On the Plus 2 which has a similar handbrake system there was an update to the central tree where all the linkage connects together to make the system more effective. This was for the early versions, had to modify my 1967 version tree and it now works fine. Did it years ago so can’t remember the exact details, but it did include cutting and rewelding to change the angles between the arms on the tree. Not sure how to confirm which version you have.