Have you tried using a rubber ball compressed between a couple of washers and a wing nut to adjust tension and therefore the diameter?  Also goes without saying, make sure you don't have any air leaks in all the fittings.

David,

Not knowing what part of the world you are, I would suggest your GOOD local hardware store.
Find some rubber washers to fit the inside dia of the pipe your using with some large fender washers. I use three washers on my 2" and 4 washers on my 3", thinking of adding another.

The trick is finding PVC pipe with smooth insides.
As Arthur suggests, no air leaks 'below' the washers. The air or water needs out above the washers.
Forget any check valves, Y fittings, just straight suck it in blow it into your bucket all in one motion.
It takes practice, but works better than any other.


Bill

In the same Isle you bought your PVC pipe there should be a mechanical plastic test plug that has a wing nut on top and a rubber gasket already installed around a plastic washer if you will, get one of these of the correct diameter, remove the hardware and reinstall your threaded rod down through the all ready drilled hole and tighten with a nut above and below the plug. You will have to remove the top plastic edge that's above the gasket with a few bumps to a belt sander so that it will fully insert into the pvc. You can then compress or loosen the two nuts you installed on the threaded rod to compress or decompress the gasket to obtain your desired results.

You can make a sure fire gasket from a tennis ball. Just cut it in half and trim down until it will slip into the tube. Sizing isn't super critical. The fuzzy side is installed facing the nozzle end. The downstroke is easy and it scrubs sand off of the walls. Upstroke flips the tennis ball half inside out sucking the rubber onto the walls. Works like a charm.