Yep, I agree with coolvt. Buy a jug of CLR at the hardware store. Shut the water down and remove the ceramic cartridge from the tub/shower valve. Soak the cartridge in the CLR for about an hour, rinse off and reinstall after inspecting the interior of the valve for any calcium/lime buld-up. As for the leak.....forget about it. You don't have one. If you did have a leak, either you or your neighbours would know it. And it's not a water pressure problem, it's a water volume problem. Something is restricting the flow of water to the tub. Take your shower head off and soak it in the CLR too. Showerheads are notorious for lime/calcium deposits clogging them up.

This is an old thread but it helped me. We had a leak under the kitchen sink, somewhere. There's a leak it turns out on the hose for the sprayer. 'kind of where it forms a 'U' near the weight. We have a Kohler K560 CP. (one handle for hot and cold) For some stupid reason I shut off the hot and cold valves under the sink. But it was nice that after 3 years they were not frozen at all. So im using a bucket under the leaky line until I get the replacement hose. ANYWAY, when I turned on the valves and the faucet, hot did not work at all, and cold worked normal. And then I lost some pressure on the cold. So I eventually tried what was recommended in this thread. I shut off the hot valve under the sink and I detached the hot line at the valve point and put it in a bucket. The cold valve was open, and I opened the cold full blast at the sink and held my hand tight over the faucet tip. I did that a few seconds. I was glad to see water and sediment had shot out of the hot line into the bucket. I reattached the hot to the valve and opened the valve. Both hot and cold water flow were restored. So I haven't flushed out the cold line because I dont want to break what isnt broken. :)

TWO possible causes I've personally encountered. The first fix is FREE, the second one is cheap. 1) The air columns in your arrestor tubes have been absorbed over time by the passing water, and the rapid action of your toilet's automatic fill valve is causing water hammer--not just annoying, but damaging to your pipes! You simply need to drain your pipes and let the arrestor tubes refill with air. You do this by shutting off the water to the whole house, then opening the highest faucet in the house (to let air in), then opening the lowest faucet (to drain as much water down a drain as possible), then lastly, open the very lowest point in the system, which is usually a toilet shutoff valve, or an outside/basement hose bib. (You will need a pan under the toilet shutoff valve.) When the draining is complete, close all these fixtures. Note that turning the whole-house valve back on will likely kick loose a lot of sediment. This sediment can clog sink faucet aerators, and ruin toilet fill valves... SO, turn on only BATHTUB faucets, as they are full flow, and have no restrictors or screens. Let the tub(s) run until all the spitting and spurting is over, and they're running clear and quiet. If the problem was water hammer, you should be cured, otherwise on to #2. (Your time has NOT been wasted, as you should do the whole-house drain-down annually anyway, for these very reasons--to clear sediment, and prevent water hammer.) 2) Your toilet fill valve has sand/sediment in it, or some other fault, and needs cleaned or replaced. About $0 to dissassemble and clean, or $7 to replace, ASSUMING you can do-it-yourself. Flushmaster if you want an all-in-one solution. PS: Thank you for your service to the nation, and WELCOME HOME! (My brother returns from "the 'stans" on Friday.)

I'm guessing you mean the flow rate at faucet #2 is reduced? Even if you have a partial blockage in that line, the pressure should remain the same. Remove that fixture and test the flow rate of that line. Could be just debris or calc build-up in the faucet assembly like klem1 said.