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      02-06-2014, 08:33 PM   #10

Drives: 2008 135i, 2011 328i xDrive
Join Date: Jan 2008
Location: Canada

iTrader: (2)

Originally Posted by Ian///M View Post
See how quickly it flows in the video though.

I don't think dry is the same as wet with the Motive. Wouldn't fluid pressure would be different to air pressure?

The further away from the MC, the longer the line so the more you have to bleed. I recall reading that the system holds 1 litre, but the ABS unit would hold some of that, so 0.8 litre should be sufficient.

Send Dackelone a PM as he knows
Pressure is pressure - first lesson in hydraulics. Not only that but there is an air spring in the Motive (you pump it up) and there is another air spring in the master cylinder reservoir (because the motive pressurizes the air space that you left in the reservoir when you started). Conclusion: dry = wet.

The volume of fluid in the brake lines themselves is negligible. Also fresh fluid immediately displaces the old fluid in the lines. At the calipers, the fluid has to get changed by progressive dilution. That takes a while.

The first caliper you bleed also has to deal with the progressive dilution of the old fluid remaining in the reservoir when you start. You suck out all you can before starting to reduce the amount of new fluid needed to do this.

The amount of fluid needed is a judgement call, made easier if you alternate coloured fluids.

The pressure bleeder doesn't touch any fluid accumulated in the ABS unit, but that is likely too little to be worried about.

Given that you shouldn't store brake fluid after the container has been opened, 1.0 litre is "sufficient" for a full flush, unless you buy your fluid in 0.8 litre bottles ...

Originally Posted by JimD View Post
The disadvantage of putting the brake fluid in the Motive is it gets more chance to absorb moisture. Especially the next time you use it since it is nearly impossible to clean all the old fluid out. The only disadvantage of putting the fluid in the master cylinder (only) is you have to refill for each brake. That's a hassle but I want good fluid in my lines so I do it that way. Only real advantage of dumping fluid in the Motive is the convenience. I choose a quality job over convenience.
This is nonsense. The point of the Motive is convenience. And if you do run your reservoir dry by mistake, that would be really inconvenient.

Final obvious observation - if you want the fluid to flow faster, pump the brake pedal (slowly, releasing it slowly) with the Motive attached and pressurized. The only place air can theoretically get it is around the threads of the bleeder screws if your drain hose is full of fluid and immersed in fluid in the catch bottle (which is standard practice). In the unlikely event some air does get drawn in on the return pedal stroke, it will be expelled immediately by the pressurized flow from the Motive, even if that flow is slow.