Hi, the tyre isn't popped or anything, or at least I don't think so. Right now there's a problem with even pumping the tyre up. I noticed the rear a bit flat after a break in riding for a bit, so I went to a servo to pump it up. Nope, wouldn't work. Thought the machine was farked. Went to a different servo and still didn't wanna inflate. I checked if my front tyre would inflate, and it did. Then I checked if my rear tyre would even deflate. It wouldn't. Are valves expensive to replace? Is it worth just getting a new rear tyre?