Hello MesoTronik !
I hope most goes well, considering.
As for some former posts u made, 6k Dollar is way 2mutch.
I would recommend replacing parts (if needed) and patching up the air-conditioner u have.
Ill first describe the part where there are no leaks in the system followed later for when there are leaks;
Assuming there are no leaking holes in your system, use a replacement coolant for your system.
To test if u got the right stuff; tap a little bit out of the system and into a cheap small bowl,
go outside and ad a little new replacement coolant into the bowl.
If no bubbling flashing hissing or whatever happens, they are more than compatible,
and thus u don't need to drain your system and replace it... u can simply add the new ''replacement'' coolant.
Some people would advice against this, but that is why u test whit just a little bit, and also this happens all
the time everywhere, including your car during service or even factory's where there is a lot more than
just heat-exchangers and air-conditioners.
Ignore commercial talk like; only replace x form brand x, whit x from brand x.
This is litteral commercial talk. This is also the reason some would almost always fall for this salesman's pitch,
out of stupidity or fear... probably both.
Example... GM motors would always advice GM motors ....would it not? This goes for all commercial talks / commercials.
That being sad, dont skip the testing part, there are incompatibility's around, altho for non-industrial stuff most unlikely.
U have a 1990's system? here is a link for replacement coolants usually found in your local DIY hardware store:
https://www.ctc-n.org/technologies/shift-coolants-and-refrigerants-lower-gwpSometimes it even says on the bottle for what time''era the replacement coolant is.
As for any rust,
Use something called a steel brush; (linky down) brush the spot of rust down, u should -rough up- the area,
( ''roughing it up'' is for surface preparation)
dont worry about the rust that remains after a good brush, for that:
Use some paint that deals whit rust. linky down.
If there are holes, Use the steel brush again this time to remove some rust if present,
than paint it whit same rust dealing paint, than,
than cringe patch-up tape will work fine, this tape is similar to ducktape, but when heated if shrinks, and thus making a cringe seal. linky down.
Apply heat whit an paint blower, or torch lighter. Open flame works2, altho is difficult, / i would not recommend for beginners in the DIY department.
If i had a case like this, i would be done for under 100 Euro (including buying a paint blower)
U should be done for under 100 Dollar ; -)
PS... check whit a ladder ontop your roof for leaks, should be obvious when u get close, a darker spot --(previously wet spot)-- is easy to see from close up.
Use your fingers to feel the bottom of any hose / hardline, if it has a wet feeling, find it visually, and track it back to the source.
steel brush example;
https://nl.rs-online.com/web/p/wire-brushes/0822658anti rust paint example:
https://www.hammerite.co.uk/product/direct-to-rust-metal-paint-smooth-finish/Be carefull here, canned coating sprays dont do the job, and anti rust paints need a rust free underground (and thats hard todo!! )
cringe tape:
https://www.gaines-thermoretractables.fr/en/heat-shrink-tape/paint blower:
https://www.thomasnet.com/articles/materials-handling/best-heat-gun-for-shrink-wrap/Edit, in your language it is called heat gun instead of blower.
And the cringe tape, could be called Preservation tape. or Pipeline Shrink Sleeve. (for waterlines) sometimes called Waterwrap; linky
http://blog.heatshrinksleeves.com/2014/12/heat-shrink-sleeves-on-water-pipe.htmlWhatever the case, it needs to be suited for plumbing. Whit a good range, your area is hot, and your
tubing transports eigther hot or cold coolant medium.
If all this information is 2 mutch ->
The savvy tech guy at your local DIY store should know what u need. Describe the situation and item + (sub)items to be repaired / take alot of pictures
And of-course, don't forget to check for holes or breaks in the system before u actually go to the store, no need to buy stuff your not gonna use.
All in all,
This information should set u up to deal whit this yourself, for a lot cheaper,
and thus, i will consider this my contribution to u ^^
Last but not least, your system uses a lot of power / energy, driving your monthy bill way up.
Instead of kicking the can down the road, read up on stuff like terracotta cooling, here is a starting linky:
https://earthbuddies.net/terracotta-air-conditioner/A system like this is possible for your house, because even tho the foto u provided is a bit shy on detail,
I could see u have an house at ground floor, most likely whit a garden, therefore this is possible for u.
These luv nature people aren't just tree huggers ya know? ;-)