Hi. I have a problem with my front tyre (CX 500 1979). The last couple of times I have been out to it the tyre has been completely flat. There is no evidence of a puncture i.e. nails etc in the tyre and I have checked the valve isn't leaking. I was wondering if it is possible to put an inner tube in the tyre to stop the leaks. The tyre is a new Avon Roadrider Mk2 tubeless. size 3.25 x 19. What size of tube do you recommend to get. Thanks in advance.