is your car running original compressor and refrigerant? you can get a one time use reclaim cylinder at most supply houses, soak it in either dry ice, or put it in a bag, then put the bag in a bucket of salted ice. evacuate the cylinder, hook up gauges, let her rip the 12 out of your system, then open up the line, change the valve and put it back together, then evacuate your car, and put the 12 back in!
a shop is going to charge you upwards of 200.00 labor. and you won't know if it was done right.
now. the valve could be sticking because the compressor is worn and leaving bits of aluminum in the oil. this makes a flush the wise choice. you'll need the correct amount of mineral oil, and a reclaim system that can separate the oil from the refrigerant. then you will need to open all lines and replace the o-rings at each fitting. and that leaves the compressor with possible wear, so do you want to plan and replace the compressor too?