On 05/04/2021 17:05, Paul wrote:
soup wrote:
On 05/04/2021 07:31, Alan wrote:
On Mon, 05 Apr 2021 06:23:41 +0000, RJH wrote:

I would err on the side of caution - a lot of pressure in those things.

Er, no, there is no pressure in there unless the lever is pulled. Even
then, it isnt much.

Agreed it's atmospheric (or thereabouts) with lever relaxed but pressure
is massive when the lever is pulled. Force isn't that much, but as the
tubes are of such a small cross sectional area the pressure is WAY up
there .

Pressures at maximum braking are in the 1,500 PSI region
Â*The tubing is designed to work up to the 2,000 PSI region.
Atmospheric pressure (depending on altitude and Temperature)
is in the 15Â* PSI region (think standard is 14.7)

Isn't there a reservoir of fluid in a bladder ?

I thought the bleed would be at atmospheric, the
reservoir provides fluid to the master, the master
provides mechanical advantage via the ratio of
master diameter to slave diameter (trades long throw
of brake lever to short throw of hydraulic calipers).

The bleed screw is not meant to be pressurized and is like
the plastic lid on the brake fluid reservoir in your car.

Hadn't thought of that the pressure in the tube will vary from
atmospheric up to 1500 PSI or so. But the pressure on the screw will
not vary so much
I've not been able to find a logical diagram of hydraulic
brake operation, nor even an exploded view of the parts
inside. Most of the articles seem to plagiarize one another,
as poor excuses for adverts.

Try page 5 of this :-
https://cdn.sram.com/sites/default/f...over view.pdf

Most are open systems but closed systems do exist but there are not a
lot of them



And the reservoir is there to feed the feed port or
accept fluid from the return port.

One article says the Tektro family uses mineral oil
and does not gather water or corrode stuff. And it
should have printed near the lever, that it uses
mineral oil. Like a car, different brands use
different fluids, and the wrong fluid is "death to
seals".

Yes I have mineral oil based fluid other stuff is called DOT something
or other

It should have a few similarities to a bottle jack,
only a really tiny one.

The bleed screw then, needs to "lock", the plastic ring
on the screw needs to maintain integrity so fluid does not ooze
past. But it really shouldn't "blow" because the reservoir
is at ambient. Some "cover screws" (my bike has some
for other purposes), they only rotate a partial turn
and are a curse (flimsy). Rather than being stripped,
I'd inspect to see what type of screw it is. Fully threaded,
or just a 90 degree turn for "closure". A kind of "cap"
and not a screw.

If it was the screw that was stripped I could probably just replace it
but it is the actual bleed port but as you say and thinking about it the
pressure in the reservoir doesn't vary much.
Just tried the brake again and it has held the fluid from yesterday
bleed screw/port interface feels only half stripped so maybe it has
enough friction (with the half thread and the wee rubber o-ring) to stop
fluid oozing out