Hello all, We have a problem with the shower in our bathroom that we hope you all might be able to help with. Our house is ~40 years old, and we think that the shower fixtures are original. This is a tub-shower combination, with a tub faucet, shower head, and knob in the middle of the two. The knob is of the type where you pull it in and out to adjust the volume of the water, and twist from side to side to change the temperature. We can turn the water on without a problem, but when either the shower or the tub faucet runs for any more than a few minutes, the knob becomes almost impossible to turn off. To push it in to shut off the water, we have to push it, hard, for several minutes while twisting it from side to side. Eventually, we can get it to shut off. During that process, if we let go, the flow of water increases on its own until we keep pushing it in. We're at our wits end here. This means the shower doesn't work, and we'd appreciate any advice you all might have on how to fix it or what's wrong.