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I'm bleeding the brakes on my motorcycle with a cheap autozone mityvac vacuum bleeder and getting A LOT of air. I don't think there's any way there's that much air in my lines (I've run more fluid through than I think it takes to refill the entire system). So I suspect that I'm sucking air in around the pistons (as far as I can tell, it's not coming in around the nipple). I haven't seen any fluid leaking anywhere, though, so my guess is that they are designed to hold more pressure in than out and aren't failing enough to leak from the inside yet.

I'm getting ready to order the parts to rebuild both calipers regardless just on general principles (since it hasn't been done since it was new in 2002), but is that going to fix it? Is the mityvac strong enough that it can overcome whatever pressure limit the seal has in reverse and I am just being too aggressive with the pump, or is this legitimately a sign of failing seals (or is there some other possible leak source I'm not considering)?

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    I've never seen air sucked in around the pistons and I don't think that's happening here. It's much more likely coming in around the edge of your tube attached to the bleed valve or around the threads of the valve.
    – jwh20
    Jul 27, 2022 at 9:50
  • I originally thought that, but there's far too much air, and I can hear the caliper "roaring" as the air goes through it. It's really weird. The rear caliper was fine, just the front has this issue. And it's been a while since I messed with it, but I'm pretty sure I was getting air in both front, but the roaring was only in one, so I'm guessing that's the one with the bad seal, if that's what it is. I guess I can get a pinch clamp and close off that side of the line and see what happens.
    – user34314
    Jul 27, 2022 at 15:30
  • I'm wondering if you are pulling the caliper past the point it should be going, allowing it to open up past a seal? Why don't you try a standard bleed instead of a power bleed? Jul 27, 2022 at 21:26
  • Not enough hands, usually. This is with the caliper and pads still on the motorcycle.
    – user34314
    Jul 28, 2022 at 0:57
  • Finally got around to testing it again, pinching off various lines, and couldn't replicate it at all, with or without pinched lines. Maybe it was just related to doing the brake job at 3am.
    – user34314
    Aug 6, 2022 at 2:29

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