by Chad Frost
The various recent comments regarding replacement Teflon / stainless steel brake lines leads me to believe that a dose of information is in order...
Years ago, I worked as a project engineer designing and manufacturing fluid systems, including just about every type of hose. Making hoses was definitely the boring part of the job, but it was our bread and butter ;-)
While the steel (or other material) braided over a hose liner does serve to protect the liner, its primary function is to _reinforce_ the liner. A plain Teflon tube will only take a small amount of pressure, and tends to kink very easily when flexed. Adding braid creates a hose capable of handling very high pressures, and actually improves flexibility somewhat.
The hose wears out when the braid wires fatigue or corrode, or if it is damaged. The Teflon doesn't age, and is unaffected by brake fluid (and just about everything else). The Teflon hoses are favored in high-performance applications because (a) they have an effectively infinite working life, if the proper hose is properly installed, and (b) the Teflon liner doesn't "give" under pressure like a rubber liner does, so hydraulic systems function more crisply and efficiently.
In racing, military, aircraft, etc. applications, hydraulic hoses and systems get pressure-tested, typically to 1.5 x working pressure, on a regular basis to find bad components before they fail in service.
The pressure-testing on your Bus happens every time you step on the brake pedal... and the consequences of failure could be dire. DOT certification gives peace of mind for a couple of reasons: you can be sure that the hose has the correct pressure rating for your vehicle, and is sufficiently high that failure is extremely unlikely; the manufacturer proof-tests the hoses to verify that they will take pressures well in excess of what they will see in your vehicle; and if there's a failure and an accident results, the manufacturer has liability insurance.
If you want to specify a Teflon / SS hose assembly to use for your brake lines, then by all means be sure that you get them from a reputable supplier, and that they are tested at proof pressure. There are plenty of little shops around that will slap together hose assemblies of very poor quality, and I'm sure you'd agree that it's not worth taking chances with your brake lines. Teflon hose assemblies are quite a bit trickier to build than are rubber hoses.
A final word -- be *very careful* installing Teflon hoses on your Bus -- Teflon hoses will kink very easily, and once kinked they shouldn't be used. They are not as forgiving as rubber hoses. Doing a brake job? Replacing ball joints? Watch out for those brake lines!