Lots of good advice here. First point is that what you've got is rubbish, and so doesn't work. Serious equipment isn't cheap, but it can last for ever (virtually) and does work. There are a number of good suppliers (some mentioned already) and I've tried a few, but the best I've come across are from
Welcome to PumpToys! - a bit of a misleading name I always think, as their products are anything but 'toys', but highly professional; and if you e-mail Dan for advice on any pumping matter at all he replies promptly and in full and helpful detail. And there's a great deal of pumping info on the site.
One of their best accessories is the silicone seal, with a size to fit every cylinder: amazingly comfortable and gives an airtight fit. They also sell the Mityvac pump with guage - very efficient and accurate, so that you avoid the danger of overpumping.
I have four penis cylinders of different sizes at the moment. I work up through these (in 1/4" diameter increments) on a long pumping session. But my best is what they call the 'Long Johnny' - a 4" dia straight cylinder for cock & balls pumping - the effect of this is both enjoyable and very impressive. And with the silicone seal, very comfortable to use.
Their full range is much greater, however, and can be seen on their website. Highly recommendable if you're serious about pumping. But if you're not, don't waste money on the cheap stuff.