On 2018-06-11 11:29, David Scheidt wrote:
Joerg wrote:
:On 2018-06-11 08:39, David Scheidt wrote:
: Joerg wrote:
:
: :Water in DOT boils out. That's what happens in the open systems on motor
: :vehicles. Unfortunately bikes don't have those but if you were truly
:
: What motor vehicle has a brake system open to atmosphere? It ain't
: the fifties, man. They're sealed systems. Have been for decades.
:


:What do you think that little hole is for?

What do you think the diaphragm is for?

:https://i.stack.imgur.com/lFHT8.jpg


It separates one air volume from another (from the big one). But air is
air is air.

IOW if air bubbles boil up in a car's brake system they have a chance to
exit the fluid and hiss into the air between the reservoir level and the
diaphragm. On bikes that regions simply isn't there.


That's a cap for a clutch master cylinder, but the principal is the
same. the rubber diaphragm is in contact with the fluid, kepeing the
system sealed. the hole in
the cap lets the air in and out of the air space between the air tight
rubber gasket and the cap. That lets the fluid level flucuate with
temperature or air pressure changes, etc, while still keepoing a
sealed system. Brake systems open to the atmoshpere went out with the
60s.

Bikes, by the way, use the same system:
https://www.parktool.com/assets/img/...gure_11-12.jpg


You will notice that the fluid goes all the way to the diaphragm, there
is no air underneath. Hence the "wet look". You either fill it to a high
level, gently put the diaphragm and cap back on and quickly clean off
the overflow (that's how I do it) or you leave the diaphragm on there
and instead top off at the upper bleed screw. The key is topping off.

In contrast, on a car you are not allowed to top off the reservoir,
there is a min and a max fill level. The level is not allowed to get to
the diaphragm whereas with bikes it's supposed to.


: Brake fluid
: in motor vehicle systems is not recirculated much, and the the fluid
: that's in the calipers tends to stay in the calipers. It is not at
: all unusual to discover a caliper is full of water, while the fluid at
: the master cylinder is normal.
:

:That would be a vehicle in a very harsh state of neglect. Probably sat
:in a yard for a couple of decades. Water is supposed to mix with the
:brake fluid which is why that's DOT and not oil. This lowers the boiling

Brake fluid only carries a small amount of water, when that is
exceeded, you have water and water-logged brake fluid. Because there is
very little actual movement of fluid in conventional juice brakes, if
the fluid in the caliper is overwhelmed, it seperates out, even if the
amount of water involved is low enough the fluid volume of the whole
system would carry it. It requires neglect or abuse, but I've seen it
on working trucks.


Key is that air bubbles can vent off either into the outside air or a
sizeable room below a diaphragm. On a motor vehicle it can, on a bicycle
it can't.

--
Regards, Joerg

http://www.analogconsultants.com/