Leaking radiator Peterbilt 379

If the radiator is mostly aluminum and a few years old that could be the issue.
Or if radiator is aftermarket some are made poorly. Or poor recore work. Have radiator leak tested.

 

Leaking from the breather line? You mean overflow tube? If so, could be a bad cap. Or could be a pressurized coolant system. Sign of a more serious problem. Rule out any heater control valves, on cab or bunk that have an air line running to them. Air over electric,( or electric over air?) heater control valves were used in 99. Not sure on Peterbilt’s or not. Turning off water supply valves from water pump, will isolate it if that’s the problem. When air controlled valves go bad they will pressurize the system, often resulting in a mis- diagnosed head gasket or bad liner problem. Sounds like you need to get system pressure tested. See if it’s making excessive coolant pressure. Any shop can do it.

 

When i had my 3406-e it would burp out coolant on a hot day as the coolant rose to a higher level in the tank. Even though it wasn't overheating. If i put the engine fan on anyway, it solved the issue. Also may add that topping the level to the very end of the filler neck made it do more burping until the level got down to its normal running amount of coolant. All other great advice, but i am hoping its something this simple like this.