I have a 91 Dt466 that I replaced the liner O rings. The engine has over 200,000 miles on her. The district didn't want me to do an in frame due to cost.Ever since then when I come back the surge tank is bubbling inside.I pressurized the system to 18 pounds and let it sit. No loss . After I also noticed coolant on the springs and frame. So I started bus up and no pressure build up exceeding 16 pounds. When when I removed the tester I touched one of the hoses leading to the tank it released air bubbles into the tank. What could this be ? I do notice when the bus is being run hard the heaters will work and then go cold and then heat up again. help !!
Some things I would try: Have you tried pressure testing after the engine is warmed up? Or doing a complete drain/refill to be sure you get all the air out?
Are you opening up the top of the system when adding coolant to bleed the air out? Does this bus have manual heater control valves? Are you opening them when refilling the coolant? Any water on the oil dipstick?