How long does it take to vacuum down a previously open system

[Dano 500]

I have a lot of questions in mind, but I'll try to address at least some of them here.

Given the situation, I would have replaced everything with new parts, including the the evaporator, because you never know what's lurking inside a system that has been open for years—including gunked-up oil mixed with water, and even insects, that might not have been entirely eliminated by flushing.

If you're evacuating the system and the vacuum fails to hold, it's possible that the leak could be someplace other than in the system. How are your Manifold Gauge Set, hoses, and adapters? Are they relatively new, of good quality, and all seals intact?

Also, what type of vacuum pump are you using? An electric, rotary-vane pump containing fresh oil and good fittings, or an air-operated pump?

The vacuum leak could also be in the evaporator (or anywhere else, for that matter) if you didn't remove it, clean the fins, and carefully inspect it for signs of corrosion or evidence of oil leakage.

Did you replace all the o-rings and/or seals?

As for how long to evacuate, a shop will usually evacuate a system for around one hour at most due to the "time is money" aspect. If it's my own vehicle, and I'm not in a hurry, I'll run the vacuum pump for about 8 hours, or overnight.

Without any leaks in your equipment and system, the low-side gauge should drop almost immediately to around 25 inHg and continue in a few minutes down to about 29.9 inHg if you're located at an elevation of sea level or slightly above. For higher elevations, the gauge reading will be less than 29.9 inHg.

But to achieve the best possible evacuation, a micron gauge will be needed. A good vacuum pump is a given. The goal is to draw-down the system to about 500 microns or below, which might take several tries depending on how much air and moisture need to be removed and how much of the same the system might be outgassing.

Lastly, because the vacuum in this system doesn't hold yet, and you're pulling in additional air and moisture through the leak(s), you'll need to replace the R/D again once you achieve a system that is tight and leak free. The R/D contains a desiccant that becomes saturated with moisture under leakage conditions.

Thanks for taking the time to help me out.

So, the vacuum is an oil 4.3 cfm and the manifold/gauges and pump are all brand new. To check if there was a leak in the hoses, connections, manifold, or pump, I left pump running and closed valves on high/low ports. The gauges immediately went down to -30hg. I turned off the pump and it stayed at -30hg, so I guess that means the leak is in the system still. I did replace all o-rings/ seals. The only part that isn't new is the evaporator. I was just wondering if the moisture in the evaporator would prevent vacuum from going down to -30hg until it's all boiled off. Maybe the evaporator is somehow damaged. So, once I get the leak figured out, I have to replace the RD? before charging system? What is RD? Is that the dryer?

[JohnHere]

So, the vacuum is an oil 4.3 cfm and the manifold/gauges and pump are all brand new. To check if there was a leak in the hoses, connections, manifold, or pump, I left pump running and closed valves on high/low ports. The gauges immediately went down to -30hg. I turned off the pump and it stayed at -30hg, so I guess that means the leak is in the system still.

If I understand correctly, you closed the thumbwheels on the service-port adapters, and the vacuum held. So you are correct that the leak(s) must be in the system itself.

I did replace all o-rings/ seals. The only part that isn't new is the evaporator. I was just wondering if the moisture in the evaporator would prevent vacuum from going down to -30hg until it's all boiled off. Maybe the evaporator is somehow damaged.

Moisture inside the evaporator probably wouldn't cause a loss of vacuum as you described. It's possible that the evaporator has corroded-through from the outside over the years, particularly if leaves and debris have been sitting in the evaporator case, which retain moisture and promote corrosion.

So, once I get the leak figured out, I have to replace the RD? before charging system? What is RD? Is that the dryer?

Correct...the Receiver/Dryer (R/D) will have to be changed again because the system has been sucking-in air and moisture due to the vacuum leak.