Radiator caps go bad?

[NikFu S.]

You learn something new every day, as they say :/

Does it seem like my questions are easier than everyone else's or what? :p

Not but 10 minutes after my last auto-x I noticed a coolant leak from a distance and found the area under the battery wet.
Checked some things... nothing out of place.
No biggy. I drove really hard, something probably got temporarily wacky.

Today, I mobbed out to an inspection, noticed coolant leaking from about where the radiator is.
Checked it out, could not find any leaks or even any wet spots aside from the oil pan, which was odd.

Drove slowly and solidly afterwards, and made an apointment with Dan the man.
We leave it running, he pops the cap off, stares at the drink for a sec, tells me to shut it off.
Shows me the radiator cap.
He says a good cap should take about 15-20 lbs of pressure to depress that spring.
He was compressing it fully and with little effort from his thumb and guarantees getting a new cap would probably fix the problem.

I never knew.

Drove home slowly, no leak.

Thoughts?