Brake Master Cylinder Lube??

[screwylouie]

Just finished my custom brake caliper fix. I paid very close attention to the details this time when I disassembled the master cylinder. I noticed right away that the piston was sticking bad at about it's last 20% of travel when I released the brake lever. After I drained the fluid and removed the master cylinder assembly from the handlebars I pulled the piston out and there was a LOT of crusty black stuff stuck on the walls of the bore and on the O-ring (again). I'm now convinced that the "lube" that comes with the rebuild kit is drying up and making the o ring stick in the bore. I cleaned everything up real well again and reassembled the piston dry, definitely no good, wouldn't slide in the bore at all. I removed the piston and coated it with brake fluid and tried again, now it moves nice and free in the bore. Put it all back together, filled and bled the brakes and now it seems to work great again. Lever snaps back nice and smartly, I think even better than new. My spring extension is no doubt helping here too. I will post on this again in a month or so and report if this is going to be a permanent fix. I think the root cause is the lube in the rebuild kit drying up and making a sticky mess in the master cylinder piston bore. Time will tell.

 

[screwylouie]

I'd say the verdict is in and the HD assembly lube is guilty, it's been at least a couple of months now since I rebuilt my front master cylinder and it hasn't been sticking at all! Also, when I squeeze the lever I can see the fluid squirting against the inside of the little view port in the master cylinder cover every time.(it used to be intermittent). It would seem that the assembly lube was hardening a little and plugging up the relief ports and making the piston drag too. Someone said to use only brake fluid to lube the parts at assembly and I believe that was the solution. The lever snaps back like a champ every time now! YEEEEEHAAAAWWWWW!!!

 

[SE6A]

Great tips guys, I have the same problem with a 99 softail with 40k on the clock, have a re-build kit to fit tomorrow, I was wondering about lube, the kit , (Drag Specialities), didn't come with any so think I will go for cleaning it out and using the DOT5 fluid as a lube...