B2500 heater/AC frustration.
 

OK got an issue I've never rnacross and need some help, here's the story.
2000 Mazda B2500 son's first truck. Few years ago the heater core started leaking so a mech. by-passed heater core. After that AC won't work so son just dealt with it. I have the truck now, been parked a couple years so recently I decided to get it going for an extra truck. Replaced the heater core, thermostat, diverter, poteinometer and repair some vacuum lines. Truck runs good but no heat. The funny thing I can understand is the AC always seems to run even when in heat. Tonight I rechecked the vacuum lines, isolated between the switch and first plug and it would not hold vacuum, then from the plug at the firewall into the engine, still wouldn't hold vacuum. All the lines seem good. I unplugged the AC condensor and an obvious raise in interrior temps from the ducts. Why would the AC run when not in AC but heat? TIA for any suggestions.

200 is same as a Ford Ranger truck, ask at those sites.