I dont remember if I complained about the front brake before. It would pulse or surge when applied. I re-surfaced the rotor and it helped for a little bit, but it returned. I found a new front rotor for about $30 so I ordered it. Safe stopping is important. I got it changed, but then the sticking master cylinder issue was exaggerated. Pulling the brake lever would begin to engage the pads but then the lever would stick. I'd have to pull harder to get through the sticky part and it would apply too much brake. I considered a whole new master cylinder unit for $40. I thought I should at least look at it first to see if it was something simple.
I pulled apart the top of my scooter and pulled the master unit off. One circlip and the entire brake was apart. Very simple device! A plunger with two cup seals and a spring go inside a cylinder. I could see the spot on the inside of the cylinder where the plunger was rubbing. My guess is the screw on the handle had dug a divot in the end of the plunger. Pulling the handle would then force the plunger to one side of the cylinder and make it rub on the wall. Over time it wore the cylinder and plunger.
I made up a hone and cleaned up the cylinder smooth again. I lightly touched up the end of the plunger. Clean everything really well and re-assemble. I made sure to turn the plunger so the screw would not line up with the divot again, and put a little grease between them. Put it all back together again. It took me A bit over an hour. Now the front brake is smooth and firm. I didn't realize what I had been missing out on!
I'm sure I'll need to replace the plunger, and now I know what size I'll need. Ride safely!