I have encountered an AC service port that has piece of round plastic blocking the schrader valve (apparenty the cap broke off). Any recommendations on how to extract it wihout damging the threaded bore and not conaminating the system ? - Thanks
How much is still left? If the top broke off but the bottom half of the cap is on the treads then you'd use a set of channel locks. If you have less than half of a cap then you'll need to maybe use a pick to gently remove it from the threads.
Can you take a picture?
Whatever you do, don't use brute force to repair this.