On Tue, 23 May 2017 11:14:05 -0700 (PDT), trader_4
wrote:

Here's a baffling problem. Over maybe the last year or so, the water
flow in the shower has decreased. It's happened slowly, so not sure
when it really started, but lately it's become a problem. It's not
the shower head, I removed it and the flow right out of the 1/2"
pipe from both the hot and cold are way below what they should be.
But here's the bizarre part. If I open just the cold all the way,
I get a flow of X, which is slow and way below what it should be.
If I open the hot, I get the same exact slow flow. If I open them
both, I still get exactly the same slow flow, ie no improvement.

To me that's weird, because it implies that there is some obstruction
after the two valves. But this is a Grohe faucet from 25 years ago,
ie it's not one with a tempering valve or anything fancy, so I
would assume the water just comes from each valve into a simple T
and is combined. So, I'm at a loss as to what could be wrong.
If it were debris, I could see it lodging before or in a valve,
but how could there be anything after the two valves?

Water pressure into the separate bathtub is fine, plenty of
water flow there, many times what's coming out of the shower.
Only other odd behavior is that the cold side of the sink,
which is also a Grohe, once in awhile with the faucet wide open
or near wide open, there will be little or no flow. But close
it once or couple times, and the problem goes away.

I haven't take the faucets apart yet, I guess that's a next step,
but have been reluctant because I didn't know how the handles
come off. Just found on the internet that they just pull off.
So, I can investigate that way or alternatively I thought about
rigging up my air compressor and blowing back into the shower
line.

Any ideas?

Any chance the valve handle is slipping, not working as it should?