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Old 08-24-2021, 08:22 PM   #1
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Vacuum Pump for 38' Mh with two evaporators?

So, I have an old Harbor Freight single stage 2.5 cfm vacuum pump.

Should I be able to achieve -28 vacuum on my 134a AC gauges with this pump with @ 126 running feet of ac hoses and two Evaporators and driers?

Currently it is reporting, and holding, -10 vacuum max.
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Old 08-24-2021, 08:37 PM   #2
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probably not with a single stage pump.

But, you should be able to get WAY more than 10 in of vacuum. Apparently, you have a leak or two. Why not pull a vacuum on something small, not 126 ft of hose? See what you get then.

I am an old HVAC tech and contractor and we always used 2 stage pumps.
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Old 08-24-2021, 10:01 PM   #3
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Quote:
Originally Posted by Pogo View Post
But, you should be able to get WAY more than 10 in of vacuum. Apparently, you have a leak or two. Why not pull a vacuum on something small, not 126 ft of hose? See what you get then.

I am an old HVAC tech and contractor and we always used 2 stage pumps.
Because I have nothing small that needs a vacuum and refill.

And, I need to fill the subject hose.

So how much more than 10 inch of vacuum should I be able to achieve. What do you think is the margin of error?
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Old 08-25-2021, 07:58 AM   #4
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There isn't a margin of error as you say. 28in is necessary. Get a better pump (2 stage as recommended) or at the least replace your pump oil as old oil can be a cause.

-10 tells me you have a heavy leak as well. Good luck
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Old 08-25-2021, 10:19 AM   #5
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This is embarrassing................

I posted this question about 15 months ago with the exact same statistics.

(My brain age is going to do me in!! Getting older is a challenge.)

Reviewing that post,

1. My gauges and pump do not work right.

2. The set from Auto Zone did.

3. I filled the system with $60 of 134a (7 cans) and it worked.

4. But we did not use the AC system due to limited travels last year.

Now the system is empty, so even though the -26 vacuum held for two hours back then, I still have a leak.

So I have a path forward.

Thanks to all that replied to my question. I apologize for asking my redundant question.
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Old 08-25-2021, 10:24 AM   #6
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It's all good Dean.
7 cans is A LOT. Most motorhome systems I have seen are about 4 pounds. (4 cans?)

Many say stay away from stop leak products, but my system still had a leak after many attempts at detection. My final resolve as I didn't have the thousands of dollars shops wanted to possibly correct the problem was to put in about $100 worth of Blue Devil's Red Angel stop leak. That was a year ago. Sealed my system up until recently when I put in a half a can as the discharge temps went up to 58 from 48 degrees.

Good luck.
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Old 08-25-2021, 12:05 PM   #7
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Rich, thank you for the Red Angel reference.

I just happened to read about it a few days ago in another thread. It sounds good from anecdotal evidence.

Typical me, the more popular brands of 134a products have a similar but cheaper product that I will try.

We just are not using our rig much due to our age and Covid.

But, I will refill the system to see if the sealant stuff helps. They say it is not a permanent solution on the can label I read. Perhaps this is to encourage future sales to current customers.

I have the original label on my radiator coolant reservoir for the over-the-road ac system which specifies the quantities of 134a and pag oil required. 5lbs of 134a is required.

The positive in this thread is I inadvertently verified that my current gauge set is consistently wrong...............
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