There should be a return spring around the outside of the cam spindle housing on the brake plate. Unfortunately, I had one break recently on the RH500 and found that they do not seem to be in production at present but I managed to modify a slightly different one to suit. Try both Draganfly and Singles Spares.
36 4G, 37 VH, 54 KH(A), 75 Healey 1000/4, 52/53 ex ISDT KHA (project).
Former Machine Registrar & Archivist, General Secretary and Single Spares Organiser (over a 25 year period).
Now Archivist once more - but not Machine Registrar.