Cruise control inop and speedometer reading 15 mph faster than I am running.

Seal, They don't last for ever, the O-ring in the valve has prob gone bad.

 

Are you sure its leaking and not a leak in the air system it self ? Example, if I have a bad tlr brake can and I engage the air then hit the brake it will sound like my assembly inside the truck is leaking. When infact its just the volume of air moving thur the system unable to pressurize. Might be worth a check. Heck it could be a hole in a air line that only happens when air applied to that line.

Edit,
Misread your post did not see the brakes were already "set" sorry bro I am at a loss on this one without hearing the problem my self.

 

ECM Programing, wrong gear ratio progrmed at a guess

 

your air leak issue sounds like the "anti-compound quick release valve" has failed, it is designed to allow application air to the spring brake chamber when parked so not to compound torque into the slackadjuster/brakes. when it fails you will hear air leak out of the dash valves exhaust port.

the speedo issue could be a loose trans output yoke nut or bad wiring from the speed sensor to the ecm, a rubbed bare spot on the wiring can send faulty signals to ecm.

 

IIRC, That's normal. The brakes are already set, with spring pressure holding the shoes against the drums, if you step on the pedal the air system is set up to vent the pressure rather than overstress the brake drum/shoes. Threw me for a loop a couple times too, it's okay as long as it doesn't leak when you're not stepping on the pedal, or when the brakes are released.

Were you talking about the valve in the dash? That would be weird. I read it as the brake valve on the chassis.

 

Somebody may have changed wheel/tire size causing your speedo to read wrong. Is it reading steadily over what you actual speed is? Or under it? Or is it erratic?

 

Could be some one replaced speedo and the DIN switches are set wrong.