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Discussion Starter #1
Hey guys, I'm having a heck of a time stopping a weepy brake fluid leak on my 65 convertible. I am just finishing a total restoration, and as part of that, I got a new master cylinder for the factory power brakes. The reproduction master cylinders have two ports on the front, one with a flare-fitting seat, the other with a pipe-thread style opening, no seat. Obviously, the line has to go into the one with the seat, and that seals fine. The other however is where I am getting a leak from. The MC came with a small plug for this port, but no matter how tightly I put that in, a drop of brake fluid will form after a day or two. Despite reading lots of information to the contrary, it looks like a thread sealant is absolutely necessary in this case. I tried some PTFE tape, which did very much slow down the leak, but not completely. Now it's maybe one drop per week. Plenty to mess up my lovely engine-compartment paint if I can't stop it. Has anyone else had this issue, and what is safe and effective to use to seal out brake fluid leaks. I'm tempted to try something like "Right Stuff", but seems like brake fluid can find its way through about anything.

I'm assuming this port exists for cars that have a brake switch mounted on the MC, but it's beneath the line fitting. If my memory serves, the originals with that setup had the switch on top, no?

Thanks for your help!
 

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If it is NPT then you could use some ptfe thread sealer the type in the tube not thread tape. Apply to the male threads only and It will help lube the threads so you can get it tight enough to stop leaking.
 

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Discussion Starter #3
The plug itself may be the issue (although it came with the thing, so you'd think it would be right). I can tighten it all the way until the hex bottoms out on the casting, so I don't think it's not getting tight enough. It puts up plenty of resistance prior to that point, and having removed it a couple times, no sign of damage to the threads or anything. It just looks like the fluid finds a way around the threads, so I need something that won't dissolve in there to keep the fluid in.

I was just reading that Loctite is brake-fluid resistant....thoughts?
 

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A petroleum-safe pipe sealant like used for LPG fittings should suffice. That's what I use on my NPT fittings. If it leaks with THAT, then I'd question the plug or the hole in the master.
 

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That was my first thought too. I've used it for fuel lines. Have not used it for brakes.
Obviously you wont use the full bottle. I've found that if I take all those loctite porducts put them in a ziplock and stick them in the fridge it extends their shelf life.
 

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Was the supplied plug made of steel? If so get a new brass one, file 1/16” off the bottom of it just to ensure it’s not bottoming out, then use an approved hydraulic thread sealant to install.
I like to use the brass plugs that have a hex key hole in them rather than the ones with a square head.
Good luck!
 
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