Power Steering issues after rebuild

Hi all,
This is driving me crazy, I converted this 68 barracuda from manual to power steering. installed all new hoses, a rebuilt saginaw pump (used original res). The box came from bone yard car. Before i put the box in i set it up and flushed the box out, i got dark fluid out of it unit it was clean. Then i put in a new pitman arm and shaft seal. Installed everything and it looked like the factory put it there. Filled the system and turned the wheels left and right to purge the air then started the car and did the same (car was up in the air for all of this). Everything seemed fine the pump ad little whine to it. Took it for a drive the pump noise seem to get louder and then the steering shaft blew. Got it home pushed the seal back into place for now. Refilled the system and the noise is worse. while i get the noise somewhat especially after driving the pump with low or out of fluid but my question is why did the shaft seal blow out? I really don't want to pull this box again.