What Pinto5.0 said is correct, dunno what I was thinking. My post on trapping air is correct but if you mount the core with vertical outlets he is 100% right. I'm an idiot.

'You want the flow going in the lower inlet & out the top to avoid air pockets in the core'

X2.

My ex's Villager minivan core wont bleed trapped air unless a dealer does it. I installed one of those flush/fill caps in the top hose & I use a mustard bottle filled with coolant to fill the core while running it until I have heat. Works perfectly for $10

I put the filler in the upper hose. I run the engine up to temp then shut it off, unscrew the little fill cap, squirt coolant until the hose fills then cap it & run the engine some more. 3 to 4 repeats of that & the heater core is full. I wasn't too clear in that last post but eventually I get the air out. Dealers pressurize the water in to get the air out but I'm not paying for that.

Problem with GM is waiting for the thermostat to open before you get any heat

My dually's an 89. I don't get heat till the temp hits about 160