Brake problem, need help...rear brake cylinder, custom fwd controils..push fast and hard, the brake works and stays put....push too slow and the brake sinks to bottom and will not return on its own with internal spring

 

That's a weird one, sounds like 2 seperate problems. The piston seal in the master will expand and seal under quick application but can still leak on slow push, bottoming out. The seal can be worn or damaged. As for the sticking at the bottom, check the angle of the pushrod at fully depressed, it could be forcing the piston against the cylinder causing a physical bind. That can be adjusted by loosening the cylinder mounts and angling it a little one way or the other. Sounds like a disassembly and rebuild kit is in order. Brake caliper is not probably the cause.

Good luck, hope this doesn't come too late

 

I had an issue with my fwd control rear brake. I didn't realize the extent of the problem at first. When making right turns in city/suburban riding conditions, it felt like the bike was falling out of a tree! I would at times stab my foot to the road to keep the bike from falling. Left turns seemed OK. The problem was very simple,.. The pivot at the brake pedal was binding up. I could lift the pedal by hand with a bit of resistance but it wouldn't fully release on it's own. I disconnected the brake rod and found that the pedal level was kinda tough to move. I tried spray lube but it didn't change anything. I disassembled the pedal from the bike and I had a hell of a time getting the pivot shaft from the pedal. I had to use a brass hammer to get things apart. I dressed up the shaft and Teflon sleeve with fine sand paper and scotchbrite. I needed to go at it twice to get the thing to work smoothly again. Right turns require more of a bike lean than left turns. With the bike still slowing down in a turn, gravity really has a chance to work. Now the pedal releases fully to the stop with some authority. If you can reach down and pull up on pedal by hand to the stop, it's most likely the pivot at the pedal. Dirt and grime will make it's way in. Keeping things lube is one thing but eventually, it should be taken apart for a true cleaning and inspection.
Hope this helps you.