The rear caliper sits inside the rear triangle - seat stay and chain stay - so as far as I know, there is only one model of caliper that fits - a rarer than a photo of Mike Tyson in a library Hayes model.
10 years on being fed up with rims gound away by mud and grit, I'd thought I do a conversion to discs. My standard disc brake - 4-pot XT - will not fit in the space but so happens that Magura Marta front caliper will - just. The rear one, with longer mounting arm will fit but needs 180 mm disc instead of a 160 mm.
First photo shows the evolution of making the caliper: starting with a paper template, thin aluminum template to a 3 mm steel bracket. The steel bracket, not only being heavy, was difficult for me to bend to be square and get flat so I made a more accurate one from 6 mm aluminium angle.
Having measured other caliker mounts, they tend to be 8 to 10 mm thick so I made it once again from 8 mm aluminium angle as the 6 mm may be too weak.
Last photo shows it painted and fitted.
Only draw back is the bleed nipple sits below the pistons so it is not possible to bleed the caliper - easily remedied by undoing the 2 bolts and positioning it behind the seat stay.
