A/C holding vacuum?

[Soundkillr]

I have replaced every part in the system now except the small hose going from the condenser to the dryer. I JUST got done replacing the condenser, so I thought I would vacuum the system out. I pulled 29 inches for 1.5 hours and stopped. Let it sit with my gauges on it for two hours. Dropped after two hours to 28.5 inches.
I decided to let it sit for five days, as I planned to recharge this weekend. Hook the gauge back up and it's at 16 inches. I know some vacuum was lost when I unhooked, rehooked gauges, but not that much. Question is: has anyone pulled vacuum on the delorean system and let it sit. How long would it hold vacuum, assuming the system didn't leak.
I know in theory no vacuum should be lost, but based on my research it simply doesn't work that way. Some systems lose vacuum but when charged wont lose pressure. I also know this isn't a good way to determine if I still have a leak, but I would like some feedback.

[Bitsyncmaster]

Try it again. You may just be pulling moisture out of the system which would drop the vacuum.

[David T]

If you don't pull a hard enough vacuum for long enough, any moisture in the system will boil and you will lose vacuum. The other common thing is very often the test connections you hook to the system leak. If you replaced all of the stuff why didn't you also do the little hose too? If any hose leaks or breaks you lose all of your refrigerant. Using vacuum to test for leaks is not a good strategy. If you have a leak you draw contaminants in and you cannot find leaks. Pull your vacuum again and then immediately fill with a dry pressurized gas (like Nitrogen) to 200 psi. NOW you can look for leaks with some bubbles (soapy water or leak test solution). Fix any leaks you find, put your oil in and then vacuum again for at least an hour to 70 microns. Then fill with refrigerant. BTW using vacuum is only negative 15 psi. Using positive 200 psi you can find much smaller leaks and much easier.
David Teitelbaum

[Soundkillr]

I pulled vacuum for 1.5 hours, plenty of time per recommendation. I know pulling vacuum isn't a way to look for leaks, I already stated that. I pulled vacuum because I needed to evac the system after the change. I happened to notice it was down 5 days later so posted the question. i don't have access to nitrogen, and don't wanna refill if I am out of spec. That's why i wanted to see if the vacuum would hold in a healthy delorean system. Several websites have stated that it will, and it won't. I am hoping for feedback from someone who has done this. I have not been able to source a new small hose with the fittings for low and high pressure switch. Really wanted to avoid a nos one, but if that's all I can do, I guess I could replace it. I know the larger lines are prone to leak, but haven't heard many horror stories on the small line. I have a 40 year old Toyota with original lines, and it doesn't leak at all. (They are also not 10+ foot lines) hence my being (as) unconcerned with the shorter line.

[Bitsyncmaster]

My guess would be you can't keep your connections on the test gauge tight enough to use vacuum to determine leaks. The schrader valves only get a good seal when under pressure.

[nullset]

I think you need to let the vacuum pump run for a while longer. You also may need better gauges.

I have a digital vacuum gauge I use for this, and a good deep draw vacuum pump. Your gauges may not be accurate at pressures that low - my digital gauge reads in microns!

As you pull a vacuum, moisture in the system boils off. The deeper the vacuum, the more moisture will boil off. As that happens, the pressure increases, because the water is now a gas. You have to keep drawing the vacuum down until that stops happening.

Are your valves new? They tend to leak under vacuum, so make sure they're capped or you have the gauges hooked up securely.

--buddy

[Ron]

Other things to consider-
How often did you have to re-fill originally.
How much did it hold before you changed out parts.
Removing fittings right before/during test.
You are at 1000ft (reduce .5).

I think you are good to go from the history I know about it. (Maybe go ahead and change the small hose)
The closest you are going to get to being assured of a tight system before charging with refrigerant is to follow David T's suggestion (and putting a single gauge on one side then remove all other connections.....a bit of over kill imho)

[Soundkillr]

Thanks guy! The valves are new, the gauges and the pump are both really nice robyn air units. The pump is a high dollar commercial unit. I will try a deeper vacuum. Wouldnt suprise me the valves pull air in, but fwiw they were capped tight when done.
The system originally leaked about a can a year. The low pressure switch was suspect and replaced along with the high switch at the same time I did the condenser. I have some time this weekend I hope, so ill run a three or four hour vacuum.

[nullset]

Thanks guy! The valves are new, the gauges and the pump are both really nice robyn air units. The pump is a high dollar commercial unit. I will try a deeper vacuum. Wouldnt suprise me the valves pull air in, but fwiw they were capped tight when done.
The system originally leaked about a can a year. The low pressure switch was suspect and replaced along with the high switch at the same time I did the condenser. I have some time this weekend I hope, so ill run a three or four hour vacuum.

Let me know if you'd like to borrow the digital vacuum gauge.

--buddy

[David T]

The compound gauges on the manifold gauge set cannot accurately read deep vacuum. You need a special gauge for that (to read in microns or Torr). The proper order is to do your repairs, fill with dry Nitrogen, pressure test, repair any leaks, retest, repeat till you have no leaks. Then you do a deep vacuum and fill with refrigerant. If you do not have Nitrogen you can use shop air at 100 psi but it should be very dry. That means a dew point below -20. To get there you need a coalescing filter and/or a chiller. You pull a vacuum till you no longer see the vacuum rise when you shut the pump off. If the pressure rises (and you fixed your leaks) it means you still have moisture in there boiling and increasing the pressure. You should hold 40 microns for at least 1/2 hour with the pump off. As I said, leaks are usually your test connections. BTW, to pull a hard vacuum you should be using a valve core remover so you don't try sucking through that tiny Shrader valve.
David Teitelbaum