Brake bleed

[sixpack577]

Bleed the mc or caliper down, watch the fluid drop in the mc resivor, close the bleeder...and watch the fluid level in the mc rise...That is the air between the mc and abs pump pushing the fluid back, and bleeding the exit lines is how you get that out.

 

[Flows]

I built a couple of MC reservoir caps for the front and back with quick connects to my pressure filler so that I can pressure blead down initially with loads of inexpensive (but compatible) fluid that I recycle (after a time on the vibrator to remove any bubbles). I then pressure bleed up from the calibers and syringe out of the MCs with the fancy fluids. Also use the cable tie method and can confirm that despite it being physically impossible I do get more air out and a firmer leaver. I then ride and 'activate' the ABS then cable tie again and bleed again. I haven't blead at the ABS module yet but will do that when I swap out the MCs as I usually have to redo the final stages after ABS activation.

In theory there is no difference between theory and practice but in practice there is.

 

[dcnblues]

I don't remove any of the lines...
I bleed calipers, master cylinders, and then the abs pump.

When you say you bleed the ABS pump, you mean you open the exit bolts, then open the calipers, and pump until the bubbles have traveled from the ABS all the way out. Is that right?

 

[dcnblues]

I built a couple of MC reservoir caps for the front and back with quick connects to my pressure filler so that I can pressure blead down initially with loads of inexpensive (but compatible) fluid that I recycle (after a time on the vibrator to remove any bubbles). I then pressure bleed up from the calibers and syringe out of the MCs with the fancy fluids. Also use the cable tie method and can confirm that despite it being physically impossible I do get more air out and a firmer leaver. I then ride and 'activate' the ABS then cable tie again and bleed again. I haven't blead at the ABS module yet but will do that when I swap out the MCs as I usually have to redo the final stages after ABS activation.

In theory there is no difference between theory and practice but in practice there is.

I keep asking mechanics, Auto and bike if they have a flushing system like this. It makes so much sense. None ever do!

 

[dcnblues]

Bleed the mc or caliper down, watch the fluid drop in the mc resivor, close the bleeder...and watch the fluid level in the mc rise...That is the air between the mc and abs pump pushing the fluid back, and bleeding the exit lines is how you get that out.

Oh f*** I get it now! I love playing this game if I have kids in a car and a helium balloon: I roll up all the windows and ask the kids to predict which way the balloon will move if I stomp on the gas or brakes. The result is counterintuitive when the balloon flies forward under gas and backwards under brake. And the balloon moves left in a left turn and right in a right. If you've got a smart kid, he or she will figure out that the air is denser than the helium, and that's why the balloon moves opposite to where the air is being slammed.

So in a motorcycle brake line, the bubbles will travel in the opposite direction of gravity, down, if the master cylinder is putting constant pressure into the line. The vertical up down of gravity becomes irrelevant but the pressure in the line becomes directional and the only thing the buoyancy of the bubbles cares about.
The pressure must dissipate to some small extent with distance, and the lines are narrow enough and the fluid dense enough so there's very little internal current, so the bubbles will move away from the compressing cylinder to get to the minutely lesser pressure area at the end of the line, and buoyancy from gravity is less than the buoyancy from the change in pressure! All the bubbles need is a little time! Mind blown! But I get it now, and it is counterintuitive, but it explains the why of your way working. I get it now!

*And this invalidates the gravity benefits of reverse flow bleeding...
**But gravity will pull contaminants from the calipers down to the end of the brake lines, and those contaminants should never be forced up into the ABS pump because they will destroy it. So the reverse flow bleeding should never happen without a conventional flush first.

 

[sixpack577]

When you say you bleed the ABS pump, you mean you open the exit bolts, then open the calipers, and pump until the bubbles have traveled from the ABS all the way out. Is that right?

No, everything is tight and closed, all bleeders, and all line bolts.
(This is After you have refilled the lines by vacumming fluid through, and fluid is coming out of all bleeders when they are opened).
Bleed the front calipers, then the front mc, then the abs exit line bolt(open/close the line bolt as if it were a bleeder screw. Wrap a rag around it while you do it, to keep the small amount of fluid from getting on the bike).
Bleed the calipers, they should bleed as normal.
Bleed the MC, it should appear to bleed as normal, only when you release the brake lever, you will see the fluid level in the MC resivor cup rise right before your eyes...and that is the air in the line from the mc to the abs pump that is causing that.
So pump your front brakes, and then bleed the abs exit line bolt.
After you bleed the calipers, the mc, and the abs pump, then go back to the mc, and finally the calipers again.
I prefer to tie the lever to the bar for several hours and bleed again.
When doing so, I bleed the calipers once each, and the mc's once each, and it's done.
Do the rear brakes the same way.
It also does not matter if you do the front or rear brakes first.
Just keep in mind that abs brakes bleed the same as non abs brakes, only that you need to bleed the abs exit line bolts after you bleed the mc's.
And, if a mc does not have a bleeder, simply use it's brake line bolt as the bleeder instead.

 

[dcnblues]

It's called friction pressure drop. As a viscous fluid moves through twists and bends (and an ABS pump), friction with the interior of the tube generates heat and slows the movement of the fluid. So by the end of the line (at the slave cylinder) the pressure is lower than at the beginning of the line where the Piston is compressing the fluid. So any little bubbles in the line will migrate to the end of the line where the pressure is less. And gravity is no longer a factor. (Unless you dawdle after removing the zip tie and cracking the bleed valve, so don't do that).
Pressure drop

 

[dcnblues]

WTF!?!?

4. Are all brake fluids equally compressible? No, not all brake fluids are equally compressible. DOT 3 and DOT 4 brake fluids are typically less compressible than DOT 5 and DOT 5.1 fluids. The latter two are silicone-based and have higher compressibility, which is why they are not recommended for vehicles with anti-lock braking systems (ABS).

Reference: Fluid Dynamics question -- compressibily of brake fluids "oil"

Okay that's good to know...

There's another part going on which is compression waves. At the master cylinder, the cylinder has to compress all the fluid in the line. As the compression wave moves down the line it has to compress less and less fluid. So in relation to the length of the line and the compressibility of the fluid, there's less pressure at the end of the line. The lower pressure area would suck any air bubbles towards it.

 

[Flows]

Loving the nerding out going on here! Actually DOT 5.1 is glycol based like DOT 3,4. DOT 5 is Silicone.

 

[dcnblues]

Damn good catch! And phew, I've got 5.1 in there now, didn't really want to drop down to 4, and feel bad about posting inaccurate data.