You make it very tempting to not say a word and watch the sort of replies
you'll get to your question. :-)
You learn some things working on motorcycles, especially since their front
brake line runs straight down along the fork tubes, and the clutch line runs
nearly straight down to the engine.
It's not that air rises just in closed hydraulic systems, it's that air rises
in any fluid. From an aquarium bubbler to the fizz in soda. Air simply rises
in liquids.
Now, if you can mechanically shove an air bubble down faster then it floats up,
you can purge the line this way. This is how a garden hose works the air out.
Takes a heck of a lot of fluid, moving fast, for quite a while. Especially if
you were to compare brake bleeding to a garden hose. Look at how long a garden
hose spits air before it extablishes a steady fluid stream.
So yes, it is possible to pressure bleed brakes this way, opening the bleed
nipples. It's going to take a lot of fluid, moving fast and steady, but it can
be done. Accidently empty the master cylinder doing this, and you get to start
all over again.
Far easier is to crack the lines.
Let me use the motorcycle example, because it's simpler to visualize. You've
got a master cylinder up high on the handle bars, the caliper down low at the
wheel, and a line in between them. We're replacing the line. So you install
the new line. Air in the line, fluid in the caliper and the master cylinder
(if you didn't accidently drain the master cylinder).
Leave the line connection at the caliper tight, but crack it at the master
cylinder. Depress the master cylinder, and you will hear some air escaping
from the losened fitting as the master cylinder squirts fluid into the line.
When you stop hearing air coming out of the cracked fitting, tighten it and
slowly release the master cylinder. You slowly release the master cylinder to
encourage it to suck fluid from the reservour, and not air from the line. Keep
on repeating. Eventually you will start getting fluid coming out of the
cracked joint. It will snap crackle and pop as air bubbles come out with it.
Finally, you will get nothing but fluid. At that point the line is bled.
What is happening is that as you depress the master cylinder you move fluid
into the line. The fluid dribbles down the inside of the line to the bottom,
displacing the air upwards. Ie, you are filling the line from the bottom up.
You crack the fitting to allow the air to escape as the line fills up with
fluid. Repeat, repeat, repeat. Eventually the line is nearly filled up, and
you start getting the froth of both brake fluid and air coming out of the
cracked fitting. Finally, you get nothing but brake fluid.
Some folks prefer to have the line tight, depress the master cylinder, and then
crack the fitting so they can more easily hear the air being expelled (tends to
make a pop sound this way). I'm one of them.
At this point, with the line thusly bled, you'll probably still have a somewhat
soft pedal. The reason is perfectly demonstrated with a glass of soda. There
are little bubbles of fizz stuck to the side of the drinking glass. Same
problem occurrs in brake hydraulics. There will be air bubbles mechanically
stuck to the sides of the lines, the master cylinder, and the caliper or wheel
cylinder. These bubbles do tend to get mechanically worked down the line to
the caliper by you activating the brakes when driving. If there are enough of
these air bublbles, they may get worked together until they form a large enough
bubble that it breaks free from the side of the line (or master cylinder or
caliper) and rises up through the fluid to the highest point. If it's in a
line, and that high point is just a bend in the line, it's tough to work it
out. If it rises to a fitting, cracking the fitting will allow it to be
expelled. In the caliper, it will rise to the bleed nipple.
This is why you go back a week later and rebleed the brakes, you will find a
little air in there. Even though you completely bled them when you last worked
on them. The little air bubbles stuck on the side of things have been worked
down to the caliper.
Those little air bubbles are part of the reason you don't want to be moving the
master cylinder piston fast when first bleeding. It creates froth, which is
nothing but little air bubbles. Makes it darn hard to get a firm pedal. But
you do want to work the piston fast at the end, to help jar those little
bubbles lose.
You also may want to examine the routing of your lines. If you've got a brake
line that has high spots that are not joints, that line is nearly impossible to
bleed. Bend it, move it, splice it, but get it so that the fluid rising would
rise to a joint that can be bled.
And that was probably a lot more reading then you wanted to do. :-)
>>> <frandrum@voicenet.com> 06/13 9:59 AM >>>
Hi Listers,
I recently read a Nolan Penney statement that air rises in a closed hydraulic
system
making it difficult to bleed the system.
Since I am in the process of replacing my OEM 72 Spit lines with SS brakelines
should I
anticipate difficulty in bleeding the system? Will a power bleeder solve the
problem?
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