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Been at it for it seems like a long time. I had rebuilt the master and both calipers for my dual disc system. But the bugger just won't bleed completely. I am using a mini vac which basically uses the bleed screw, you pump up vacuum and it sucks the fluid out of the system when you open the bleed. Been at it for a few hours, gone through a ton of fluid and when you pump the lever up, it gets firm but slowly sinks to the handlebar .
Any wisdom to share on the process?

Try bleeding it without the vacuum pump. They suck air into the system. Just; pump, hold, bleed and repeat. Some here have mentioned wrapping a bunji cord around the lever overnight, as a method to get the last bit of air out of the system.

My favorite was the speed bleeder.
I also clamped the brake lever overnight, with a zip tie.

http://www.speedbleeder.com/

Well I tried it without the minivac pump. Seems to work better, at least I am not seeing the bubbles I was before. Did get the lever to pump up and hold, so I tied it off to see if it get better overnight. The lever was inconsistent in pumping and holding firm, but now at least it does. When I used the old fashion method there seemed to be less bubbles, just fluid moving through the clear hose to the catch container.
For awhile there I had visions for the fluid pushing past the seal in the master.

jagarra wrote:
For awhile there I had visions for the fluid pushing past the seal in the master.

Where else can the fluid go if 'x amount' of gained lever pressure drops, and the lever comes back to touch the bar ?

If lever pressure drops it will show as a leak somewhere, or its getting back to the mc reservoir.
Even with a little air in the line, if you're able to pump up x amount of pressure, it should stay pumped up like that if the lever pressure remains the same.

What size piston is now in your rebuilt master cylinder? What size is your master cylinder?
A too small piston seal could produce the symptoms, or the seal on backwards if that's even possible.

What condition was the mc bore?

Bamboo812 wrote:Try bleeding it without the vacuum pump. They suck air into the system. Just; pump, hold, bleed and repeat. Some here have mentioned wrapping a bunji cord around the lever overnight, as a method to get the last bit of air out of the system.

Vacuum pumps do not suck air "into the system." Where would that air come from, and how would it get in? Any leak that allowed air to be sucked in would spew huge amounts of fluid at braking pressures.

What vacuum pumps DO do is suck air into the the tube that is attached to the bleeder valve downstream of the system, so you can't tell if you're still getting air out of the system or if you're just seeing air that has been sucked in at the bleeder.

I haven't used the speed bleeder valves, so it may not be a problem with those, but with regular bleeders I have found that I have to wrap the threads with Teflon tape to keep air from being sucked past the threads. This does not impact the seal of the bleeder as that seal is created by the cone shaped end of the bleeder forced into the caliper - metal on metal, but you have to make sure you do not allow any of the tape to get down there on the end - only on the threads.

The overnight handle squeeze is definitely the best way to get any air trapped at the top of the system in the Master Cylinder. I've found that no matter how well I bleed a system, that last step makes a difference. Once after bleeding and then one more night after riding usually makes an even bigger difference.

As far as the M/C size, it is 16mm. The bore was good, there was some pitting at the very end, but that was in the area where the spring lived, the working surface was good and I honed it. I am pretty sure I put the seal in correctly, but sometimes you question what you did. I am going to let it burb overnight, we will see how it feels in the morning.

Depending, I would expect to see a disturbance of some sort on the surface of the mc reservoir if fluid was pushing back past the mc seal 'when the lever was pulled towards the bar'. (not looking at disturbance from the lever when releasing)

Gerard, I assume you are talking about the under-tank MC. I understand that there is a 'corner' that will hold on to a bubble of air and cause sponginess. The fix is to undo the fixing clamp and prop up the rear end of the MC while you have the lever tied back. This allows the bubble to rise and be released.

16mm does seem big. I guess you will soon find out if there is a decent improvement. Good luck.

OMG Charlie, you'll have me throwing it over my shoulder next. I will give it a whirl.

Thanks