Dual calipers, dual errors
 

Hey guys!

So I rebuilt the master cylinder and the front calipers with a bit of trouble but still. Today I bled the system and refilled it like I've done before. I got the hand brake to work but there were something spongy about it so I removed the calipers from the rotor to see what was happening to them and I noticed a weird thing. On both sides the lower (when mounted properly) pistons on each side moved nicely but the upper ones did not move as much. This means that it was only the lower piston on each side that actually connected to the rotor. So I filled it some more. Bled it again until no air came out, just fluid. Same thing happened. I thought I had failed the rebuild but then I realized that the chance hat I had failed with the top pistons of both calipers at the same time was slim. I tried holding the freely moving piston back and then the other one, the one who didn't move as much, moved more. Conclusion: fluid travels downward hence the movement of the lower pistons, right? And that means that there are air trapped behind the upper pistons. I tried knocking with a rubber hammer to release air bubbles. Did not work. I tried moving it around a bit. Didn't work. And the nipples (when mounted) is lower than the suspected air behind the upper pistons which means that I cannot really bleed it out of the system since it will only bleed fluid.

Maybe I could try bleeding the calipers removed from the rotor lying flat somewhere? Or any other suggestions? I am thinking of buying a compressor and these bleeder tools to it, but can't really afford it at the moment. Any advice is appreciated.

Are the pistons different sizes? Some dual piston set ups are designed so one piston activates just before the other.

Are the bleeders on top of the caliper? Bleeder on the bottom will never bleed.

Keep at it. Air can be stubborn sometimes. When you think you got it all out, there still will be some trapped in there.

Tried to bleed the system again, no use. So I opened up one of the calipers and noticed that one of the seals that I just installed had snapped when pushing back the piston. So with some effort I redid the rebuild without snapping anything. Now I'm going to try to bleed the system again and hopefully it will work this time.

74demon, since it's like that on both calipers and I have swapped the original brake line from one-line-branching-out-to-two-lines for a two-line-from-the-master-cylinder it could be a design thing that the lower piston grips first and the upper one as soon as the lower one has stopped moving (being filled with fluid). Anyone who can confirm/deny this theory?

good stuff!

I have lately learned to really rely on the late apex on the street, especially on blind corners. For that matter nearly every corner on the street could be thought of as blind because who knows if there is gravel, or a dog, or a fuel spill etc?

Plus, subscribing to hear more

What does this have to do with a 5 year old brake thread?

In another 5 years, someone will answer this question. You just have to wait for it.

how the hell?

sorry. Obviously a glitch in where it posted... Supposed to be in a thread about riding technique...

Haha, for what it's worth, I only use one caliper at the front wheel now... :)
https://cimg5.ibsrv.net/gimg/cbrforu...cb786efa75.jpg
https://cimg8.ibsrv.net/gimg/cbrforu...1e501fa734.jpg

Well, it was nice knowing you...:D

Haha, after having tried the creature I might go back to two calipers...