HELP! - Stuck brake inner pistons after caliper rebuild - Triumph675.Net Forums
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post #1 of 13 Old 12-07-19, 17:31 Thread Starter
Neanderthal
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HELP! - Stuck brake inner pistons after caliper rebuild

I just rebuilt my front brake calipers using OEM seals. Everything came apart fine and went back together fine. Cleaned everything so well you can eat off it. Filled with new fluid and new pads.

After pulling fluid through the calipers with a MityVac and doing a ton of hand-pumps to the lever, the outer two pistons (in each caliper) are going right up to the rotor, however, the inner two pistons are not budging, they are recessed all the way down into the caliper. I've unbolted the calipers and clamped a 3/16" steel plate to prevent the outer pistons from expanding out, giving the inner ones plenty of room to move. No dice. Any suggestions? MacBandit or MGFChapin?

Last edited by Neanderthal; 12-09-19 at 09:27.
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post #2 of 13 Old 12-10-19, 11:51
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Would be very odd for both calipers to have the same problem, so I'd guess is that there is still air in the calipers. Which calipers do you have (are they monoblock or can they be split)?

Are the dust seals directional? If so, i'm wondering if the seals are on the wrong way on the pistons that are moving...wrong way would mean they would move out more easily.

My last resort to check them would be to use compressed air to see if that will move all the pistons (keeping a block in the middle to prevent them from pushing too far out).
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post #3 of 13 Old 12-10-19, 12:15 Thread Starter
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Quote:
Originally Posted by keeena View Post
Would be very odd for both calipers to have the same problem, so I'd guess is that there is still air in the calipers. Which calipers do you have (are they monoblock or can they be split)?

Are the dust seals directional? If so, i'm wondering if the seals are on the wrong way on the pistons that are moving...wrong way would mean they would move out more easily.

My last resort to check them would be to use compressed air to see if that will move all the pistons (keeping a block in the middle to prevent them from pushing too far out).
They are the Nissin 2-piece calipers (from 2006 Daytona 675). Seals are OEM, and looking under a magnifying glass, I could not see any directionality on the sealís profile.

Yeah, Iíve already drained all the fluid back out and my next thing to try was to blow air back through the bleeder valve. If that doesnít make the inner pistons move, I canít think of anything else but to split the calipers again and try to pop them out.

Just a guess here, but Iím thinking that the inner caliper pistons can not be fully seated all the way inside the caliper bores after re-assembly. Perhaps there is just no mechanical way for fluid to get behind them if they are in all they way? Thatís a design flaw IMO if thatís the case. The pistons are rather hard to push in by hand and once they clear the seals, they just ďpopĒ down the rest of the way. Not sure how to prevent that from happening....
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post #4 of 13 Old 12-10-19, 23:36
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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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post #5 of 13 Old 12-11-19, 07:42
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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.

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post #6 of 13 Old 12-11-19, 09:39 Thread Starter
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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.
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post #7 of 13 Old 12-11-19, 13:11
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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.
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post #8 of 13 Old 12-12-19, 06:51
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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.

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post #9 of 13 Old 12-12-19, 09:38 Thread Starter
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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.
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post #10 of 13 Old 12-12-19, 11:25
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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.
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