Re: radiator flush
Well it all has to do with gravity and package space. The rear heater core is nearly at the same level as the high point of the system in the front. If the rear core sits higher or even close to the same level say when filling it, ( such as nose down in a driveway and/or on an incline) then sometimes the air is trapped in the top of the rear heater core.
It was a known concern to the engineer who designed the system. There just wasnt any room back there to get the heater core lower in vehicle and maintain the comfort requirements. I errr he did try his best get it as low as possible.
Seriously any decent dealership or garage will check for air trapping and some can fill it under pressure something they also do on occasion in the assembly plants.
BTW your son's 94 T-Bird highest point is the fill neck and the radiator is also higher then his heater core which sits, I believe, about sternum level inside the Instrument Panel, This makes air trapping a lot less likely to occur. I dont know the design engineer on that vehicle btw.
All sealed systems will eventual work the air bubble out but then you dont have enough coolant in the system filling your heater cores from top to bottom and this reducing their efficiency.
Check the coolant level and if it's down add more. Make sure to put it on an incline and nose high. Idling it will help get it purged. Let idle for awhile ( 10-15 minutes). If this still doesn't help a trip in to the dealership is in order but be sure to mention the radiator change out just in case you got one stubborn bubble.
Glad to help,