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Thread: HELP! - Stuck brake inner pistons after caliper rebuild Reply to Thread
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  Topic Review (Newest First)
12-16-19 21:12
keeena Thanks for posting up what you found! Glad you got it sorted.
12-15-19 06:01
2BRacing Wow, good thing you took the calipers apart again before resorting to "drastic" measures to force the pistons out!

I never realised that the calipers could even go back together with mismatched halves.

Thank you for reporting back.
12-14-19 17:23
Neanderthal Well ...... It was a classic case of supreme dumbassery on my part. Even after laying everything out in order, I somehow managed to swap the inner caliper halves - RH inner got bolted to LH outer, LH inner got bolted to RH outer. It was immediately evident as soon as I broke them apart. The two inner halves are identical except for the fluid passage hole, so maybe I got distracted during the critical moment of mating the parts, lol. It's all back together and bled. I'm chasing the elusive rock-solid lever, so for now I have the lever tied back against the throttle tube.

Just for any future reference - it is impossible for the piston to be all the way into the caliper, there is a small protrusion at the bottom of the caliper bore that holds it out far enough for fluid to get behind it.
12-12-19 11:25
keeena Interested to see what you find w/ compressed air and/or splitting them. Make sure ya use a soft material (wood) between the pistons and that its wide enough that the pistons on one side can't come all the way out if the other side remains stuck.

@Chirp - forcing brake fluid into the system from either end is fine; they make bleeder kits which are designed to push fluid thru the reservoir (typically in automotive applications). The benefit of pushing it thru the caliper's bleeder is that it pushes air up...the direction the air naturally wants to go. However, I did mistakenly say to zip-tie the lever when forcing fluid from the caliper. You do not want to do that because you wouldn't be able to push the fluid past the M/C piston. Nothing bad will happen...just won't be able to get brake fluid into the system due to the pressure.
12-12-19 09:38
Neanderthal All the fluid has been drained. Iíve tried all the above things except for shooting a blast of air through the bleeder nipple hole. If that doesnít work, Iíll have no choice but to split the calipers. For no better of a description, itís almost like a ďvacuumĒ has formed between the piston and bottom of the caliper bore.
12-12-19 06:51
Chirp
Quote:
Originally Posted by Neanderthal View Post
keeena,
I did break the calipers apart and thoroughly cleaned everything, you could eat off them, lol.

Chirp,
Lever was still squishy, but did take considerable effort (not like zero amount of fluid in system). I soaked the seals, pistons and bores in brake fluid during assembly. I did remove the calipers from the fork and hung them upside down, while both bleeding and just sitting overnight with the lever tied back against the grip. Now that the air is out, I'm going to remove the bleeder nipples and try a blast of air directly into the nipple hole and see if that moves them.

I've rebuilt these before and never had anything like this happen. Weird.
I'd be very wary of using compressed air with brake fluid in the system. DOT 3/4/5.1 is one of the best and fastest acting paint removers available. Feel free to ask me how I know. I'd also be wary of forcing fluid into the caliper with the lever ziptied for the same reason. Keeena may have more to say on this.

As keeena notes, upside down is not what you want. While I can't visualize your caliper at the moment, the idea is to get the bleeder pointed straight up relative to the air pocket. Tapping and rotating the caliper can also help. Another thought is to use a syringe to suck the fluid through the system.
12-11-19 13:11
keeena
Quote:
Originally Posted by Neanderthal View Post
keeena,
I did break the calipers apart and thoroughly cleaned everything, you could eat off them, lol.

Chirp,
Lever was still squishy, but did take considerable effort (not like zero amount of fluid in system). I soaked the seals, pistons and bores in brake fluid during assembly. I did remove the calipers from the fork and hung them upside down, while both bleeding and just sitting overnight with the lever tied back against the grip. Now that the air is out, I'm going to remove the bleeder nipples and try a blast of air directly into the nipple hole and see if that moves them.

I've rebuilt these before and never had anything like this happen. Weird.
Ah - if lever was squishy when trying to pump the inner pistons out: I wouldn't try compressed air yet. Just means more air you'll have to get out later.

A couple things to try before going the compressed air route:
  • When you did the zip-tied lever: did you gently tap the calipers with a soft-faced mallet? That can help free up air pockets.
  • Try raising the calipers higher than the M/C, but not upside down. You want the bleed nipple to be the highest part of the system and can help by having air rise to the top for bleeding.
  • Get a large syringe filled with brake fluid and pump into the caliper thru the bleed nipple with lever zip-tied. This may help push any air out of the caliper and up the brake line...then bleed.
12-11-19 09:39
Neanderthal keeena,
I did break the calipers apart and thoroughly cleaned everything, you could eat off them, lol.

Chirp,
Lever was still squishy, but did take considerable effort (not like zero amount of fluid in system). I soaked the seals, pistons and bores in brake fluid during assembly. I did remove the calipers from the fork and hung them upside down, while both bleeding and just sitting overnight with the lever tied back against the grip. Now that the air is out, I'm going to remove the bleeder nipples and try a blast of air directly into the nipple hole and see if that moves them.

I've rebuilt these before and never had anything like this happen. Weird.
12-11-19 07:42
Chirp Fluid can get behind them even if they are fully seated. If the lever is squishy, it's air. You'll need to bleed them while rotating the caliper to make sure the inner chambers are higher than the outer chambers. If the lever is not squishy, then there is a likely blockage although is seems unlikely that would happen on both. If you have to take them apart again, use red rubber grease on the seals when reinstalling them.
12-10-19 23:36
keeena Seeing that they are 2 piece: did you split the calipers as part of the rebuild? If so: maybe something blocking the porting to the inner portion of the caliper? Is it possible that the inner halves are installed on the wrong sides thus preventing proper alignment of the porting? I'd highly doubt this would even be possible, but ya never know.

I'd be really surprised if the pistons could be bottomed out to the point where hydraulic fluid couldn't get behind them. You did exactly what I would've done by locking the outer pistons to isolate the inner ones. How did the lever feel in that case? Was the lever solid or did it go to the bar/squishy?

Never heard of a problem like this so I'm just throwing ideas out there - sorry can't help more!
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