You should finalize the cab adjustment before you adjust anything brake related.
The master cylinder is mounted to the frame. If the cab is not in the correct position on the frame, you will run into issues like you are having.
Adjustment of the pushrod involves obtaining a specific distance between the brake lever and the toeboard (you will need to look that up in the Chevrolet Truck Repair Manual). If your cab is not positioned correctly, fore and aft, up and down, and side to side, that gap between the brake lever and the toeboard will be meaningless and may adversely affect the manner in which the master cylinder functions.
