Has it always been this way?
Does the water at the kitchen sink act the same way?
Does the hot water have a good sustained flow rate when that is all that is used, or does it die off after a few seconds?

Randy, thanks so much for responding.

My father says that, yes, it has always been hard to adjust the temperature, but not as hard as it is now. (I realize that equivocal answer may not be that helpful.)

No, it does not happen at the kitchen sink. However, there is a lavatory in the same bath as the shower. I never noticed this before, but it is similar; if the hot at the lav is open fully, it only takes about an eighth of a turn of the cold to make the water tepid. (In the shower, it only takes about 1/16th of a turn to make it too cold to shower.)

If left alone (i.e., hot only), the flow does not die, but continues at same rate (decent flow) indefinitely.

The other thing that may or may not be relevant is that when you turn on the hot water in the shower, water leaks out around the cold water valve handle. (This also happens when you turn on the cold.)

Thanks again.

I would first change the washers and valve seats in the shower controls and inspect the inside of the valve body for debris or any other situation that 'looks wrong'. If the valve stems are corroded and deteriorated when you remove them, replacement of the stems might be advisable. The same would apply to the lavetory.
For the water leaking around the cold water valve, the packing nut around the valve stem may need tightening or the packing may need to be replaced.

about debris in valve and the similar problem with lavatory:
60 years old? galvanized supply piping was still common in budget homes mid-1900s. but it should have disintegrated by 1990. :-)
perhaps most of galvanized was replaced, but this part of the house was not replaced?

and if water flow is strong when *only* hot is on, then huhh?? :-(

this sounds like two-handle faucet.
for example, pp was very common (handle, and escutcheon 960-110 style may differ from yours.) blowup on: http://www.diychatroom.com/f7/price-pfister-3-valve-shower-replacement-water-issue-46953/

Thanks for the additional thoughts. First, an update. I did replace the washers, inspect the seats, and replace the gland/packing material. The good news is that the leak was fixed, but the bad news is that balance problem persists.

The pipes are copper, all 1/2". The cold side looks original (heavily oxidized exterior), but the hot side looks like it may have been replaced at some point (less oxidized). (Or maybe that is just from greater condensation on the cold?) The water is not hard, and there are no buildup problems elsewhere. It is indeed a two-handle faucet that looks very similar to the diagram to which you linked.

I removed the shower head, and get a very strong flow from both the hot and the cold supplies (Many many gallons/minute). However, even with no head, turning the cold on just a bit kills the water temperature.

It seems to me to be a balance/pressure problem, right? That the cold side has a higher pressure that suppresses the flow from the hot side? I contemplated patching in a pressure reducer on the cold supply to that bathroom, but did not do it. Does that sound like it would be in order?

THANKS!

Have you checked to see if there is a pressure regulator/reducer on the incoming side of the water heater or between the water heater and the bathroom? Is there perhaps an isolation valve somewhere that is partially closed?

Randy,

Thanks for your continued help. Unfortunately, this chapter is closed for the time being, as I have returned home. They were happy with the shower as I left it. I put in a handheld shower so they can at least adjust the temperature without being in the stream. They also report that the new washers seemed to help the adjustability a bit.

There is just one quarter-turn ball valve in front of the water heater, and it was open. I did not check for a regulator or pressure reducer. I will do that next time I am there, in the summer. I don't believe there is any isolation valves, as they would be under drywall, but I am sure you have seen crazier things!

Thanks again,
A_D

60 year old galvanized steel lines will likely be nearly completely clogged by now, especially the hot water line.

This will make it very hard to adjust the temperature since the slightest change in back pressure from the valves (especially cold pressure into the hot line) will alter the flow.

I have seen hot lines so close to blocked that simply turning on the cold water stops the hot flow almost completely (the cold will flow back into the hot water line).

About the only fix for old galvanized lines is new pipes.

Brickeyee,

Thanks for weighing in. As I have noted upthread, the pipes are all copper; it was ccdry who thought they may be galvanized. They have very good flow, both hot and cold.

I was very glad to have replaced all of the 75 yr old galvanized in my own home five or six years ago! It was a backbreaking 3 days, but totally worth it!