water tank valves

I have recently purchased a 2012, 25' Eddie Bauer and I'm perplexed about draining the fresh water tanks. The first time I drained the tank, I simply opened two, red handled valves near the roadside and accessed between the tires. Now when I tried to drain the full tanks, I opened the same valves and nothing but drips came out. It wasn't until I turned on the pump that it forced water out of the tanks. I'm confused and wonder what I'm missing. Why won't the valves simply drain when I open them?