I recently did some work on the front master cylinder of my 86 R80G/SPD+. Bike has 90k and I've never rebuilt the caliper or the master cylinder.
I wasn't planning to rebuild anything because the brake was working fine. But as is usual for me, a simple job turns into a nightmare through a comedy of errors. It all began with a frozen screw holding the reservoir cap on. I was not able to remove it, so I bought some used parts on the IBMWR Marketplace... a complete master cylinder (part #23 below), piston and reservoir.
Put it all together and everything worked fine. Well... there was a very small leak from what appeared to be the cylinder. After closer inspection, the leak was coming from the junction between the reservoir and the cylinder (part 23 below). I used some assembly sealant to fix that, re-assembled, and everything worked fine.
Then, to make a long story short, a job involving an upgraded handlebar resulted in the need to purchase a new part #11.
When re-assembling everything, and prior to filling with brake fluid, I mistakenly squeezed the front brake lever and the piston got stuck, hard. So I disassembled, unstuck the piston, reassembled, filled and bled the system.
Now I am having the sticking problems. One problem is that the brake will stay on after releasing the lever. Another problem is a very sticky action when applying... squeeze/nothing... squeeze harder and something breaks free and the brake is applied.
The cylinder *looks* fine... it is smooth and clean.
I have two pistons (part #12) spare... one from this bike and one from a previous bike, so I am first going to try one of those. Neither looks worn, they both look the same as what I bought on IBMWR, but I am not sure what I am looking at.
Not sure what to do if the same problem happens afterwards. A new piston and cylinder is $200.
Any advice would be appreciated.