Fill the brake reservoir and park the car on a dry spot of pavement. Keep pumping up the brakes over and over again dozens of times. Check to see if the fluid level drops. If it does, move the car and check for fresh brake fluid on the ground. There should be plenty if you have a leak.

mason rocket, you probably have air in the lines. When you replace the Master Cylinder, you have to bleed it, sometimes extensively, and THEN bleed your brakes. Follow your service manual for bleeding instructions, but in layman's terms, start at the farthest wheel from the cylinder, get a friend to mash the brakes as you crack the bleeder screws until no air comes out at each wheel, and once there's no air, move one wheel closer to the master cylinder.

If you don't bleed the master, you may need to go through the contents of the brake fluid reservoir a few times to get it all out. Don't let it run dry while you're doing it or you have to start the process over.

If there's no air in it, and it's not leaking somewhere, the pedal will not fall. If you can't find leaked fluid, and the pedal still falls, keep bleeding it.