A/C blows warm, weird problem, 99 astro

this is the problem :

the a/c was cooling great with just the front on ,and the frt and rear a/c on. always cold .
then one day the front a/c started to lose its chill (over a 2 week period) , but when turn the rear switch on the front gets cold and the rear is cold still. so i drove for 2 weeks with both frt & rear a/c's on then they both lost there chill again .
after i filled the system up and it blew cold for 1 week .
i looked at all the lines and found a return line from the rear a/c that goes over the engine and down the side of the block and has a orange rubber hose clamp attaching it to the block,it had let the metal bend in the return line rub on the engine block . but no leaks or oil drips that i could see with my eyes. that doesnt explain why the rear was cold and the frt a/c was warm till the rear was switched on.
i think maybe the frt evaporator has a leak and is clogged and the rear one runs both frt/rear a/c's till the freon is gone. ???????? im not a pro at a/c . could someone give some imput on this . thanks

There is only one system, so if refrigerant was lost, it would be gone everywhere. There is a possibility that the orifice tube in the front evaporator is clogging, causing poor refrigerant flow in that coil. If system pressures are normal, that may be something to investigate. However, that requires evacuating the system.

ok, does the rear evaporator have a orifice tube also?

and it sounds like the system needs a cleaning ,how do i clean this large system right and what spray do i use?? and what about oil replacement ,how much and what kind.??? thanks

If when you add refrigerant it cools much better- then you are losing refrigerant- due to a leak-- that rub-through area on the back of the block is a TSB item- saw it the other day-- I would look at that area really good- if in doubt -you can have it sniffed for a leak- the only good fix- is to replace that $200 hose set- or remove it and aluminum spot weld the rub-through area- I have done about 7 or 8 this way- works good- just time consumimng- removing that massive hose assembly-- this is of course assuming this is where your leak is. I also highly suspect the compressor shaft seal and belly o-rings-- look under the body of the compressor- is it oily/greasy? or clean? Look around the radiator hose assembly for "splatter" of oil from a leaky shaft seal of the compressor- is the front of the compressor oily? is the radiator hose oily? Just some thoughts--- remember most a/c failures are due to leaks somewhere in the system.........

the a/c compressor is oily on its body but the whole engine is grimey, none on the hoses have oil on them . i think its the rub in the line . should i just change the whole compressor/and lines rather then just the frt seals, and o-rings ??

i was going to weld the rubbed line up, i have a tig welder and years of practice.

Having access to a good welder is great- I continue to find and lose them- due to company downsizings-- so the compressor re-sealing is a good idea- for someone who has done a few before-- but special tools and experience are extremely necessary- so a new compressor may be your rout there-- spot welding the aluminum pipe at the rub-through is a huge savings- just go price that 3-legged hose assembly somewhere.....sure wish you lived near me-- I can re-seal those HT-6 compressors in my sleep....and could use your welding ability.......