2020 International HX520 parking brake slow to engage

Possibilities:
internal air leak in the parking brake valve
Could be an air leak in the spring brake
Sticking at the valve
Air dryer issues excessive moisture buildup.

 

Likely a quick release valve sticking

 

I 2nd this.

 

No, it's a valve under the fifth wheel that releases all of the air in the brake chambers.

When you set the brake, the dash valve sends a signal to the quick release valve, the quick release valve then dumps all air on the supply side of the brake chambers.

It's easy to change and reasonably cheap

 

When you pull the knob you should hear the air exhaust rapidly out in the general area of the drive axles. If not, see the above posts.

 

It will look like this,