That drain you're seeing fluid come from is actually for the a/c evaporator, to let condensation from it escape to the outside. That's just a side effect that it'll also allow coolant to drain from it, if the heater core ruptures, albeit a cool one.
As for the radiator cap: it needs to be rated 13-16 pounds because this is what keeps the coolant in the system from BOILING! which is a VERY bad thing to have happen inside your engine.
Remember, water boils at sea level and one atmosphere, at 212 degrees Fahrenheit. Your coolant certainly gets hotter than this inside the engine; 195 degrees is when the thermostat
starts to open. I can't remember the exact number, but it's around the neighborhood of raising the boiling point by 13 degrees for every pound of pressure you put on the system.
Many people think that the lower pressure caps will spare their failing systems from blowing more holes, but honestly it's not helping that much. If your heater core can't handle 13 psi, it's not going to handle 7 psi very long.
Okay, enough from me, I'm gonna go watch the fireworks outside the neighbors are lighting off.
