coolant system bleeder valve screw broke off

I was recently having air in my coolant system, and my boyfriend had been bleeding the air out by using the bleeder valve. But when he was tightening the screw back, the top of the screw broke off. and we have tried getting the screw out but the screw is to soft. so we are going to replace the whole hose/pipe or whatever its called. but we can't find the right name for the part/parts that we need. and it is making me go bonkers trying to find anything out. my brain is hurting. so please help point us in the right direction.