81 corvette can't getr a hard brake pedal . I have changed the master and bleed . still no hard pedal

Question edited for clarity, wrong type of brake/break.

If you brakes have ABS, you have to bleed the brakes from the caliper up to the master cylinder using a pressure bleeder. It also works better on normal brakes because it pushes the air upwards which is its natural way, air rises in liquid.

Does the brake pedal go to the floor? Or does it stay up, and is hard to push...? Did you bleed the master cylinder? Also, you will need to bleed all 4 wheels connection to the brake wheel cylinders...you must start with the rear, farthest from the master cylinder, then other rear, then front farthest from the master cylinder, then other front...the bleeding of each line is a must, since you switched the master cylinder.

Fill master remove bleeder have somebody rapidly pump brake pedal while you hold your finger over the hole. within ten pumps (usually) fluid should come out.By holding your finger over the hole you will feel air pressure let that pass Then do your manual bleed.If still nothing check the rubber line sometime they get messed up inside and fluid will not go through

possiable brake booster. if you hear a hissing sound and truck runs rough if not possiable master cylinder.

You may have to bleed the slave cylinder to get it to work. Bleed it just like brakes-pedal going to floor, open valve. Close valve, pump pedal and pedal to floor again with bleed valve open. When fluid comes out in steady stream with pedal pressed, done. Watch the plunger on the slave cylinder when clutch is pressed-it should move out to move the clutch lever.

How did you bleed your brakes or did you even do that? their should be a little nipple on your calipers or even the brake booster to bleed the system.

try bleeding the brakes again and then with the cap off of the brake fluid tank, pump the brake pedal tell it gets hard

First check each wheel brake cylinder for small leaks. If no leaks then the brakes have not been bled properly. Liquid will not compress like air, thus the reason pumping the pedal makes the brake pedal come up. If you had all of the air out of the lines then you pedal would not be soft. With your pedal going to the floor either there is air in the lines or fluid is slowly leaking.If it is a small leak on one of your wheel cylinders, it may be hard to detect without pulling the wheels off and looking. Small leaks on your wheel cylinders may never drip off of the car.

It's very possible to be the master cylinder, the o-rings on the piston could be worn, letting the fluid seep back through instead of getting full pressure through the lines. Could also be the brake booster.
Try changing the master cylinder first, which is cheaper and easier, and see if there is a difference, you'll need to bleed the brakes again.
To check the booster,
With the engine off, pump the brakes until the pedal is hard, then hold pressure on the brake pedal and listen for air escaping, or the pedal goes slowly to the floor, Do that after the master cylinder has been changed, at least that will be eliminated.

You need to bleed this system with the special tool that holds the ABS solenoids open, it plugs into the ABS brake controller, if you don't do this there is no way to get the air out of the system, that is why your pedal is low.