You posted this in May, but I will answer anyhow. Just had the same problem with my 1999 HD. There should be a thin O-ring seal on the perimeter where the strainer part pushes back on. It is very hard to do from that back seat. Also, you may want (or need) to unscrew the fitting that goes to the strainer cap. I cleaned my strainer and the drain still didn't work. Once I got water forced through the drain line (with a rubber hose) it started working.....BUT! Once the pump dried again it wouldn't self prime. I bought a new ShurFlo pump head (model 208
for about $50. You take the floor of the wardrobe out (easiest way to service this pump) with three screws hidden under the carpet. The pump head has 3 screws attaching it to the motor. There are two red wires that clip into the pump head assembly. Mark them, unplug them, unscrew the two pipe lines, and screw the new pump head on. The power drain should work like a charm. I poured a two gallon bucket of water into the shower pan and it gurgled it away in about 6 seconds!!! I believe that the diaphragms dry out on the shower pumps mainly from lack of use. I use my shower and it is actually quite roomy if you do it right. It is best to leave the bathroom door open while showering. If you are quick and efficient you should use no more than 2 1/2 gallons - which is about one shower pan full. That drains to the black tank and barely make an indication which means you can take several showers self-contained. Good luck and post your results!