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deandec · 05-22-2020, 09:16 PM

I need to pull a vacuum on my 38' DP to test for leaks before installing the new 134A.

My Harbor Freight Vacuum Pump only pulls to -10 vs the -29 recommended for Auto AC systems.

Once the system is at -10 , the vacuum prevails for a 2 hr test period.

Do I have good reason to think that my AC systems is probably free of leaks?

The extra 60' of AC hose from front to back and return vs an automobile would seem like my simple vacuum would struggle to pull a -29 vacuum.

Jim18655 · 05-22-2020, 09:39 PM

Which pump are you using? Even their $16 air operated pump is rated for -28.3".
The length of the hoses should only increase the time to reach maximum vacuum. Could your gauge be wrong?

RichKimball · 05-23-2020, 08:11 AM

I rebuilt my system last summer with no issues pulling to -29. Either your gauge is wrong or your pump is bad or you have a huge leak?

96Bounder1st · 05-23-2020, 09:33 AM

Quote:

Originally Posted by deandec

I need to pull a vacuum on my 38' DP to test for leaks before installing the new 134A.

My Harbor Freight Vacuum Pump only pulls to -10 vs the -29 recommended for Auto AC systems.

Once the system is at -10 , the vacuum prevails for a 2 hr test period.

Do I have good reason to think that my AC systems is probably free of leaks?

The extra 60' of AC hose from front to back and return vs an automobile would seem like my simple vacuum would struggle to pull a -29 vacuum.

If as you stated the vacuum holds at -10 you are likely good. As the others have stated your gauge may be wrong.

Good luck,

Terry

twinboat · 05-23-2020, 09:39 AM

If you trust your gauge, holding at 10 only means it starts leaking above ( below ) that.

Are you chasing a refrigerent leak ?

You can charge the system with argon and monitor pressure.

MVD · 05-23-2020, 09:54 AM

Even if there are no leaks in the system you need to pull the vacuum down to 29 to get all the moisture and "air" out of the system. Otherwise you will have a partially charged system with contaminants in it which will decrease efficiency and damage the compressor over time. I use a Robinair vacuum pump with fresh oil and after I know I have no leaks I pull the vacuum at ~29 for at least an hour before the system is recharged. It needs to be clean, clean, clean. If your pump wont get there, or you still have a leak and after you fix it, barrow someones pump for an hour to get the system empty.

deandec · 05-23-2020, 12:59 PM

Quote:

Originally Posted by twinboat

If you trust your gauge, holding at 10 only means it starts leaking above ( below ) that.

Are you chasing a refrigerent leak ?

You can charge the system with argon and monitor pressure.

Thank you. Your point of a potential leak at below -10 inspires me to obtain a different pump to test the result my pump delivers.

My AC system tests empty on the manifold gauges which indicates I had a refrigerant leak.

Dav L · 05-24-2020, 06:54 AM

Bad pump, bad gauge or more likely a leak.
And you are looking at the system because no AC?
And there was no freon?
Sounds like a Leak.

Look for dirt collecting around a fitting. Sure sign that the fitting is leaking and the oil in the freon is attracting the dirt.

One of the few repairs I don't power wash the system before diagnosis

You can test the pump and gauge by closing off the end of the hose and turning on the pump for a minute. Should immediately go to -29ish.

deandec · 05-24-2020, 10:56 PM

Quote:

Originally Posted by Dav L

Bad pump, bad gauge or more likely a leak.

One of the few repairs I don't power wash the system before diagnosis

You can test the pump and gauge by closing off the end of the hose and turning on the pump for a minute. Should immediately go to -29ish.

Thank you for the pump and gauge test recommendation.

While evacuating the MH systems, I had both gauges deliver a -10 result and the system held that result for a couple of hours.

The pump is a 2.5 cfm capacity.

Closing the coupling valves on the blue and red hoses, the Vacuum Test result is -10 on the blue (lp) hose and -30 on the red (hp) hose.

This vacuum has held for over 30 minutes.

The result is the LP side cannot report a -30 vacuum but the HP side can.

Neither side leaks down. There was no MH AC system leak in the test to cause the LP -10 limit.

(I have applied teflon tape to the threads of the Blue LP hose which did not change the result and firmly tightened those fittings.)

Any guesses out there whether my problem is with the pump or the LP manifold gauge?

deandec · 05-25-2020, 02:51 PM

Ok, I lucked into a Loaner gauge set from AutoZone to test my owned gauge set issues.

The loaned set was able to pull at least a -25 (I quit a bit early) reading and then hold it on the Blue LP connection. As before the Red HP connection went to -10 almost immediately.

Conclusions:

My gauge is not reliable. My Vacuum Pump is.

I replaced the LP hose adapter on my Manifold with some success.

1. The LP side of my Manifold Gauge will still only report -10.

2. The LP side of my Manifold Gauge reported correctly for charged pressure levels on my old Jeep AC system after I installed a new adapter on the old hose.

3. My Vacuum Pump reported vacuum of -25 LP and -10 HP with the loaner gauges.

Now I will tackle the MH dash AC system with working tools!

Thanks to all who assisted in resolving this diversion.

TXiceman · 05-25-2020, 04:08 PM

The vacuum is primarily to get the moisture out of the system, not as a leak check. If under a vacuum and you still have moisture in the system, the pressure will rise due to any liquid water going into the vapor state. First thing to do is pressurize the system and use a refrigerant leak detector to find the leaks.

Once the system is leak free under positive pressure, break the vacuum with the refrigerant and start charging the system.

Ken

deandec · 05-25-2020, 10:34 PM

So, if I pull a vacuum that holds for an extended period of time, I will still have an unidentified leak that I cannot detect via that vacuum?

If so, will that be a major leak or slow leak?

Dav L · 05-26-2020, 06:49 AM

I don't know why one side is showing it can be pulled down to a deeper vacuum than the other side...especially when you say the vacuum holds stable (ie: no significant leak).

You should be pulling a vacuum when BOTH sides are opened to the vac pump. So they should have identical vacuum.

Using a vacuum pump is to rid moisture, to help draw freon from the can, AND to check for a leak.

Basic tutorial on pulling a vacuum:

https://www.youtube.com/watch?v=kUdCa3feGvg

Old Scout · 05-26-2020, 07:44 AM

Agree with Dave L....I am by no means an expert on AC but do have a degree from the
"school of hard knocks" ….a vacuum is a vacuum--don't recall ever seeing a different level on high vs low ports...don't know the specifics, but an AC system is a loop--so a system at rest will equalize [hi/lo ports[pressure or vacuum]]over a period of time....also thought low side gauge was the only one with a vacuum reading????]..think you have something more than a small leaking going on here--this might get a bit expensive???????