I have not yet completely diagnosed my 603 diesel overheating issue but I did learn a little something in the process so far.

If you have not yet found the exact location of your coolant leak on your 126 chassis, check out this leak that I found in my coolant system.

http://www.peachparts.com/shopforum/showthread.php?threadid=33244

I'm sure that any leak will prevent the system from operating at the required level of pressure so I would think you have to determine if the system will hold pressure and if not the exact location of any pressurized leaks first.