Any plumbing advice to give? Low pressure on one faucet

I thought I knew how to fix this, but it turns out I was incorrect.  We have this jacuzzi tub in our master bath.  It's a two handle system where the two handles are over on one side of the tub and the faucet itself is over on the other end.  Worked fine for the last 20 years. 

All of a sudden, though, the hot tap will push a good slug of water when you first open it and then the flow immediately reduces to almost nothing.  Cold flows fine.  Hot water flows fine to every other faucet in the house.  What's weird is that if I turn on the hot taps for the sinks in the master bath they flow strong until I turn on the bath hot tap.  As soon as I turn on that one, the water basically stops flowing from the sink.  Turn the tub off and pressure returns.

I figured it was the cartridge, so I just put a new Delta one in.  It doesn't leak any more, but it doesn't change the pressure problem.  I opened up the panel to look around under the tub and there's no water leaking beneath. 

Does this mean that I've somehow gotten something clogging up the supply line?  How does one go about fixing that?  Any ideas?

Hmm. Sometimes there's a short galvanized pipe between the copper supply and your shutoff valve. That can have crud build up in it that will restrict the flow a bunch. When the faucet is off, full pressure builds up between the shutoff and the faucet so an initial burst of water is pushed out upon opening. But not much volume. Then the restriction keeps the flow low from then on. Do you know if there's a galvanized stub out behind your shut off valves? Can you see behind them at all?

Definitely sounds like a blockage issue. Super common. Sometimes hard to find. Sometimes it's as easy as pulling the supply lines off and just flushing them out a bit. Think of it like chasing voltage. You obviously don't have a problem at the other fixtures, so something is wrong nearer the tub. Makes sense too, as that's likely the lowest point for hot water lines. Any chance for pictures under the tub? Stubouts, supply lines, and the fixture from both top and bottom?

Shutoff valve?  Ha, that's funny.  As best I can tell, I have to turn off the water to the whole house. 

Here are the pipes as they come through the floor:

And then they go along to the faucet handles:

And up into the handles and out to the actual faucet spigot:

I do not know where the sinks and shower get their water supply but wherever that is, it branches off before the first picture.

What's weird is that there's no water pressure problem at the sinks until you turn on the hot at the tub.

I wish there was a shutoff valve close by.

I have taken the guts out of the faucet then turned the water back on (deflecting it into the tub) in order to flush a line.   In your case it would take two people with phones to work in co-ordination.

Also, with water off, pull guts out, and run a pipe-cleaner down through the valve assembly.

In reply to Purple Frog (Forum Supporter) :

And here I thought pipe cleaners were just for third grade arts and crafts time!

It wouldn't be too tough to get someone down by the shutoff valve while I tried to contain old faithful.  You're thinking that by not having the restriction of the faucet and everything, a blockage might get dislodged by the increased flow and come flying out?  I get that.  Which, actually, now that I think about it, when I turned the water back on there was a bunch of air in the pipes - as expected - and as the tube faucet was clearing that out, it did shoot some "stuff" out that wasn't water.

Do they make a tool like a snake but that's for supply lines and not drains?  I cannot fathom why in the heck they didn't do the supply like every other faucet in the house:  pipe with a valve on it then a threaded hose up to the faucet.  I mean, yeah, I can fathom it:  It saved them about $5 on building the house plus about an hours' of the plumber's time.

Is there an aerator in the faucet? It could be clogged with deposits if you have hard water. 

I have to soak my master bathroom faucet aerators (not sure why just those and not the 8 other faucets/shower heads in the house) in CLR every couple of years to clear the crud out of them and restore flow. 

Do you have a water softener? When the resin goes bad you will loose water flow through the softener at higher volumes. 

You know what I hate?  When I search the internet for a problem, find someone else that has the exact same problem, and there's no follow-up on how they resolved it.  Well, internet people from the future, I'm not going to do that to you!

The problem was the water heater.  I am assuming that it is very filled with sediment.  I'm actually a little bit surprised that I was able to ride that thing out this long, but the water pressure for hot water got worse and worse to the point where if the washing machine or the dishwasher was running, the best you could get out of any other hot water faucet was a bare trickle.  Yesterday, the 13-14 year old water heater sprung a leak so with the help of an amazing friend, we now have a brand-new Rheem unit and there's full hot water pressure everywhere in the house.  I can even take a shower while the dishwasher and washing machine are both running AND someone flushes a toilet.