XS500 Master cylinder/brake line

Yes, it does appear that your torque figures are way off. Gary has provided a corrective thread, which includes general torques specs in relation to bolt diameter, which is your fall-back.

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When they began rounding I thought maybe I confused the outer caliper bolts with the bridge bolts, but I'm fairly certain the bridge bolts are the smaller allen headed ones that drive from the center of the bike out. The 17mm bolts are fine, and required about half the torque which I also found strange since they are much larger.
 

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The caliper is halfway rebuilt. I changed the seals on the outer caliper half, but the inner piston didn't budge with compressed air. My hope was pressurizing the line with fluid would get the stuck piston to move, but no dice there. Good tip on prefill, hadn't thought to try that.

Hi minus,
try higher pressure. Thread a grease nipple into the caliper's fluid connector. The average side-lever grease gun puts out some thousands of PSI.
 
Sorry to be dense, but reading though this thread it's not clear to me which is the correct torque value for the smaller allen key bridge bolts.


According to this ^ post I was torquing within the correct range, as 9.5 m kg = 825 in lb = 69 ft lb. If that's the case, I'm hopeful the threads in the caliper are fine and something just went awry at the head, as I did round out the heads a bit at that spec. Appreciate anyone clearing things up a bit more.
 
If I was in your shoes I would go with the torque figures recommended by Gary in post #23.
I'd feel OK about using the lower figure 7.5 m-kg.
I suggest getting those old allen bolts out and tossing them.
Replace with properly hardened bolts from your Yamaha dealer or from an industrial fastener outlet which focuses on nuts and bolts.
Make sure your allen socket wrench is high quality and in good shape.
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edit: the bolts are discontinued from Yamaha. There are some NOS available on ebay. Your local fastener store will likely have hardened items.


BOLT 3​

306-25826-01-00​

by Yamaha
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Front brake lever is firm, new rear brake shoes are installed and I put some new tires on too. As advised, patience went a long way and I was eventually able to bleed it. The sweet spot seems to be squeezing the lever just past the point of making contact with the master cylinder piston. Stopping and holding at that point even produced a steady stream of bubbles in the reservoir in my case.

Once the lever firmed up, I was able to get the seized piston to move and then finally take it the rest of the way out with the air compressor. After cleaning and installing new caliper seals, I sheared one of the seals seating the piston :doh: Thanks to my lack of patience and a tendency to throw money pretty carelessly at this project, I already had another replacement caliper and other misc parts on the way so that's what's installed now.

Strange thing is the brake fluid turned a murky black during the bleed process with the new caliper. I'm low on brake fluid now, after I get more I'm going to flush it out and hopefully that will resolve the issue.
 

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Front brake lever is firm, new rear brake shoes are installed and I put some new tires on too. As advised, patience went a long way and I was eventually able to bleed it. The sweet spot seems to be squeezing the lever just past the point of making contact with the master cylinder piston. Stopping and holding at that point even produced a steady stream of bubbles in the reservoir in my case.

Once the lever firmed up, I was able to get the seized piston to move and then finally take it the rest of the way out with the air compressor. After cleaning and installing new caliper seals, I sheared one of the seals seating the piston :doh: Thanks to my lack of patience and a tendency to throw money pretty carelessly at this project, I already had another replacement caliper and other misc parts on the way so that's what's installed now.

Strange thing is the brake fluid turned a murky black during the bleed process with the new caliper. I'm low on brake fluid now, after I get more I'm going to flush it out and hopefully that will resolve the issue.

Congrats on making progress.
The black stuff is likely to be just rubber particles.
Ideally, I would start the thorough flush by removing all the bad fluid from the master cylinder reservoir using a suction, like a turkey baster. You could also soak it up using toilet paper or such.
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