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Master cylinder swap question.. Brain-pick?


SR71BLACKBIRDXX

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I wanna change out my front brake master. Is there a way to swap it out without re-bleeding the whole system? Yeah, I know it's no big deal and I've done it countless times. I was just wondering. Cant imagine that the air pocket would travel down the front brake line into the caliper or the secondary master on the fork. I thought it might be possible to hook up a syringe type apparatus to a front bleeder and push fluid\air back up into the reservoir. Ordinarily, I'd just rebleed.. but I literally just flushed the entire system. Took me hours, but it was well worth it. 

Anyway, thoughts?

 

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I've been able to do this on a bicycle and then bleed by tapping the lever hard and fast.  It shakes the bubbles back out.  A motorcycle should in theory work better since the lines and holes are much bigger.

 

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I think speed won't matter.  The bubble will stay up top.  And the master has a way to reverse-bleed bubbles out.  Oh, and on many masters it will spurt when you move the lever so watch for that.

 

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2 hours ago, SR71BLACKBIRDXX said:

I suppose I'll just try it and see. I'd like to think if I transfer the line over quickly enough, the air bubble should be confined to at or near the banjo bolt. As long as I don't pump the lever, it should stay there.

Yes, you are correct. Once masters  are swapped try to stick flathead screwdriver between brake pads and rotors. Push pistons in calipers all the way in. It should take care of air bubbles in brake line near master cylinder. Fill reservoir with fluid and pump the brakes. Keep close eye on fluid level, because if it drops too low the air will get in, and you will be fucked.

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Time won't matter, don't rush.  When the new master is installed turn/tilt the bars so that the outlet end of the master is a hair lower than the handle end, add fluid, take slow short pumps and you'll watch the air come up as the fluid level drops.  Fast pumps might shoot fluid up and out.  When the bubbles stop you should be done.  If you want to be absolutely sure there's no bubbles hiding in the banjo area, use a screwdriver or something to leverage a brake pad pushing the pistons in, pushing fluid and any air up into the master.  Do not push the pistons in before bleeding the master, if you choose to push them in it should be done after you think the master is bled.

 

I would do this even if the fluid was old and needed to be flushed, then I'd do the flush after the master was air free.  Trying to bleed an empty master at the caliper bleed bolt is a useless battle.

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