My 70 M-20 has developed a new habit of popping out of fourth gear when applying the brakes about half of the time. I rebuilt the trans about 5K miles, 25 years ago with new synchro and bearings etc. I am concerned there is wear on the input gear that is causing this. Not looking forward to dropping the trans after just installing the new 461 6 months ago. I installed a shorter 3/4 lever and made sure the shifter bump stops were correct, and they are. I even removed the bump stop thinking it was not fully engaging the gear, but seems to have full travel.

Does anyone have any ideas if there is something else I can check before pulling the trans?