Brake pedal issues

I am having a bit of trouble on my 2014 2500 with a squishy brake pedal after the engine is shut down. I decided I would flush the old fluid and after the flush the pedal will sink to the floor when the engine is not running. Brakes work normal with the engine running.

So far I have bled the master cylinder, bled all four calipers, bled the abs unit using AlfaOBD.

Rear pads are worn but not near end of life or anything. I think one rear caliper slide pin may be seized. I have new pads and a set of slide pins ordered.

And ideas what could be causing me this trouble?

Just an update on this. I replaced pads and fixed the seized caliper slide pin. Braking performance is already improved.

However, the soft pedal after shutting down the engine is still there. I can pump it two or three times and restore the firm pedal. So maybe still air in the system somewhere? Or maybe it’s a vacuum issue?

Does anyone have advice on how to test the master cylinder and hydroboost unit?

One thing I have noticed while troubleshooting is the power steering cap was making a hissing after I shut the truck down. I started the truck, ran the wheel back and forth a few times and shut down the truck. I noted the fluid level, and then pumped the brakes several times. After pumping the brakes I noticed the power steering fluid level to be much higher. Is this normal operation?