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5.0L (1979-1995) MustangTechnical discussions on 5.0 Liter Mustangs within. This does not include the 5.0 from the 2011 Mustang GT. That information is in the 2005-1011 section.
Beg, borrow or steal a cooling system pressure checker.
I suspect you have a leak. It may not be letting fluid out but taking air in when you shut it off and let it cool.