W C,

1. Yes it's still working. It's a filter drier. The filter is on the inside. If the outer shell of the drier looks "wet" signifying oil leakage when there has been no rain in awhile then it is no longer working. There are other reasons why these fail beside leaks, but leaks are 99% why they fail.

2.The best time to replace them is when they are in this condition when they are beginning to show this kind of wear. But before they actually start leaking. Refrigerant costs will only increase the cost of repair. This is not DIY. (contents under pressure)

If this job is not done right it will most likely lead to equipment death, not cool properly and a whole list of ha, ha, ha. Told you so sort of rhetoric. If the equipment is older than 15 years you may not escape the equipment grim reaper for long anyway. (Equipment doesn't last forever).

Again: This is not DIY repair. Do not attempt. I don't care if you got an A report card from YouTube school of hard knocks. If you attempt it, screw it up and then call me... you won't like what I will tell you then.

CONTEXTUAL NOTE FOR CLARITY: ( I know the OP lives in San Fran and I also know that others will read this post 2 days, weeks, years later. I see all kinds of nonsense in this profession -- you have been warned.) You call me from Katy, Texas I am coming to fix it, not play around with it. If I put my hands on it, it is going to work not just when it's 80F outside but also when that temp hits 100F as well. I have built a reputation of providing solutions that actually work, you nor anyone else is going to derail with foolishness what I have built with my own two hands. (Those that live in my area that call me)

3. In a normal poor mans cost of living environment probably around $300 per drier (which will vary due to access not to mention where the drier is located within the system).

Provided that: Access is easy. if someone has to man handle refrigerant repair equipment up on a roof or hard access area costs go up for that because it takes more time (labor).

Refrigerant: if refrigerant is needed OR equipment can not be pumped down and refrigerant has to be reclaimed prices go up for that.

Some HVAC equipment can not be fully pumped down (it varies) costs to do this are directly tied to those kinds of problems. Because you're in a HCOLA otherwise known as High Cost of Living Area -- you should expect to "PAY MORE" than the traditional poor man environment.

4. Once they are brazed in and the repair is complete you can paint them with rustoleum rust prohibitor. BUT, this sort of thing is only a temporary protector in a salt spray environment. It might give you a few years more of protection but at some point even that too will break down in a heavy corrosive environment.

I service the Katy, Texas area.

@W C,

This looks nearly identical to your post from 7 months ago https://www.houzz.com/discussions/6354383/replacing-filter-driers-on-rooftop-diy-or-hire-professional#n=3

Placing blame where it belongs , that was caused by some sunny beach alright but not the one 2 miles away. That drier is in that condition because it was heated with a torch. Not incidental heat from brazing but intentional heat the installer thought would drive moisture from it allowing it to be reused.