Twice in the past month I have had occasion to drain the cooling system on my 77 300d. Once to replace the water pump and then again today to replace the ACC servo. In both instance it took forever to get the air out of the cooling system. I filled the block through the top radiator hose, filled the radiator, removed the vent bolt on top of the thermostat housing, turned the heater on full blast and ran the engine. Both times it took a long time to get the air out and get coolant in the heater core. It took about a hour of fooling around to get the air out. I'd run it, shut it off, drive it around etc. in an attempt to get it full. I'm still not sure why the coolant finally began circulating through the heater.
I figured out in the process that if the temperature goes up when you run the heater, it means there is air locked in the heating loop of the system.
Anybody have any hints on how to speed up the filling of the system? Would loosening one of the hoses high in the heating loop help get air out of it? It looks like there is a small heater line high up on the firewall. I've never had this problem with other cars I have owned. The fact that both radiator hoses exit the block at the top seems like poor engineering to me since it means you can't really fill the engine by pouring coolant in to the radiator.

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1977 300d 70k--sold 08
1985 300TD 185k+
1984 307d 126k--sold 8/03
1985 409d 65k--sold 06
1984 300SD 315k--daughter's car
1979 300SD 122k--sold 2/11
1999 Fuso FG Expedition Camper
1993 GMC Sierra 6.5 TD 4x4
1982 Bluebird Wanderlodge CAT 3208--Sold 2/13