front brake XS2

tmikeyb

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I am still working on getting a nice working front brake. My new pistons were so tight in the caliper that I had to drive them in with a mallet. They would not release once they were installed on the bike. A member here told me this was too tight and that the pistons should not be that tight. I disassembled it again, and spent a lot of time scraping out the seal grooves. I dug a lot of rust specs out of the grooves. I cleaned the walls again. Then I rubbed the walls with 600 grit sandpaper and light oil. I finally got one side to the point where I could push the piston in with my hands. I worked on the other side for at least as long, but could still not push the piston in by hand (with the seals in place). I finally got it to where I could tap the piston in pretty easily with light taps from a mallet. Now it's all back on the bike. It is much better, but the brake still doesn't release completely. I have instant brake, with no play in the lever. Is this normal? Then the brake will back off only very slightly when the lever is released, just enough that I can push the bike with a good bit of force. The brake does not completely release. I have no experience with disk brakes, so I don't know if my piston{s} are still too tight, or if the master cylinder is somehow not releasing pressure as it should. I find no help in the manual. I'm not sure where to go from here. Any tips, advice, or info is much appreciated.
 
WITHOUT the seals in place can you freely slide the pistons in and out?
They should move freely.
If they do then your seals are swelled or not to spec.
Heavy contamination with fork oil or other petroleum based products can swell brake seals. Did you soak them in something?
 
Once the brake system is filled/bled the master cylinder should not block the pistons retracting, you should see brake fluid going back up into the reservoir, push too fast and it might squirt up/out of the reservoir. Piston retraction will require more than thumb pressure once it's got brake fluid in it. With an old set of pads installed twisting a flat blade screw driver between them should retract pistons. Conversely if the bleeders are opened the fluid should drain from the reservoir down and out through the bleeders til it is empty. This might take 5 or ten minutes.
Have you rebuilt the master cylinder? (a must do)
 
OK guys, that definately tells me that my pistons are too tight. They are brand new, so there's not much I can do about that. I don't see any way that my caliper could be any cleaner. No, I did not soak the seals in anything. The first time I assembled it, I used a disk brake grease, which was made just for this. The second time I used only brake fluid. I have a 72 parts bike that I'll take the caliper off of, and hope for better results from it. Thanks for all of the help and info.
 
OK guys, that definately tells me that my pistons are too tight. They are brand new, so there's not much I can do about that. I don't see any way that my caliper could be any cleaner. No, I did not soak the seals in anything. The first time I assembled it, I used a disk brake grease, which was made just for this. The second time I used only brake fluid. I have a 72 parts bike that I'll take the caliper off of, and hope for better results from it. Thanks for all of the help and info.
That's a poor outcome for you, sorry to hear. I suspect the caliper bore isn't truly round, distorted in someway.
 
This has been a long ongoing fight, and for now I have lost. I finally gave up on my front caliper, assuming that it might be warped, as one member said. So, I took the caliper off of my parts bike, (also a 72). After taking it apart, it appeared to look slightly better than the first one. So, after cleaning it up very well, I cannot push the new pistons into this caliper with my hands either, even with no seals in place. I smacked them in with a screwdriver handle, which was slightly easier than in the first caliper. They still fit so tight that I had to push them back out using a grease gun. So, I have new pistons, and have tried 2 identical calipers. I had the same results with both, so I'm stumped, and need advice. I don't give up quickly or easily, but I don't know what to now. I'm beginning to wonder about my new pistons. They look nice and smooth. They are not out of round. They measure 1.894 inches, but I don't know what they're supposed to measure. I don't have a real good way of measuring the bores in the caliper. Do any of you folks know what I need to try now? Any idea what the pistons and / or bores should measure?
 
I get 1.892 with a mic and 1.894 with a caliper. Do not take that as gospel as I am not a machinist.
What do your old ones measure?
 
I can dig my old pistons out of the trash and take some pics tomorrow. I’ll also measure them. My diameter measurement was done with a starrett micrometer, but it’s as old as the hills, so I’m hoping it’s still accurate.
 
I can dig my old pistons out of the trash and take some pics tomorrow. I’ll also measure them. My diameter measurement was done with a starrett micrometer, but it’s as old as the hills, so I’m hoping it’s still accurate.
cough, when things don't fit, check new against original parts
 
Here is a picture of the original (thick wall), and new (thin wall) brake pistons. Here's the really important thing though. The diameter of the original is 1.892 inches. The diameter of the new is 1.894 inches. I feel pretty stupid for not checking that first.
 

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Here is a picture of the original (thick wall), and new (thin wall) brake pistons. Here's the really important thing though. The diameter of the original is 1.892 inches. The diameter of the new is 1.894 inches. I feel pretty stupid for not checking that first.
That makes the old one 48.05mm diameter. The new one 48.1mm. So 0.05mm (or 0.002") difference in size. That just might be enough to give you a problem.
 
Does your old one work?
The old ones each have a couple of rough spots on the outer side wall surface. I thought about cleaning them up and seeing how they fit. I threw away the new seals that I had bought because I pushed the pistons out several times using the grease method and I figured that the grease had probably contaminated the seals to the point where they would swell up. So, I can check the original pistons for fit without seals, but I can't actually check the operation until I get new seals.
 
Yes, I cleaned up the old originals and they slide right into the caliper very easily, with no binding or friction. So, this entire problem has been caused by the new pistons. I have contacted the vendor about it. We'll see what kind of customer service they offer. The originals had a few rust spots on them. I used a soft wire wheel to smooth the spots down. I don't really know whether or not to try to use them, mostly I'm afraid that the spots would cause undue wear on the seals. What are your thoughts about that?
 
Yes, I cleaned up the old originals and they slide right into the caliper very easily, with no binding or friction. So, this entire problem has been caused by the new pistons. I have contacted the vendor about it. We'll see what kind of customer service they offer. The originals had a few rust spots on them. I used a soft wire wheel to smooth the spots down. I don't really know whether or not to try to use them, mostly I'm afraid that the spots would cause undue wear on the seals. What are your thoughts about that?

pics?
very fine paper then a buffing wheel to round the pit edges if the rust pits are minor has worked for me more than once.
 
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