bleed tool

[gstallons]

I will have to think about it . I went back to stock clip-ons w/my red frame and had to go back w/a stock length of brake line going down to the tee where the line splits . I had ZERO trouble bleeding the system .  do you have the bike on the sidestand and the bars turned all the way to the right ?  

Are you still using the set-up Phil recommended with the air pressurized cap ? 

Have you thought of gravity bleeding the system ? Leave the cap off , top the reservoir w/fluid and loosen the l.h. bleeder screw (1/2 turn is plenty) and wait for 10 mins and keep the reservoir full . Watch for bubbles then a steady flow of fluid. tighten the bleed screw and go to the r.h. side and repeat .

[docc]

This happened to mySport replacing the front brake master cylinder with the bike on a lift (standing vertical) and stock clip-ons. Seems the trap was the small 90º elbow at the M/C. Tying off the lever made the difference.

[Gmc28]

Interesting.  And some good things to try on my end.

yes, dot 4.  Tying off the lever I’ll do, but maybe letting it gravity flow as suggested first for a while would be the thing.  Then tie it off.  All free and easy things to try.  

sad thing is that if that works, then I likely replaced a part or two for no good reason. Perhaps the original issue was just air in the system from it sitting outside for so long and maybe a loose fitting somehwhere.  C’est la vie.  But will see how it goes.

[GuzziMoto]

I have seen enough air in the system, like with new parts, cause the master to be able to overcome it and get the system pumping fluid. Car guys will often bench fill a new master before installing it and trying to bleed the system. I prefer filling the slave with fluid and using it to force fluid backwards up through the system. That way you are filling the system in the same direction air wants to go (vs filling the system from the top down while the air in the system wants to go the other way).

As to the trick of tying the brake or clutch lever down overnight, since it seems to remove trapped air and not simply hide the air by forcing it to temporarily dissolve into the fluid I would say that is not likely what is happening. What seems to happen is like docc said; the higher pressure does two things, it causes the smaller air bubbles to collect into fewer larger bubbles, and due to the higher pressure those bubbles tend to rise to the top of the system (into the master cylinder in this case). Then, when you undo the lever and remove the pressure they vent out the top to the reservoir.

[audiomick]

4 hours ago, GuzziMoto said:

... What seems to happen is like docc said; the higher pressure does two things, it causes the smaller air bubbles to collect into fewer larger bubbles, and due to the higher pressure those bubbles tend to rise to the top of the system (into the master cylinder in this case). Then, when you undo the lever and remove the pressure they vent out the top to the reservoir.

Yes, there is a lot of truth in that, but I would care to expand on it.

It is not so much that the smaller bubbles collect into fewer larger ones, although that may happen, as much as that the bubbles will all become smaller due to the higher pressure. This can cause bubbles that are lodged in "traps" in the lines to dislodge, and rise to the master cylinder.

The factor "the air dissolves in the fluid" also plays a part. Maybe not all of the air, but some of it will go into solution at higher pressure. The chances are good that when it comes back out of solution when the higher pressure is released, it may do this in a part of the system from which it can then easily rise to the master cylinder to be vented. So my informant's opinion that the fluid should really be flushed with fresh fluid to remove the dissolved air can probably taken with a grain of salt.

Whatever, I too have had success with tying off the lever overnight. I didn't flush the system again afterwards, and didn't have the feeling that the pressure point deteriorated with time, as my informant predicted it would. Maybe it did, and I didn't notice, maybe it didn't. I had useable brakes, and didn't delve into the issue any further.

Edited Tuesday at 09:35 PM by audiomick

[docc]

What is the explanation, then, that my system "bled" after chocking the master cylinder ("tying the lever") even though there was zero pressure when I tied it (the lever came to the completely to the grip with no resistance/pressure at all)?

[audiomick]

On 1/21/2025 at 1:31 AM, docc said:

... At the time I thought the rationale was that the master cylinder, in the "activated" position, allowed air to rise into the reservoir,...

I'm speculating a bit now, but that isn't plausible. When the lever is "activated", it is pumping, so how could anything, fluid or air, get back past the piston into the reservoir? There is, however, a way back past the piston when the lever is released. One can often see fluid swirling back into the reservoir when the cap is off and the lever released after a pumping stroke.

As far as "zero pressure" and "no resistance" goes, I would suggest that those are relative terms. If your braking system really was creating "zero pressure", it would not have worked after the subsequent successful bleed. A bit like the guitarist sitting in front of his Marshall who claims he "can't hear anything". Of course he can, and of course your braking system was creating pressure, just very little.

Given that the system must have been creating pressure (even if it didn't feel like much at the lever) because it subsequently worked without any other changes, one can assume that the bubbles in the system where a little smaller overnight when the lever was tied off. Maybe that helped, as well as maybe some of the air going into solution.

Or maybe the bubble would have found its way up to the reservoir anyway, and tying the lever off was just voodoo.