film build up under radiator cap? on 1991 Pontiac Bonneville

i let my car sit in the garage for a year and a half ,
did an oil change and it runs fine, but there is a brown film,
under the radiator cap? will a flush work?

Asked by for the 1991 Pontiac Bonneville
I would flush the cooling system and possible remove the thermostat while flushing the system and replace the thermostat and radiator cap and antifreeze when servicing the cooling system When using a cooling system flush product like Prestone cooling system flush I like to flush the system out well and add a few table spoons of baking soda afterward to ensure acids have been fully neutralized.
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