Why don't brake lines need gaskets?

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Most every other metal part that is joined to another metal part and seals in a fluid, uses some gasket. Why don't brake lines with bubble or inverted flare fittings use a gasket? I think I know but what do others say?
 
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Theory, the pressure in the line actually tightens the seal.
I expect the line expands a bit under pressure and that presses the flare harder into the flare nut and fitting.
But that's just my theory. Maybe someone who designs hydraulic systems can confirm or deny.
 

4WD

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Theory, the pressure in the line actually tightens the seal.
I expect the line expands a bit under pressure and that presses the flare harder into the flare nut and fitting.
But that's just my theory. Maybe someone who designs hydraulic systems can confirm or deny.
Many metal to metal seals have a ball and seat profile/interaction whereas they “coin” when made up tight …
 
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Flare fittings are a type of compression fitting.

With pressure applied by a tightened flare nut, the flared tube deforms to match the profile of the flare fitting, making a leak-free seal.

It can hold a lot of pressure. Hydraulic lines are also typically a compression fitting. They also can hold massive pressure.
 
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Mini-split air conditioners are installed with flare fittings with no gaskets and operate between 100 and 150PSI on the low side. (The metering device in a mini-spit is in the condensing unit, so the high side doesn't exist outside of the condensing unit, so all the flare connections are on the low side. This is also why BOTH lines of a mini-split need to be insulated).
 
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The slight 'crush', wedge or interference fit (I'm a glorified accountant....engineers/auto techs please supply the correct term :) ) of a flare or olive type fitting forms the seal and eliminates the need for a separate seal. Akin to a tapered thread...sort of. That said, many of our older vehicles also use copper crush washers in the hydraulic pressure circuits.
 

TCL

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I wonder what kinds of fittings are they using on the Artemis1 moon rocket that's been delayed due to leaks?
 
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Still metal to metal.


Water pumps, exhaust manifolds, oil pans. Their gaskets are failure points but they still use them.
Those are large items that are prone to warping or poor machining tolerances. At least with brake lines they are sort of malleable and mesh together to form a tight seal.
 
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High pressure fuel line connections on 30,000+ psi diesel fuel systems are metal on metal. Sometimes can be a pain to seal up if there are any imperfections on the mating surfaces.
 
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Lots of high pressure hydraulic hose fittings are metal to metal.
JIC 37degree. On our power plant Electro Hydraulic Control Unit that supplied oil pressure for main steam turbine control and stop valves, all the connection were Parker Four Bolt flange connection that used an O ring. 3000 psig.
 
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