From the Yahoo FT500 Ascot Owners Group: 
https://groups.yahoo.com/neo/groups/honda-ascot/filesRear Brake Calipers that can be used to replace the FT500’s rear caliper.
Below is a list of verified suitable replacement calipers for the FT500’s rear brake. Also, at the bottom is a great tip for unsticking a badly stuck brake caliper piston.
Assemblies where 43101-MA3-006 is used
 1981 CB750F A - REAR BRAKE CALIPER
 1981 CB900F A - REAR BRAKE CALIPER
 1981 CBX A - REAR BRAKE CALIPER
 1982 CB750F A - REAR BRAKE CALIPER
 1982 CB900F A - REAR BRAKE CALIPER
 1982 CBX A - REAR BRAKE CALIPER
 1982 FT500 A - REAR BRAKE CALIPER
 1983 CB1100F A - REAR BRAKE CALIPER
 1983 FT500 A - REAR BRAKE CALIPER
Submitted by “wesley112977”
 the caliper from a vf500 fits - i know this because i've done it Submitted by “martinrook”
 CB1100F-D 1983 1100 CB1100F Street
 CB750F-B 1981 750 750 Super Sport Street
 CB750F-C 1982 750 750 Super Sport Street
 CB900F-B 1981 900 900 Super Sport Street
 CB900F-C 1982 900 900 Super Sport Street
 CBX-B 1981 1047 CBX Street
 CBX-C 1982 1047 CBX Street
 FT500-C 1982 500 FT500 Ascot Street
 FT500-D 1983 500 FT500 Ascot Street
Submitted from Zedder parts cross comparison software.
To unstick a badly stuck caliper piston: A friend of mine turned me onto the grease gun trick for removing stuck caliper/master cylinder pistons--works awesome. You really don't need an adaptor to make it work. Just unscrew the end from the grease gun and carefully thread the grease gun into the caliper/master cylinder. You'll only be able to get a couple of threads started, but that is enough. Several pumps of the grease gun and the stuck piston's out!!
Jointly submitted by “oldguzziguy” & “Antonio”
You can also add the 1987 to 1990 Honda CBR600 rear caliper to the list of rear calipers that will fit the FT500 Ascots.
Submitted by “oldguzziguy”
Part # = 43101-MA3-006
Part Description = CALIPER, REAR Model Count = 9