When the cars on, the pedal goes to the floor and there is very little stopping power. A few days ago when I last drove it it was fine.
Fluid is full, no noticeable leaks coming from anywhere. Doesn't harden at all if I pump the brakes when engines running. If it's off it'll harden after a couple pumps.

Most likely master cylinder?

Sounds like a bunch of air in the lines or the master cylinder is done. When was the last time any work was done to the brakes?

No clue. Bought it from auction a little over a month ago and I haven't messed with the brakes at all yet. They have been working fine since I got it, just noticed it last night when I drove it after it sitting over the weekend. If it was air in the lines. Is there a particular reason why it just happen all of a sudden without doing anything to any of the brake system?

Something is leaking. It doesn't take much air to mess your pedal pressure up. It could be small. It could be a bad booster or master cylinder too. Bleed the brakes fully with fresh fluid and see if that helps. If it does you are sucking air somewhere.

Checked out the rear and pretty much everything is shot. One cylinder is leaking. Still think the master cyl is toast.
Kind of in a shitty situation. My accords transmission is fucked and needs to be replaced. Been deciding what to do with that.. 1200 for a used one + install (not sure if i'll be able to do it or if i'll have to take it in)and risk it not working. Or spend 2500-4000 for a rebuild.
The van is my only transportation atm. Think if I replace the master cyl it'll help fix the issue at least to where it's driveable? Front brakes seem to be good, rears are shot. If I run the risk of something in the rear seizing up and it'd be unsafe to drive I won't fuck with anything. But think I could get a little more use out of it for the 20-40 bucks spent on the master cyl?

if you have a leaking cylinder on the rear there is absolutely no point in putting a master cylinder on if you arent going to fix the leak first. fix the leak then bleed each brake to make sure none have air in them. after that change the master cylinder. dont half ass the thing that keeps you from slamming into other vehicles or objects.

Totally understandable. Not really familiar with drum brakes but one of the guys that looked at it said that shit would start falling apart if I tried to replace the cylinder how things are now. Thanks for the insight though.