Heater hose tee (connector/fitting)

I have a cooling issue with my car. I have replaced the head gasket about a month ago and had replaced the water pump shortly before that. I just replaced the coolant plastic fluid reservoir last week due to a leak at the hose connector next to the reservoir cap. With this new reservoir and the pressure being sealed on (at least I think), there is not much pressure escaping, so yesterday after about forty five minutes drive alternating between A/C and no A/C I pulled up into the parking lot and as I was coming to a stop the heater hose tee broke (again). This is the plastic connector near the firewall that connects the hose coming from the top of the fluid reservoir and the two other hoses towards the bottom read of the engine. It seems there was pressure build up and since this was the weakest point, it broke. My question now is should continue to replace this connector with a plastic connector such as the Dorman #47147 or should I use a metallic part such as the Murray #84544? I am afraid that if I use the metal part, the pressure will go elsewhere and break something harder and more expensive to fix rather than this $5.00 part.
Also, the heat gauge on the dashboard remained at the half-point the whole time, which tells me that the engine was not overheating or anything. This is now the second connector I replaced, but as long as the car is in motion, or I turn on the heat inside the vehicle, I can go longer without experiencing issues.