Thanks for the suggestions. So it was leaking a lot, and when I removed the cap from the PVC this time, water started gushing out. I put the cap back on, and came back with a bucket, removed the cap, and let all the excess water spill I the bucket. It was still full until the top, (not sure why I did this, but) I replaced the cap and removed it again, and more water spilled over. (I guess the suction pulled out more water). Once I saw this, I kept doing it, just so as to try to remove as much water when I could. Suddenly, I heard all the water rushing out of the pipe, and it was suddenly clear. This was a few weeks ago, and I haven't had any problem since, so I assume the clog is gone. It must have been forced out by the pressure, like plunging a toilet. (Afterward, someone else suggested using a shop vac to suck out the clog).