There is n-o manual reset option for the late 200 brake junction block (don't know about early seventies). The piston is self-centering by internal springs and will return when pressure differences in the system disappear. If it doesn't, the piston is stuck because of a clogged junction block. It can be disassembled, cleaned out and reassembled with a small amount of brake grease but sometimes you just have to replace the junction block. Always try a brake fluid flush and some exercising with the brake pedal first, though.

This is a common misunderstanding, almost certainly because of the 140/160 design which h-a-d to be reset manually (loosen/retighten the electrical switch) when the failure light was set.

Martin
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'65 121, '73 165