AMfoto1@aol.com wrote:
> If this looks like yours, the hydraulic line is best made from
> braided stainless steel AN or "aircraft" hose. Moss offers it as an
> upgrade for TR6 (584-785), probably would fit your car, too. It
> looks pricey at $46, but would cost more than that to buy the
> individual parts and make it up yourself. Plus that hose is pretty
> critical, as you've just found out!
I suppose it's time to reiterate a few things about SS braided hoses
(having worked with them industrially for more than a decade).
There's nothing inherently wrong with rubber-covered hoses, and
depending upon the material type, they can be superior to SS braided
hoses--in both wear resistance and burst pressure. As well, contrary to
popular opinion, SS hoses have an expansion rate that is inferior to
some of the better rubber-coated hoses.
Typically, these hoses use a Teflon or other fluorocarbon liner which
has very little mechanical strength and will balloon easily--that's why
there is the necessity for the outer stainless steel braid. But, like a
Chinese finger puzzle, the braid is flexible, and the line will swell
under pressure until the braid locks.
My favorite story about this was from an engineer with whom I worked who
was producing his own variant of the Zink Formula Vee race car. He
thought the SS braided hose should be better, so he replaced all the
hose and line in the brake system with it. The brakes bled out fine, the
pedal felt okay, but on his first hard, high-speed braking on the track,
the pedal went to the floor and he went sailing off into the catch
fence. Afterwards, the pedal felt fine. Tried again, and the same thing
happened. Then, he decided to look up the rate of expansion under
pressure and calculated the volume change for all the line he'd
installed and it exceeded the stroke volume. (!) Went back to the
standard VW rubber hoses and steel lines and all was well again.
The lesson then is to keep the distance short if using such hoses for
brakes and/or clutch.
The other issue is wear. One of the best materials to resist abrasion
is... yup, rubber. One of the worst... yup, SS braiding. The wires are
very small diameter, so they're easily worn through. When that happens,
the strength of the hose declines rapidly. It's very, very important
that such hoses be clipped in rubber-cushioned clips, especially in
areas where there's a high vibration level--one of the hoses laying
against a metal surface or edge can wear through in no time at all from
seemingly low amplitudes of vibration. Where clipping is not possible,
the hose should be wrapped in a hard nylon garter coil (these are
available for most hose outside diameters) to keep the braiding surface
from contacting other surfaces.
Rubber hoses do have their problems--internal cracking can cause fluid
to wick out along the reinforcing fabric. As many people on the list
know, very old rubber lines can crack sufficiently internally to form a
flap that can block the line or cause pressure not to bleed off after
the pressure is released.
But, if one wants to be convinced that there are some very good rubber
lines available, just drop by your local industrial supply house and ask
to look through their Aeroquip industrial catalog and compare specs for
various hose types.
As for the cost being lower than one can make up the hose one's self, I
wonder about that. These lines are typically -3 or -4 SAE. In bulk and
at wholesale, the line and fittings are worth perhaps $8-12. But, if
it's not something with which one has a fair amount of experience
assembling, I wouldn't recommend making your own lines--but, much of
that $46 is in labor, overhead and profit, not parts.
Cheers.
--
Never let anyone drive you crazy when you know it's within walking
distance....
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