Brake pedal goes to the floor with no pressure

You have a leak in the hydraulic system. If the level is going down in the reservoir you have an external leak. Look for a puddle under the car or wetness on the inside of one of the tires. If the level is not going down you have an internal leak in the master cylinder and it will need to be replaced. Even if you find an external leak you can expect the master cylinder to be damaged from pushing it all the way to the floor. The lip seals get torn on the crud and corrosion that build up over time in the areas the pistons don't normally travel in. Not sure what gasket you're referring to. There's no external gasket that will prevent pressure from building in the system.

Yup. If you're not losing any brake fluid from the reservoir, the lip seals inside the master cylinder are torn and leaking. You have to be careful with the new master cylinder on GM cars. Never push the brake pedal more than half way to the floor. Doing that will trip an internal valve and cause one front brake and the opposite rear one to not work and you won't be able to bleed any fluid and air out. You likely won't even notice the loss of braking ability but only two of the four brakes will be working.