This is how I burp the system:
When the engine is cool (and running)
I take the crossover tube plug out, then jam a funnel in it.
Make sure there is enough coolant so the fluid is in the actual funnel. Let the bubbles start making their way out the funnel. When sufficient time has passed, the level will usually go down and no more bubbles will be present.
If at this time the lower hose is still cold...look at the thermostat.