How do I make a square brush with opacity from pen pressure?

[CrashCarWill]

I am pretty new to photoshop. I am trying to figure out how to make a hard square shaped brush that has opacity jitter with pen pressure.  How would I do this?

[Semaphoric]

First of all, Adobe seems to have broken pressure sensitivity in the latest release, so, until they fix it, you'd need to roll back to an earlier version via the Creative Cloud desktop app.

You can, though, see it work in v21.1.2 by making a Path, and Stroke with the Brush from the Paths panel, with the simulate Pressure box checked.

That aside, you set it up this way. In the Brush Settings panel, select Transfer, with Pressure as the Control.

[CrashCarWill]

Pressure usually works fin for me but for some reason when I am pressing lightly it sometimes jumps to full pressure. is this the problem you were talking about?  Also, I am using the surface pen on a surface go.

[c.pfaffenbichler]

The problems do not affect all Photoshop users. 

Just create a square Brush Preset and set the Transfer accordingly.