Without starting the car, can you pump the brake pedal and feel it getting firm, or does it just slosh right to the floor every time without resistance? If so, your master cylinder has likely just decided to take a dump. If it only does this when you start up the car, it could be that something is up with your brake booster - either a disconnected or damaged vacuum hose (you'll probably hear a whistle/hiss sound if you listen closely), or the diaphragm has crapped out.
If it does firm up, and you push down on it firmly with your foot and hold it, does it slowly sink down to the floor again? If so, you might have spring a leak somewhere in the braking system by chance - could be a brake hose, could be a rusted brake line, could be a rear brake wheel cylinder, whatever.
Unless you damaged something by really banging around in there, or you damaged a vacuum hose that had turned brittle with age, I seriously doubt the changing of plugs, in and of itself, actually caused the problem. Most likely just a coincidence. Cars do this because it's how they show their love for their owners - the more time, money, and labor you pour into them, the more they like to give you headaches.
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