Truck Repair: air conditioning for ford 7.3 l diesel, r 134a, r134

Question
I need to add refrigerant to the system, have done so on my jeep for years.  where is the valve to allow the refriegerant to enter on low pressure line.  hooked up the can of r-134a but won't enter until i find the valve.  it was obvious on my jeepo, less so on this.  it is a 1999 f350 dually.

Answer
What type of valve are you looking for?

If you have hooked up a can of R134 then where did you hook it to?

R-134 should be charged through the suction side of the system - low side - Blue side

Please follow up --- charging R134 any other way is dangerous and can also damage the vehicle ---