Does anyone know the actual function of the hose that attaches to the reservoir in the pic? It comes off the radiator on a small fitting and ties to the tank.

Well anyway after pressure testing many times I found this connection was really loose and it appears to be why I may have had air coming into the system. The strange thing is the vehicle runs great, and only gets up there on hot days but the air pockets were just a nuisance. I can rule out hg, oil cooler, heater core after a ton of tests. I can't wait for cooler temps in the northeast.

 

that's for air in the radiator to vent to the tank via the little hose.

 

Thanks, is there a one way valve design for venting or is this basically a boil over contraption meant for coolant coming off the block?