copper fittings for use with a pressure gage

Hard to tell what is available in your area, but you should be able to find a 3/4" sweat tee, with a female thread that you can bush down to the gauge. If nothing else a 3/4" tee, with a female NPT adapter soldered to it. Greg

Home Depot has a guage that connects to a regular hose fitting. You could tee it off the washing machine using a hose splitter

I just stick mine on the drain of the water heater.

A common gage will have 1/8 or 1/4 NPT male threads. You can step a 3/4 tee down to those.

Gages are pretty unreliable, especially when under pressure all the time.

???? A device designed specifically for the purpose is unreliable??? I think the manufacturers would be somewhat surprised to hear that.

Absolutely. This is the nature of the Bourdon tube mechanism.

Many other ways of building them...

Ones one the system here last for several years at least and then it's rarely the gauge itself that fails, it's accumulated rust/sediment.

Units work in power plants for years as well...but, of course, they're industrial units.

All of the above are typical except perhaps reading high.

The least bit of corrosion from a wet or condensing environment will spoil the little gears and bearings.

If you ever have a bad one to throw out, tear it apart and have a look. Looks like the proverbial ladies watch.

Here's a pretty good explanation and diagram:

Can you quantify "unreliable'? Will they stick in one spot,? Not go up? Not come down? Read high? Read low? All of the above?