Brake pedal sinking

Sounds like there is a leak in the break system or the master cylinder is gone bad. If you are not loosing any break fluid then probably would be the master cylinder.

I have check the master cylinder not leaking & brake fluid not leaking i havent change my brake fluid almost 3 1/2 years what do u think about the brake gonna be pkease answer my Question thank u...

The master cylinder is by-passing internally. You would need to replace the master cylinder.

You dont have to have a leak, it could be a seal or "O" ring somewhere in the system & most likely the the master cylinder. Master cylinders are easy to replace, but bleeding the air out you'll need some help pumping & hoolding the peddle down while someone else lets the air bibbles out at the bleed check valves at every wheel. Old drum brakes always went bad from the little wheel cylinders going bad & leaking. And todays disc breke calipers dont do that,

do you have a brake booster in the car,that is the usual problem in a c-4 corvette

Check the brake pads, extremely excessive wear would allow the pedal to go to the floor.

Its a 4 wheel disc break, not like having rear drums to keep the shoes adjusted up to have pedal. And yes it would have a break booster, and when they go bad, the break pedal gets extremely hard to push.

My disc & brake is good only the brake pedal go to the floor im slready bleed the air out of the system why brake pedal still go down to the floor please help me thanks..

Sorry I didn't notice is was a ZO6 Corvette with a power booster.