A/C switches to defrost under load at times?

Bought the check valve, YG 360 - for my car listed above six, auto. Sometimes the A/C switches from A/C to defrost on a grade, most often it does not on same grade. Engine rebuilt at about 200,000 due to almost overheating engine, and oil appearing in coolant reserve occasionally.

Cannot find where YG 360 goes, was looking for a vacuum part diagram, or repair info for an amateur like me to see where this part is located. I was thinking it would be near firewall.