Probably just your heater tank filling itself up, just a lousy design, that's if the radiator was drained, pull the hose behind the radiator cap off- the one that goes to the radiator & suck till you get coolant (keeping the header tank full) that should get rid of any air in the system............it tastes like shit so go slowly.

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Beaten up old 98 Gen 2 Liberty, 13.0, stock V3 STI running gear.
07 Transit motor home
69 VF & 70 VG Valiants needing full restoration
Yamaha Zooma 50cc rocket hopefully going with the Transit.