When I was kid I had an upright Schwinn 3 speed with chrome wheels and there wasn't any problem getting the brakes to lock up the rear wheel and skid on the pavement...as long as the wheels weren't wet!

Without seeing the bike and test riding it I wouldn't be able to specifically say whats going on other than to guess like others are doing. You could try some brake cleaner fluid on a cloth and wipe down the sidewalls of the wheels.

However, as a word of advice and caution, you need to get rid of those steel wheels, there's a reason they were banned from being sold on bikes because they were dangerous and kids got hurt and killed on bikes with those wheels. If you haven't already experienced this you will, but even just a little bit of water on the rims would take you 10 as long of a distance to stop as you would if the wheels were dry, add rain instead of just a little water and you've got a disaster waiting to happen. So buy yourself a set of cheap aluminum wheels ASAP!