The entire brake is identical to what's on the disc brake 350 twins and the 400-4.
There are a few things that cause problems with piston retraction.
- Corroded piston seal groove in the caliper bore. This should be absolutely clean. Water gets in from the outside world and although it shouldn't get past the seal it will get in the groove and cause corrosion plus any dirt in that water stays. Inspection with a dental mirror will show what's there. A bronze Dremel brush is usually suggested.
- Corroded piston, worn seal. The piston hard chrome layer is pretty thin, and rust spots show up when it's compromised. This usually shows up with new pads when the piston is now further in and the seal is on the corroded spots. There are SS and phenolic replacement pistons available. The seal does get old, hard, and wears out. A new one is pretty cheap if your piston looks great.
- The MC relief hole blocked. Not common but they can be. Easy to check: with reservoir level full it may not be visible but with it low you see it clearly, when the lever is pulled the first bit of movement pumps fluid out of the hole and that's very visible as a bump on the surface. Be wary though, it can squirt right out and brake fluid is very bad for paint.
- Air in system. The retraction is partly from the piston seal relaxing and partly from the MC sucking back a bit of fluid when the lever is released. Any air in the fluid will be compressed on braking. On release the bubbles expand again. The expanding bubbles mean that the small fluid pullback just goes to that expansion and not to retracting the piston. One culprit is the tee joint for the brake light switch, very hard to get all the air out of there. I suggest reverse bleeding with either a pressure bleeder or by taking the caliper off, pumping the piston out but not past the seal, then forcing it all the way in while holding it bleeder up and rotating and tapping the (dismounted) tee while doing the pressure bleeding or piston forcing. A C clamp works well. Repeat a few times. Watch the reservoir for bubbles coming out the MC relief hole but be super cautious about fluid squirting out strongly and dissolving spots on your paint, a plastic tank cover is advised.
- Tired rubber lines. Similar to with air bubbles, the MC pullback is used up by the relaxing worn rubber lines. By now they should have been replaced years ago if original. Using braided SS lines is a good idea, and a single line is best. An MC switch banjo replaces the tee and switch. If you remove the caliper line fitting steel taper seat (a sheet metal screw into it then pull out with pliers) you can use a fitting direct to the braided SS line, avoiding the hard line and connection to it.
It's a very very simple brake with design problems that can't be avoided. But it can work very well and is a big improvement over SLS drum brakes (ie almost every Japanese bike of that era), and won't fade on heavy use like even DLS drums will. You won't be doing stoppies but probably can get the front tire to howl in heavy braking.
Even when ideal the pad may rub slightly after braking. Riding a bit should push the piston back a bit for no friction. But it should not be noticeable riding, just some friction spinning with the wheel off the ground. If there's a lot of friction something is wrong. Also if it doesn't go away in a few minutes riding. If the disc or caliper ever gets hot after a ride when you haven't been using the brake - like after a highway stretch then coasting/downshifting/back brake only to a stop, the front brake should be dead cold, fix that fast.
