After changing brake lines the brakes are extremely soft. Bleeding doesnt help and theres no leak. No abs

I put new brake lines in the car, put the fluid into tank and started bleeding the system. The problem is the pedal goes hard after a few pumps but gradually loses pressure and its not getting any better even when I already circulated almost 2 litres of brand new brake fluid through the system. I don’t see any leaks and keep the reservoir always topped.

There’s some hissing sound in the engine area when operating the pedal but I guess that’s just air coming in and out of the brake booster?

I also put new seals into the master cylinder which fixed the leak between cylinder and brake booster but otherwise didn’t help at all.

In the car there’s also a hydraulic clutch and when there was a leak it was immediately evident. I fixed the leak and bled the clutch with no drama, but these brakes just refuse to operate properly.

Can someone please help? And if it’s of any significance, the master cylinder was without fluid for half year while the car was disassembled. I suspect its just k.o. at this point, but there wasn’t any corrosion or scratches when I disassembled it