brakes not working properly on 1999 Porsche 911

When I push on the brake pedal it seems to lock up??? If I hit them hard they don't stick but if I just push down they slam you to the dash. Checked the lines and has good vacuum. any ideas??

Asked by for the 1999 Porsche 911
suspect there is either a caliper or master cylinder issue. when was the last brake fluid flush done? (every
2 years with super DOT4 fluid only)
ended up changing the master cylinder and booster. Any idea why the booster would have water in it????