Stuck caliper pistons - retracted
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The seal groove and seal are different in section so that when bedded, the seal feels proud to the touch at the edge furthest from the bore. This is normal. (this is the only manual info. I can find on this so if anyone can help determining which way the seals seat in that would be good. I didn't know this until someone told me they are not square section but the internal form that contacts with the piston(s) have a slight taper. Better wording needed ?). :?:
cyclegeezer
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Torontonian said:
The seals themselves are usually square in aspect, it's the groove in the caliper that's cut at an angle to push the pressure side of the seal into good contact with the piston.
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cyclegeezer said:
Thank you for that clarification. The seals were relatively inexpensive. I can't imagine that they'd have a taper.
Good time for a follow up: my friend is also out of town which means i will not have access to an air compressor after all. I see that Sears sells a 125 PSI compressor for $99, however. Is that sufficient pressure to move these pistons? If it means anything, I weigh about 160 pounds and it took a lot of my upper body weight to press the pistons back in the first place. I have other uses around the house to justify the purchase if that's the case.
Contours if you are going to take the caliper off the bike and take it somewhere to give it a blast of air make sure you put something between the pads to stop them coming out too far and falling out ( and then not being able to get the disc between them). Try a small amount of air first as they can move pretty violently. If everything is clean and new it shouldn't take much.
Use only clean brake fluid on assembly.
Also I believe the seals used to be tapered but are not now. Makes it a bit of a sod to get the pistons in without damaging the seals. :x
Try this link for some tips. Good luck. It will be easy next time.
http://www.britbike.com/forums/ubbthrea ... Post213419
Use only clean brake fluid on assembly.
Also I believe the seals used to be tapered but are not now. Makes it a bit of a sod to get the pistons in without damaging the seals. :x
Try this link for some tips. Good luck. It will be easy next time.
http://www.britbike.com/forums/ubbthrea ... Post213419
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Mark F said:
Yes, they can move very violently, so you definitely don't want your finger in there to stop the puck from moving when the the air is applied.
I once had a friend smarter than I help me with a stuck caliper, and he placed a thin piece of wood into the caliper to protect the caliper body and stuck piston. The release of the piston was surprisingly violent, and if the wood had not been there I think the opposing bore, the piston, or both, would have been damaged. So if you enlist the aid of someone to help you with this, be proactive about supervising the entire affair.
And there may be brake fluid behind the pistons that could be released in all directions when the piston is released, so shrouding everything in a towel might be wise.
Safety glasses, of course, and brake fluid is bad for paint, too, especially when said paint has been applied to a Norton.
Mark F is discussing the pucks, but I think you want them out of the equation for this operation -- I'm assuming you'll be removing the caliper from the forks for the application of the air.
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Robert_Norton said:
I have read a lot about using compressed air for this task - all the same good advise here. Yes, I do have the caliper off at this point. I will be using a piece of wood between the pads out before I do this just in case the pistons are reluctant to retract again. Funny thing is that it wasn't such a problem to remove the pistons when I initially went to clean everything. I used Liquid Wrench followed by lots of brake cleaner. Once I got them out there was only tar and slight corrosion and it was limited to the exposed parts.
I'll report back later today.
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Not saying it's a good way to do it (I'm expert at finding all the wrong ways), but I put compressed air to the caliper with it mounted on the bike, pads in place. My thinking was the tiny stroke of the master cylinder just doesn't move enough fluid to complete a bleed job that also has to move the pistons out (unless one is willing to pump for several days). Mount the master at a slight angle with the bars turned (so the M/C is at the highest point in the system). Reverse bleed (pumping fluid in at the caliper) and the air rose out to the M/C. This was followed by the often given advice of letting it sit for a few days so bubbles could make their way up the lines. To help those bubbles move I lightly rapped on the lines from caliper to M/C every time I went by the bike for a couple of days.
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Ya should tie the lever back to grip when leaving it at high point for the tiny layer of bubbles to gradually creep up off surfaces and hose. Rap-pa-tap does helps them crawl up and float free faster. Only time I get brake fade on a number of hard in a row uses was when I short cutted the air clearing, so left if open a night and could not induce fade afterwards. Too bad not yet at just bleed and go stage.
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Time for an update. It took me a while to set up and get used to the new air compressor . A slight blast at 80 PSI finally got the rear piston to budge. Neither piston is moving very easily so I'm going to disassemble and clean everything once more and use brake fluid to lubricate the seals this time. If I still can't move them with my fingers I will buy a new pair of pistons. As I stated earlier, the pistons had a lot of tar near the outer edges that I cleaned up by buffing them with emery cloth. But there were also a few tiny pits in the metal that no amount of buffing would remove.
The final question (as dumb as it may sound): is that enough reason to start with new pistons?
Thanks in advance for all your input!
Peter
The final question (as dumb as it may sound): is that enough reason to start with new pistons?
Thanks in advance for all your input!
Peter
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Gee , I guess you didn't know you need stainless. The original steel will only continue to corrode and muck things up. So that would be backtracking. This thread also let me know the seals have been changed from original rebuild kit style. Go figure.
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Torontonian said:
That's another of my questions answered - I would not have known the difference between regular steel and stainless steel by looking at them. So you are saying the fact that they are pitted means they are not SS components. Should I assume that none of the original parts is SS, too? (Got my Newbie hat on).
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contours said:
Emery cloth? How fine? Can you get a mirror finish?
Pistons are relatively inexpensive, starting at $30 ea for stainless from Old Britts.
http://www.oldbritts.com/14_061896.html
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swooshdave said:
Ha ha - well I had our machinist at my workplace turn them on a lathe while holding the cloth gently against the corroded edge. It didn't turn out too badly, but I guess that doesn't mean it's functional.
I've had my eye on a pair of pistons on eBay selling for $51 and free shipping.
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dennisgb said:
No Damage that I can see. The worst of it was the tiny particles of sealant still showing up from the previous owner's hand at flushing the brake fluid (I assume). The guy sure didn't take the caliper apart or I would not have seen this problem in the first place. I got the impression the restoration expense was tightly monitored.
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contours said:
For me brake parts are not one place I want to go cheap or buy from someone I don't trust.
If you want more help post some pictures of the bore and pistons. When you say corroded do you mean rusty or something else? How much did he have to take off the pistons to get them clean? Thousandths will probably make a difference here. Also where did you get the rebuild kit from?
No reason to try to save pistons that are anything but dirty.
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swooshdave said:
That's damn good advice! I would qualify these pistons as aged - dirty beyond cleaning and corroded enough to show pits. My inexperience led me to believe the corrosion was not on the surface that mattered, i.e., the portion that slipped past the seal. But my experience is next to NIL, so maybe it does matter. I agree, pistons are certainly affordable. I was just trying to avoid throwing money at a problem that could possibly be solved with a little effort on my part.
Thanks for the help!
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contours you can certainly reuse the pucks if the working surface still seals on the rubbers just realize they are now more vulnerable to rusting again, so keep fluid fresh and let us know if any downsides happen or new rust hindering removal again. Likely have to wait till next owner cussing at the mess. Vendors got to eat now and then too.
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One thing that can happen if the surface is turned down or there is some scratching in the finish is the pistons may cock a little and bind up in the bores...from what you described in your earlier posts this sounds like it might have happened. The only caveate would be using petro lube rather than brake fluid when you assembled. I usually wipe some brake fluid on the bores and the pistons and seals before assembling. You should have been able to just push the pistons back with your fingers on the bench...this is what tells me the pistons "cocked" in the bores...easy to do...and they will lock up when it happens. You can get away with some clearance on the fit, but need to use care when assembling. Once the brake fluid is bled and is behind the pistons binding is no longer a problem.
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