[jhv48]

Just a thought, if your car has been sitting for a long period of time, the original pads might have turned to stone. Or, if your car was set up for racing, the pads might need to be hot before they work properly.
If your pedal is solid, that pretty much rules out the master cylinder and brake fluid.
My first step would be more friendly street pads on all four corners. Easy to do, relatively inexpensive too. Then see what you’ve got.
Once again, as others have said, without a booster, you do need to stand on the pedal for best results. But then, you should be able to lock them up. If you can’t lock em up, sumtingwong.