2010 Dodge caravan no heat

We had no issues at all untill the plastic y fitting leaked. Van never over heated. Thought trapped air was the issue mechanic used a push pull flush on it to eliminate air trapped in the system and replaced the thermostat. Line going to the heater cores fron/rear hot return lines are cold. Some times at higher RPMs it will blow hot air. I and the mechanic are at a loss. He suggested plugged heater cores but they flushed fine. Don't want to spend 900 replacing heater cores if it could be something else.

Try bleeding air from system like this remove cap from overflow tank fill with antifreeze to full mark leave cap off and start vehicle and let idle you will notice level in tank start to rise this is normal once thermostat starts to open level in tank will drop add more antifreeze to full mark then put cap back on.

exact same issue, exact same vehicle...