Leaking air when I press the gas and when I release service brake

What model is your truck.

 

There are better mechanics on here than me.
If that picture is your ABS valve.
I would change it.
It might be cheaper to replace the entire assembly. Rather than just the center valve.

Also the "air leak" you hear in the dash valve could just be air feeding thru to your other leaking valve.
Good luck

 

I'm thinking what is said in post 4 is more accurate. What you say is leaking from the yellow knob is most likely not. When I've had a leak of a service brake on a trailer when I step on the brakes I can hear it leak past the yellow knob. This has always been caused by a bad grommet in the blue airline gladhand hooked to the trailer.

 

Quick release valve. It'll be a smaller valve under the 5th wheel. Gonna look like this.
Very, very common

 

No, the pic you posted is the ABS valve. The quick release valve I posted has a signal line that comes from the ABS valve.

The quick release valve fails, leaks air down the signal line into the ABS valve. Which comes out the exhaust of your ABS valve.

They can be mounted in different areas, but in general they are normally bolted to the crossmember under the 5th wheel.