Bleeding brakes

[dernst]

I’m having trouble understanding a concept behind bleeding brakes. I know how to bleed brakes both manually and with a vacuum bleeder. I don’t see how a bubble in the middle of a brake line can be removed unless there is a large flow of fluid thru the whole brake line. I can see where a bubble will travel to the highest point, but it seems to me that it would take a lot of flow to move it to the lowest point or past a level section.

Is the whole system sloped to only have possible air bubbles at the master cylinder or at a wheel cylinder or caliper? To me, it looks like there are some level sections in the middle.

Any thoughts?

 

[BeefStew42791]

the theory behind brake bleeding is that the pressure from the master will force fluid through the lines. This works in regular braking because brake fluid doesn't compress, and so the actual brakes are what become the 'moving parts' of the system. Normally, the fluid would flow back into the reservoir when pressure is released. When the bleeders are opened however, it allows the fluid to escape. This would happen whether there's air in the lines or not.

When there are air bubbles in the line however, the bubbles move down the line as well. Why it doesn't pool up at high points in the line is due to the compression of the brake fluid farther back in the line and in the reservoir in such a tight, constricted area (aka brake line). The lower fluidity rate of brake fluid also doesn't allow the air to flow backwards though like water does.

The thing is, usually air doesn't show up in the middle of the line. Actually if a brake system is left undisturbed, never corrodes, and the fluid level properly maintained then air should never even get into the system. Air will most likely develop if a line bursts, or when work is done on whatever parts, or if a seal goes bad somewhere and the system is drained to the point where the reservoir empties and air gets into the actual lines, resulting in the subsequent need of a brake bleed.
Hope that helped