C6 not shifting into 2nd----sometimes

Running a '74 C6 behind a mild 466 in a drag race application. I manual shift the trans, using a B&M Pro Stick, but sometimes when I shift into 2nd under full throttle it has failed to make the shift and stays in 1st. This gets spooky because it would be easy to overrev the motor in this situation.
Is this a valve body problem?
Trans is essentially a stock C6 with Turbo Action shift kit.

I'm no expert, but that is what happen to me. Sounds like your valve body. Are you using a transbrake? Keep your rev limiter adjusted at a safe setting.

Did you replace your valve body or isolate the problem and fix the one you were running? I am not running a transbrake, just footbrake it and manually shift. I did install a Coan servo thinking this may help the situation, but did not completely solve it...just gave it a firmer shift when it does shift.
I have been considering just going with a reverse manual body and be done with it.

thanks
Rick

I had installed a new TCI Trans brake with its valve body They are having big problems with their equipment. My trans guy is sending it back to them for repairs. I am going to sell the trans brake once it is repaired, its new never use it because it never worked. And I am going to get one from Frank Merk.