Fill the cooling system, start engine, add some more coolant,now run the engine until the thermostats open.
You will have to drive the bus to get the coolant temp hot enough to open the thermostats.
Check the thermostat housing - some have a small petcock on them to bleed air from system.
Don't go very far, it won't be necessary - stay local.
Just need to put a normal load (regular driving) on the engine to let it heat up.Watch the temp guage, at about 180 the t-stats will open(depends on what t-stats you have).
Take some coolant with you on your road test - about 5 gals.
The coolant level in the surge tank will drop when the t-stats open,
DDEC might shut down the engine because it detects a "low coolant level" - depends on how your engine protection parameter is set. Be careful when you open the surge tank to fill the coolant at this time
Coolant is now hot
and there will be pressure to be released before you open the surge tank cap.
Fill your coolant to the proper final level, run bus again, final check coolant level.
Hope this helps.Good luck with your repair.