Brake pedal goes to the floor when the car is on?
 

The car (91 Tercel) has been sitting for about a month now. Drove fine last time I had it going. Went to play with it today, and the brake pedal holds pressure when the car is off, but when you turn it on, it goes right to the floor. It does still have a bit of pressure left to it, but it locks up what seems to be 1 or 2 tires.

What does it sound like? The brakes need to be bled? Brake cylinder? Any ideas?

Could be air in the system...could be a fluid leak...

if there is fluid leaking from somewhere its that
if not its air or a bad wheel cyliner/caliper

First of all check if there's leaks..
If not, then proceed to bleed the system..

It could be one of many possibilities.. You need to work through all them from simpliest/cheapest to fix and so on..

if there was leaks wouldn't it also sink with the car off? and same with Air in the system

have a look at the brake booster