Tub/shower low flow mystery!

Situation: 1990s vintage Delta 600 series tub/shower valve with integral diverter. There is insufficient flow/pressure at the valve to provide enough water to hold the diverter in the closed position and to provide a decent flow rate at the original shower head. Flow through the spout is weak, too.

Actions taken to troubleshoot.

1. Speculation that the diverter had separated and was causing the problem.
   1. Bought and installed a Delta (not third party) replacement diverter. No change in flow.
2. Speculation that the internals of the valve were restricted.
   1. Bought and replaced the ball and installed the repair kit, too. No evidence of any corrosion, erosion, rust, lime or anything in the original ball and the valve body. Used a Q tip to swab the internal cold and hot water inlet ports. No foreign material at all. No change in flow.
3. Interestingly, the flow rate through the valve to the spout is the same whether the valve is set for cold only, hot only or an equal mix. This seems to indicate a restriction, if any is common to both the cold and hot water supplies. If it was a difference between cold only and hot only, it identifies where to investigate further.
4. Note the shower head looked ugly on the outside, so I replaced it with what I hoped would be a single spray head with the least allowable flow…ha, ha. The code determines the total flow, it may have not mattered.
   1. Upon testing the shower because of the low incoming flow/pressure, the diverter would not stay in place until the entire line to the shower head was full providing some “head” for holding the diverter shut. This is annoying, but at least the shower has enough flow to take a shower.
5. Given that the flow/pressure are low, the internals were not constricted and there is no difference between cold and hot, I have to guess there is something going on upstream of where the hot water line taps off of the supply to the house, including the piping, meter/pressure regulator or line from the street to the regulator.
   1. It is important to note that in this house there are 4 apartments and the tenants in the other 3 apartments have informed me they have no issue with any of their faucets.
   2. A pressure gauge installed at a garden hose tap in the basement shows about 80 psig with no flow in the system. All that means is there is what would be expected pressure coming through the meter.
   3. Also, water flow to the washing machine in the basement shows decent fill flow for cold and hot; the hot being slightly less, possibly due to the extra length of piping to and from the heater.

It seems there can’t be a problem with the valve internals because the cold and hot are separate until they get to the ball, which has no issues as mentioned above.

A restriction somewhere before the hot water tap off the main in the basement seems out of the question because no other fixtures in the house were reported to have a problem.

By the way, the problem faucet is on the second floor (~20 feet higher), so the pressure may be affected by the height of the water column…I’m not totally sure about how/if that makes a difference.

One thing I can do is try to measure the cold only, hot only and mixed flow rate to see how close they are to the advertised flow rate at 80 psig.

Replacing the soldered valve in the wall would be a pain, the hole in the acrylic tub surround would have to be enlarged and the back side of the wall most likely would have to be opened to provide access. Soldering is probably not an easy replacement…something more modern might better, even if it is hidden inside a wall.

Lastly, on Monday, 11/9, A person from the Public Works department is coming to try to help solve this dilemma.

Does anyone have any suggestions?