On 2018-06-11 18:51, David Scheidt wrote:
Joerg wrote:
: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.

Joerg, I've replaced more brake master cylinders on more cars than you
have. The diaphragm is wetted.


Not on our cars and they are about 20 years old, so not ancient. Or the
previous ones I had (ok, one of those was from 1969). You unscrew the
cap and there is the reservoir, totally open, with the max level marker
well below the rim.


There may, or may not[1], be an air
space in the reservoir, but it's sealed from atmosphere. The only way
air or water gets across the barrier is via diffusion. There are min
and max levels, because if the level is too low, the system doesn't
work. If it's too high, excess is forced out the cap.


Exactly. On a bicycle it can't, you have to do that manually.


If ou have air in a bike system, if it can reach the reservoir, it can
stay there, as as in a car.


I need to burp my bike's brakes about once a year. Else the little air
bubbles make it back into the system and the pressure point feels
spongy. There is a surprising number of MTB riders who don't seem to do
this where the rear brake lever can be pulled almost or all the way to
the handlebar.


[1] cars can have an air gap, because the brakes arne't required to
work upside down, or even right side up after having been upside down,
while bike brakes are.


--
Regards, Joerg

http://www.analogconsultants.com/