Pressure problem

The shower/tub spigot in one of our bathrooms has a peculiar problem. It is isolated to JUST this line. The other lines in the house, including the sink in this bathroom and the bathroom right behind it (master bath), are FINE. When you turn on the water the pressure is fine for about 7-10 secs. and then drops about in half (if you turn it off and wait about 5 secs. before turning it back on - same thing). It doesn't matter whether it is coming from the tub spigot or diverted to the shower head. I replaced the Moen cartidge and it made no difference whatsoever. Also, I removed the tub spigot, leaving the 1/2 inch pipe. With the spigot off, and water coming directly out the pipe, same result !!! Also got the same result(drop in pressure) by turning the water on when the Moen cartridge was removed allowing the water to come out of the cylinder that holds the cartridge. I am completely puzzled !! Keep in mind that it is JUST this line that has this problem !!! Any help would be most appreciated.

pressure

I am not sure where your problem could be, but since it happens with both hot and cold water at the same time it almost has to be in the valve. Do you have a MoenTrol valve which has a pressure balancing spool behind the cover disc? If so it will be a large slotted cap on the right side of the valve. If that is malfuncitioning it could cause that symptom.

hj - thanks for responding. You mention a "MoenTrol valve" but I'm not sure what that is. It is a Moen, the original that was put in when the house was built in 1976 !!! Did they make a "MoenTrol" back then??? If they didn't, that couldn't be the problem. Plus, as I mentioned, with the spigot off, the same thing happens. This may sound dumb but I'm not much of a plumber - with the spigot off doesn't the water come out the 1/2 inch pipe without going through any "valve"?? Also, my wife thinks that just buying a whole new shower/tub control will solve the problem. Is this valve you speak of, part of any new one I would buy ??? If so, she may be right !! thanks again

You mention that other faucets are OK, but maybe you just don't notice because the tub is the largest volume draw. The symptoms are classic for a bad pressure regulator valve on the main supply to the house. You can check this with an inexpensive gauge available anywhere.

Jimbo - I thought about the water pressure regulator BUT, as I mentioned, the shower in the master bath (which is directly behind this bathroom) is FINE. I assume it draws as much water, but maybe not. In fact, it has the EXACT same Moen fixture (circa 1976) with the exception that there is no bathtub, thus no spout with a diverter. I even replaced the spout (it was corroded) and it made no difference either, but I really didn't think it would. So you're saying that even if the water pressure is OK everywhere else in the house it could still be the regulator causing my problem?? Maybe it is, but I just figured that there would be a pressure drop everywhere if that were the case. While we're on this topic (regulator) can't I adjust it ?? There is a nut on the top that, I think, does that?? Thanks for your interest in my problem.

moen

They did have the MoenTrol then, in fact it was one of the first pressure balanced valves. Remove the two screws holding the chrome plate to the wall and look at the valve. If it is a MoenTrol it will have a big "nut" with a screwdriver slot on the right side. If so, buy a new one and replace it. They do not "repair" very readily. The water always goes through the pressure balancing spool if you have the Moentrol valve, which suggests that you do have one, otherwise the pressure would not change.