If you contacted Topeak, I bet they would send you a new cap free or sell you one for a reasonable price.

I have a Road Morph G that I use on my touring bike, mounted beneath the downtube. However, I don't like other mounting options for the shorty pumps and bought a full-size Topeak frame pump for my bikes with pump pegs under the top tubes. The full size Topeak pump works just as well or better than the Road Morph, altho it doesn't have a pressure gauge.