Classic Motorcycle Build

Best Thread Sealant For Brake Fittings

Article by Vintage Vinny

After replacing front brake hoses on a 1981 Harley Sportster, I was getting a slight weeping of brake fluid from the brass tee fitting.

best brake line sealant

How Tight is Tight?

Brake line flares can get damaged by being over-tightened. There should be a voice in back of your head that says, "They shouldn't need to be this tight."

Teflon Tape Doesn't Seal Brake Fluid

Before trying hydraulic line sealant, I tried wrapping Teflon tape around the brake fittings, but after sitting overnight, the weeping came back.

When I removed the brake hoses, I noticed the brake fluid was dissolving the Teflon tape. Not only does this not help the weeping, the pieces of tape could clog passages in the brake caliper.

Hydraulic Sealants

Most mechanics have a tube of blue or red Loctite in their tool box, but are they compatible with brake fluid? I went to the Permatex website to find out, and learned that neither the red or blue Loctite is compatible with brake fluid.

Out of all the hydraulic sealants that Permatex makes (and there are many), I found only one that safely works with brake systems.

how to seal brake line fittings

Permatex 54540 sealant is designed for use with hydraulic line fittings, brake fittings, and transmission fluid fittings. It works well on PTO fittings and air fittings, too.

Out of the bottle, it comes out purple in color. All you need is a few drops on the threads.

Permatex 54540 sealant for brake fittings

Once dried, it has a purple-red tint.

best thread sealant for brake fittings

In the above picture, you can see the dried sealant on the brake line threads.

The Bottle's Half Empty!

The reason the bottle seems half-empty when you buy it is, the product needs to be shaken up before use.

best hydraulic sealant Permatec 54540

It's a small 1.22 oz bottle and it's expensive, but certainly worth the money. And none of the local auto parts stores had it. I tried an AutoZone, Advance, and a Napa store--nobody had it in stock, so I had to order it online.


Are Permatex and Loctite the Same Company?

I used to think they were both the same company. Turns out I was partially correct. Permatex was actually owned by Loctite for a long time, then Permatex left Loctite around 2002. Since then, the two companies offer nearly identical products. Although there's little difference between the brands, it's important to get the right type of sealant for your needs.


There's plenty of hydraulic sealants on the market, but only one is advertised to safely work with brake systems. Permatex 54540 seals any metal thread and will not contaminate brake fluid.

Permatex 54540 Hydraulic Sealant

Shop: Permatex 54540 Pneumatic and Hydraulic Sealant


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