1989 K100rs 8v. The rear brake sticks on , even when I don't press the pedal. It has ABS. It eventually frees its self and cools down.Its happened a couple of times recently. I have cleaned all the pins and pads. Also the front brake lever seems to gain pressure as time goes on . The lever is quite slack initially but builds up.

Rear brake - one of two things:

1. There's crud in the master cylinder blocking the return hole
2. The pedal is adjusted so that the piston is permanently over the return hole.

I favour (1) because you say that it returns 'eventually'. Fortunately the m/c comes apart quite easily for cleaning and overhauling.

Front brake - there's air in the system. As you use the brake the (tiny) bubble is compressed more and more and so the brake hardens up. Left overnight, the bubble gets a chance to expand and give you trouble the following day.

Could be that your brake lines are getting old and disintegrating internally.

Motobins has decent prices on braided stainless brake lines.

I had a similar problem on the rear after doing the Chi m/c replacement. 
1st, the pedal return spring was missing (so not taking pressure off the m/c)
2nd, the adjustment screw didn’t have the prescribed gap resulting in light, albeit constant, pressure on the m/c

duck wrote:Could be that your brake lines are getting old and disintegrating internally.

I saw it on one of my bikes.  Brake lines full of mouse turds at the calipers.

What does chi m/c replacement mean

Thicklotto wrote:What does chi m/c replacement mean

Chinese-manufactured master cylinder.