Is the fitting to the heater line @ manifold clean? Had one of those blocked with excess silicone. Water pump return tube clear?
Hose delaminated causing a flap to fold down partially blocking the heater hose.
Have used CLR in the past with heater cores that were blocked up with deposits.
Still may have to burp the heater circuit again to force out the air. Is there any leaking on the floor? 120psi could cause a whole batch of new problems.

sprntpshr