The XS brake caliper inside and out.
The XS caliper is a simple one piston design with a single inner seal and an outer seal that is referred to as the dust boot. If your caliper(s) need servicing (remember, your XS is potentially 39 years old) then here are some pictures of a caliper taken apart.
This is the caliper body itself taken apart. You can see (red circle) the inner seals channel where it rests around the piston itself. This is the main seal area and the one of most concern. You can buy new inner seals from most everywhere but before replacing it make sure you clean out the channel area of any foreign debris.
The arrows (in red) point to the fluid channel from the master cylinder and brake line. The smaller one is for the brake bleeder itself.
Here is the brake caliper inner seal itself. About 10 cents worth of rubber but an important 10 cents.
This is the brake piston itself. The grove near the bottom (top of the piston) is where the dust boot seal clamps around. Removal of this seal reveals surface rust if any and a possible indication of things below. Take note of the smiley face on the piston, that is pitted chrome which could cause a hydraulic failure. This piston is ruined and should be replaced. The rust near the dust boot seal could be cleaned up but again replaced it instead.
This is the slider collar that the caliper floats on when it moves from braking. This collar is cad coated silver to prevent corrosion. Upon removal and tear down of this caliper, there was no grease inside it or on the bolt. There was also two dust seals on each end.
About 52 bux will get you a new piston, inner seal and outer dust boot, brake sleeve boot and bleeder cover. Its easiest to remove the piston when the caliper is off the bike but if the piston is stuck you might want to pull the pads and squeeze the brake lever a few times in order to pop out the piston or get it started.
I recommend cleaning out all brake fluid and debris with solvent (after caliper is bare) flushing with water and drying. Use a little brake fluid to insert the inner seal and piston but reassemble the caliper and put it back on the bike before adding more fluid.
Hope that helped someone…. cheers.