I have a two handle shower faucet by Delta that was installed when the house was built, around 1972.
Recently the flow of water from the shower head has slowly diminished and is now a trickle that barely fills all of the holes in the shower head.
It is the same with each knob turned on individually or both together.
I removed the shower head and the flow from the open shower arm pipe is the same slow trickle.
I removed the hot water knob assembly and inspected what I could see of the valve body and saw no obstructions or sediment.
I blew compressed air into the shower arm and it seemed to flow freely into the hot water knob opening of the valve body.
No other fixture in the house has a water flow/pressure problem.
I am really stuck for an explanation and how to proceed.
There is no easy access to the wall behind the shower valve.
I considered that there might be a problem with a pressure balancing device, but I don’t think a valve this old would have such a thing.
I am wondering if the rubber seats have somehow gotten old and collapsed in a way that restricts flow?
It would have to have happened on both hot and cold sides to produce this symptom.
If anyone has any insight, I would love to hear it.
Chartman

when you had the handle off and the hot valve apart, did you try turning the water back on to see if any debris had collected under the seat of the valve?
do the same for the cold..

Thanks for your reply, Packy!
The seats are so deep in the wall and awkward to get at that I didn't try removing it.
I think I will have to try that next week after the holiday.
Chartman

delta calls the little rubber cups "seats".
they have small stainless springs under them.
this is what i meant by "debris under the seats"..
[www.deltafaucet.com]

If the flow stays the same even when you turn both valves on, then the problem is in the valve, (between where the two flows mix and the shower head), otherwise the added flow WOULD make a difference, by the laws of basic physics.

Hi hj. Thanks for "weighing in".
I'm pretty much "up" on the laws of physics (Professor of Physics, emeritus )
but when I blew compressed air into the shower arm pipe, it flowed freely out the open hot water knob port on the faucet body. That's what makes these symptoms so confusing (and frustrating).
Right now I am inclined to think there is a problem on both hot and cold inlet ports; possibly with the rubber seats on both sides.
I should probably just replace the seats on both sides and see what happens.
OTOH, I just got one of those endoscope cameras to operate off of my laptop. Maybe I will be able to see what's going on in some of those tight spaces.
I won't be able to do anything until next week.
I will post a message again if I learn something new.
Meanwhile, if anyone has any additional thoughts I would love to read them.
Chartman

Hi Packy.
Yes, I have worked with the Delta seats before and I don't like them.
It's not too bad on a kitchen faucet where gravity helps to hold them in place, but the spring and gravity are not helping deep in the wall on this shower faucet. I am always afraid that the seat will become dislodged before I get the faucet reassembled. But I think I do need to give it a shot, since I don't see another productive avenue to pursue.
Chartman

I don't know if anyone is still monitoring this old thread, but here is an update.
The problem persists.
I have replaced the seals, which looked fine as they were.
I looked inside the valve when the valve stems were out and saw no signs of trouble.
The flow rate is still unacceptably slow; even with the shower head completely removed.
My endoscope idea did not work since the end of it is rigid and too long to make the turn through the shower arm and down the "up tube" to reach the valve.
My new theory is that there is sediment built up in the valve at the bottom of the "up tube".
That is the only way I can think of to explain the "slow flow", which is the same when either or both of the valves is/are opened.
The water sits in this tube when the shower is not in use; unlike in a shower/tub where the water would drain down into the tub when the shower/tub valves are closed.
The owners of the house have gone on extended vacations, so water would have been sitting there for long periods of time and sediment could have built up at the bottom of that tube causing the problem (it seems to me).
I don't know how to prove/disprove this theory. Accessing the valve and these pipes from the outside would be very difficult.
I also don't know how I might address the problem, without disassembling the pipes.
I am wondering if I could open up the valves again to drain the water out of the up tube.
Then I could close up the valves an introduce a chemical (through the shower arm) that might dissolve the sediment? ...vinegar, perhaps?
I could let it sit for a while; hours or over night?
Then flush the system with water by opening the valves.
Has anyone ever heard of a problem like this before or know if something like this has ever been done successfully?
Chartman