here

You can buy one at this site

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It seems to me that a dual circuit master cylinder in a single circuit system is not going to be very safe. The plugged circuit will still get fluid (unless you plug the inlet somehow, which would then lead to unhappy/unlubricated seals and a very short life), and will give a pedal that's too firm, potentially affecting the rest of your brake system such that you won't be able to stop properly. What you need to do is split out the existing system and convert to a true dual circuit brake system (much safer), or get a proper single circuit unit for your application.

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Don't use teflon tape. It will likely get dissolved or possibly cause contaminants in your brake system.

As far as the leaky plug; I am pretty sure the plugs are designed to prevent leaks, are you sure it is seated correctly?

I've seen people weld/solder those plugs before.

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