[Fot] Brake line question

[Michael Porter]

On 1/22/2014 12:15 PM, robertten1 at aol.com wrote:
> Compression fitting(s) in this application will not hold under the pressure
> generated in any hydraulic brake system.  Unfortunately you will have instant
> brake failure.  Follow the advice of our fellow racers on the list responding
> to your question.
>
>

Ah, well, let me just throw some sand in these gears for a moment. I 
think you're right if you mean _ordinary_ compression fittings, i.e., 
those suitable for plumbing work (nominally 100-150 psi) and _ordinary_ 
steel lines.  Those together will not produce a safe and satisfactory 
outcome.  But, to make that blanket statement is probably incorrect.  
There are compression fittings and lines available from most major 
manufacturers, SwageLok, Parker-Hannifin, Aeroquip, etc.,which will 
greatly exceed the pressure generated by any master cylinder.  
Compression fittings are typically used on CNG fuel systems with ratings 
well above the working pressures of 5-6000 psi, five to six times the 
maximum pressure generated by most brake systems.  I've seen compression 
fittings in high-temperature supercritical fluid applications that 
worked well at 600 deg. C and nearly 10,000 psi.

It's a question of cost, and the size of the overall installation. The 
fittings are expensive and must be used with precisely-made seamless 
tubing of aerospace alloys with very small circumferential TIR, but for 
the brake lines in a car, would not be horribly expensive to do--just 
unnecessary, since there are safe and suitable alternatives at much 
lower cost.  I think it also would be a pain in the ass to find the 
necessary adapters from SAE or DIN to British flare to mate the lines to 
the operating bits, but not impossible.

Cars, for the most part, have the benefit of not being very long, so 
single pieces of tubing can be used throughout, requiring no splices if 
the tubing material is malleable enough, a necessary feature for cheap 
mass manufacturing, so brake system maximum pressures have usually been 
designed with that in mind. In fact, single, double and bubble flares 
are only possible because the line material is relatively soft and 
fairly malleable (and therefore not capable of higher operating 
pressures).  Line materials and fittings have generally followed that 
practice from the advent of hydraulic brakes. That's what makes the 
usually available equipment suitable for the task, rather than 
compression fittings being unsuitable.


Cheers.

-- 


Michael Porter
Roswell, NM


Never let anyone drive you crazy when you know it's within walking distance....