This problem is very much noticed on many Toyota vehicles.After some years of usage the Rear Side A/C lines gets rusted and gets cracked.Replacing the lines is only option to get complete cooling.The cost to replace is A/C line assembly is approx $1500 including labor charges.
But there is no guarantee that this same problem will not reoccur after some years.

Mostly the problem is noticed on the lines going back from the front unit to the rear unit.

THE LINES TO THE REAR HAVE BEEN AN ISSUE EVEN THROUGH THE Present latest MODEL YEAR.

It is a corrosion issue and the muffler on this year has a tendency to rubber on the low side line and wear a hole in it. The fitting at the rear cooler unit is an issue also. The front can not be isolated by any conventional means. In theory you can just run the front which many repair techs will recommend.And that is correct. This would require someone that can weld aluminum efficiently. Up on the lines where they branch off from the front would have to be cut and crimped then welded over at the seam that was crimped. There would be no side affect it would just have to be charged with a little less Freon.So the line dont gets over-pressurized by ample of Freon.

This details will help.
Thanks.

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Car AC is very important part of your car.On hot days while just sitting in the car or while driving,AC is very much required.Without AC ,you cannot feel comfortable while driving.
Suddenly while driving your car,you turn ON your car AC.You set it on MAX cooling, but the cooling is not proper.You travel by same car daily,so you know exactly about how much your car AC is cooling.
There is cooling, but you notice, its not that better like regular.

This low cooling indicates, that your car AC needs maintenance and service.
Also once in a six month check your car AC components.So you know exactly about your AC working.

You can yourself,do this maintenance or in a low cooling, you can check out AC parts and do a troubleshooting.
This are few easy steps,that will help you to perform the AC maintenance and troubleshooting.

1) First start with bolts.Check all the bolts mounting on the AC components.If any of the bolt is came out completely or just got loose.Then lubricate the bolts and re-tight them.Please note that dont over-tight the bolts.

2) On AC system there are two ports.One port is called high port and other is called low port.There is cap on this ports.Check that cap is properly seated on the service ports.This cap is to stop dirt/dust to get into the port.If the dust enters the port, then AC cooling problems are noticed.AC refrigerant flows through this ports,if there is dirt, then refrigerant will not flow freely and this will cause cooling problems.

3) There is a compressor clutch on the AC systems.This clutch engages when you turn on the AC.The compressor starts running after the compressor clutch engages.If the compressor clutch is not engaging then there is problem with refrigerant.Either refrigerant is blocked or low.Or there is electrical problem with clutch switch.

4) If you notice compressor is cycling on and off rapidly or you hear clicking sound from compressor then this due to low refrigerant in your car AC system or compressor clutch problem.On low temperatures below 38 degrees,the clutch will not engage.

5) If you hear knocking , rumbling popping type noise from compressor with engine ON,but AC off, then compressor clutch needs to be inspected.

6) If you hear squeaking / squealing type noise when AC is turned ON,then in that case inspect the AC accessory belt for cranks and getting over stretched.

7) If you notice belt is vibrating with engine running and AC ON.Then in that case the belt placement needs to be inspected.It should be tightly routed through the pulleys,if the belt is got loose ,then it will vibrate.But if belt is again getting loose,then belt tensioner is faulty and needs to be replaced.The tensioner is not creating required tension to the belt, that's why it gets loose.So tensioner needs to be replaced.

8) If you notice dirt getting accumulated or you notice oil getting collected on the fittings and on the AC lines,then this indicates problem with AC system hoses.This hoses are cracked or worn out or got very loose.In that case the hoses needs to be replaced.

9) Dirty condenser or clogged condenser coils can also cause low cooling.Many a times dirt like leaves,papers,insects,small cloth gets stuck up on condenser or get clogged in condenser coil and this causes the problem.inspecting the condenser will also help.If you notice any obstruction,then get the condenser cleaned, you can clean condenser with garden hose.That is what mostly used and it works well to clean condenser.

This troubleshooting will help you to solve your problem,or at-least help you know your AC no cooling or low cooling problem.While AC routine maintenance ,you can check the mentioned part for cleaning and servicing.This will make your AC last longer.

To get more help for your car AC troubleshooting:---
You can go through this links.