1976 peugeot 10-speed bike correct tire pressure?
First, and especially with a tire that old, check it for wear.  If it's cracked and dry-rotted, replace it.  If there are visible rips in the sidewall, also replace it.  If neither of those are the case, look at the sidewall of the tire.  Most tires should have a PSI recommendation molded in the rubber of the sidewall somewhere.