Radiator drip

After changing out the coolant on my '96 Silverado (350V8 with 124k), which requires opening the drain petcock, there is now a constant dripping from this area. I removed the petcock last week and replaced the o-ring but still the drip persists.

The drain valve basically installs and rotates about 45 degrees and locks. Are there maybe some other parts that are causing the problem?

thanks,

rpg