Originally Posted by Frisco
NOT so! There are several brake line fittings (T connections, thru frame connections, fittings for hydraulic brake light switch) that use 1/8" NPT threads. If the fittings are quality made brass material, no thread sealant should be required. Definitely no locking compound. I have used a single wrap of teflon tape on occasion.
Where a flare connection is used, then NO tape, lubricant, sealer, etc. should be used. A correctly formed double flare will seal without excessive torque being applied. For the poster who asked about tightening (or over tightening) a flare connection, check the flare of the tubing and the seat in the fitting. If the flare is off center it will not seal. If the seat of the fitting has been crushed from over tightening it will not seal and should be replaced. If you are using a single flare for non AN fittings, this style of flare will crack and not seal and is illegal for use on brake lines (with the exception of AN fittings) in the U.S.
A brake lite switch is not a brake line it has pipe thread and is designed to seal with the threads. If you are connecting a LINE to it it needs a flare. All the through the frame fittings I have seen are flared on both sides, how else would the line seal.
Brakes are nothing to screw around with, get the right parts and do it right or you are risking your life