I was able to shut off cold
water to the sink. I could not, however, shut off hot
water completely. Even using an adjustable wrench, I could shut it off almost entirely so that when I pulled out the handle, there was a small trickle. Anyway, afterwards I took a shower. It seems that temp did not drop as fast as it did before, but I think it still dropped. Also, I am not sure whether it was because I shut off the sink faucet or because in the past I was using the shower shortly after someone else in the family did. By the way, I don't know whether it makes any difference, but the sink faucet is in the downstairs bath while the shower is in an upstairs bath. I am thinking about buying a water thermometer and measuring temp drop more accurately. But I don't think the problem has been solved.
I dug up the manual for the shower valve. It's Moen L82691. The install manual (http://www.moen.com/shared/pdf/instr...ts/ins898d.pdf
) shows a temp limit part but I don't see any parts that control pressure. Also the spec sheet (http://www.moen.com/shared/pdf/l82691sp.pdf
) states that "Showerhead is limited to 2.5 gpm (9.5 L/min)". Is this a lot compared, say, to an average valve made 10-15-20 years ago? I am asking because I am wondering if we are out of luck trying to increase water pressure.