Spongy brake pedal

[Redyaris09]

]Hello, I have a 09 auto Yaris with 175k km on it. The last year I haven't been using it much and I'm realizing that my brake pedal feels soft. When I inspected the front brakes, the guide pins where seized so I got them out, cleaned everything, put new boots and guide pins and put everything back together with brake grease. The pads and disk were still in good shape.

Sadly, still soft.

Changed the rear drums, pads, cylinders and bled everything. (need to readjust a little and will do soon).

Sadly, still soft. It brakes just not as firm. If I pump the brakes while engine is off, it gets hard. No leaks anywhere and no loss of brake fluid.

I remember before the pandemic, my car would make a sound (like a pressure release / blow off valve) every time I started the car (except when car was already warm) and it doesn't do it anymore.

What could be next? I've read that some people have changed the master cylinder and it still was soft. Also, what made the pressure release sound?

Thanks!