m-12 transmission

Does it have a vacuum modulator and is it working OK? Is there a cover on the tail housing for access to a governor which could be sticking?

They seem to shift into second at very low speed unless you have the boot into it.

If the trans. is an original M12 Shift Command then the shifter sequence should be P R N D 2 1. If the trans. started life as a Flash O Matic then the quadrant sequence would be P R N D2 D1 L. With the Shift Command quadrant you get first gear start in either D or 1. With Flash O Matic you get first gear start in D1 or L. Note that if you have a Flash O Matic setup with a Shift Command quadrant in "D" then you'll be starting off in second gear.

Since you said that the trans seems to immediately shift from first to second then i would suspect that the vac. modulator pressure is set way low. The pressure is set by inserting a very thin, flat tip screwdriver in the back of the modulator once you remove the rubber hose. Turing the adjusting screw inward (clockwise) will raise the line pressure and the shift points. It takes quite a few turns to make a difference. Keep in mind that the factory TSM trans. pressure specs were set for comfort not for performance. We'll assume the vac. modulator is the correct part and is functioning. It also will need to have the correct length pushrod but unless you changed these items out these should be correct if they're the original factory installed parts.
If you suspect the vac. mod. is out of adjustment then you'll need to experiment with the pressure setting adjustment until you feel you achieved the proper shift points vs. throttle opening. If you set it too high the trans. may shift very late on the 1-2 and very hard.