Udate: i swapped the master cylinder out from my daughter’s car. Before swap mine soft hers hards. After swap mine doft hers ( the one from my fiero). Brake booster?? If i pump
The brakes they get hard. If i drive a bit brakes are soft.

So yesterday I changed my rear brake pads. I was super careful with the brake lines. I put it all back together and now the brakes won’t hold pressure. I’m hoping the master reservoir lid isn’t on tight. Yes I used the correct tool to spin brakes back in. I have done brakes many times this is the first time this has happened. If not the cover, then what?. I don’t see and fluid on braking so far.

The master cylinder lid has zero impact on brake performance, so move on front that.

If you press the brake pedal and once it is hard, it continues to drop, then you either have a leak or a seal leaking by.
If the brake pedal goes nearly to the floor before any resistance, you either have air in the lines, or the rear brake pads are too far from the rotors and all your travel is being taken up just moving the rear pads to the rotor. The gap should be about the thickness of a playing car. Since you rotate the piston back, I suspect it was rotated too far back.