It appears that the charge has leaked out from age.

There is no need to change components or get worried about excessive moisture unless the system has had its lines fully open to the atmosphere for up to 30 minutes in a low humidity climate. Ten minutes in a high rainy day one. Rule of thumb.

I would look at all the fitting connections for any sign of oil and dirt accumulation. Sometimes they can be just tightened.

You could have a small leak at the accumulator housing. On my '91 there is a factory only Volvo fitting on it. It is under a large black plastic cap. I have found that with age they leak. The black cap contains it well but not over all the years. I seal it off with a steel disc and super glue and replaced the cap.

Air is a non-condenseable gas. It carries moisture. It is the enemy!

If you have zero pressure assume at least minimum air. Any positive pressure you are probably OK!

If you can pull a vacuum on the system. It's best. Let it sit for ten minutes with a vacuum gauge. If it doesn't move up. You are OK to recharge. Having a vacuum will suck 80 to 100 percent of the liquid charge in the system upon refill using a manifold gauge set.

They are worth their salt. Better vacuum and refill to both sides. You can read temperature and pressure of both sides. Essential for diagnois of blockage or component operation. Like a voltmeter for electrical work.

If the gauge does move up you either have a leak or have gotten moisture in the system. In a case of extreme wetness you vacuum a lot with heat or change the drier, then both.

Put in the require amount by weight in pounds and ounces from the label on the car. This type system is charge by weight to achieve balance for optimum performance.

You are correct to not overcharge!

Hope this answered questions you had.

Phil