1993 Dodge Dynasty Can't fully close radiator cap.

Engine Cooling problem
1993 Dodge Dynasty 6 cyl Front Wheel Drive Automatic

Just recently replaced firewall side head gasket which followed new radiator, thermostat, electric fan repairs. First event was damaged radiator caused from fans not ever coming on and severe overheating. Installed new radiator after fixing fan problem and new thermostat, but when I filled system and started engine after warm-up excessive pressure blew block freeze plug after which compression sounding air noise began untill new plug was installed. Eventually water was coming out of tail-pipe.I removed spark plugs one at a time, locating the cylinder the leak was in and replaced that head gasket. Reassembled after replacing gasket, and remedied water leak and changed water-mixed oil with new oil&filter. Still I can not close cap fully(I leave cap in half or vent position instead). Engine totally cold, first turn over without starting it pushes water out with every rotation. No leak observed from water pump ever. Chacked all spark plugs for deposites or discoloration and none abnormal. Backflushed system and clear. Could pressure result from Faulty water pump even if no leaks?Usual current or circulating pressure usually visible with cap off and engine running is not present, just up and down of still water at idle and burst over flow when throttled. Also, could catylic converter be problem factor?Please help!