Air leaking from ABS modulator valve

no such light. There is an "ABS SLIP" on the overhead console but it doesn't light up. need to check for blown bulb thanks

 

So I had this same issue that we spent some times diagnosing. That abs valve is supplied through the relay valve. Air goes from the relay valve to the abs valve on driver and passenger side of the tractor then T’s off to another valve on the drivers side which has another set of T’s. This is your quick release valve. This is what allows supply pressure from your service side of your brake can to your emergency side of the brake can. On top of that valve should be a yellow 3/8” airline. That is your control pressure. When you release your parking brake air pressure flows through that yellow line down onto a piston. When that piston is pushed down it allows supply pressure through to release the brakes. If that valve goes bad and doesn’t open correctly, you get that air pressure back feeding through your service line and back through the abs valve and it gets pushed out of the valves exhaust.

 

Would be a QR-1C quick release valve with anti-compound.