Have I overlooked some minor step in the standard procedure of draining (copper) lines, to get water out of the vertical air chambers? Most "how to's" for this seem pretty much same.

In the past, for *rare* "minor" water hammer, just shut off water at the street, opened all faucets - inside & out, flushed toilets;
allowed to drain maybe 15 - 30 min, then turned water back on. Don't remember specialized steps in the process.
After those steps, was good for many yrs.
Now, having problems eliminating it (100.00%), and seems it's worsening (slowly) after days / weeks, vs. yrs.

Copper lines to 2 baths, ~ 70 ft run, from where supply enters garage, across the attic, to the baths.
Longest run - more inertia.

2 wks after last draining, some "bumping" increased enough, to think I overlooked something, or there's a new problem .
After that last draining, it was almost gone. Only a very faint "bump" if held an ear to the wall.

I assume original plumbers installed vertical hammer air chambers at EACH bath sink stop (hot & cold), and both toilets.
** Were air chambers typically installed at / near TUB or SHWR faucets?

Could try blowing lines w/ small compressor. Or possibly an air chamber is partially blocked.
If so, either have to open walls for repairs (not good), or install arrestors - under cabinets / behind toilets, as required.

Thanks.

Why not install some mechanical arrestors at your washing machine and dishwasher? The quick stopping of the solenoid valve is what causes the hammer, and mechanical arrestors don't get waterlogged.

Thanks. That's one option, though the hammer happens at opposite end of house. Not near the DW, washing machine, kit. faucet.

The hammer sounds like it's in the wall, between the 2 baths (share a common wall).
I imagine any arresters or added chambers would need to be on that end of house.

But not sure where the most effective location is to install an arrester (or a pair, under a vanity), or if definitely need them in multiple locations.

Modern 1/4 turn faucets make the shock worse.
(Sometimes) I hear some bumping even when toilet fill valves shut off.

I know basically how several of the lines run in those 2 baths (not 100%).
For arrestors, question is how to determine the best spot, when you have 2 small baths, close together?

I believe the lines going to each bath, branch off main lines coming across attic.
It's NOT like the supply lines go 1st to one bath, then on to the 2nd bath, farther down the line.
At least, not for both vanity faucets or the MBR shower.

But turning off either vanity faucet can cause the hammer. Sounds like the hammering point is generally the same, turning off either faucet.
And that "point" isn't right by either vanity, nor directly between them.

I can probably find / remember which vanity & which toilet is farther toward end of their respective branches.