You’ve pulled a deep vacuum, isolated the pump, and started the decay test. But the numbers on your micron gauge start climbing, turning a simple job into a frustrating leak hunt.
Your HVAC system is not holding a vacuum due to either a leak in the system itself, moisture trapped inside boiling off, or a leak in your equipment (hoses, gauges, or pump). A methodical process of elimination is the only way to find the true cause.
There are few things more frustrating than a failed standing vacuum test. You're near the end of the job, and this final check throws a wrench in the whole process. In my 10 years in the vacuum industry, I've learned that the first, most important question to ask is: "Is the problem with the system, or is it with my setup?" Blaming the AC unit before confirming your own tools are perfect is the fastest way to waste hours chasing a phantom leak. This guide is the exact methodical process I use to find the real problem fast.
Why is my vacuum pump not pulling a vacuum?
Your pump has been running for a while, but the micron gauge just won't drop below 1000 or 2000 microns. You start to question if your pump is even working.
Before blaming the pump, you must isolate it. A pump that won't pull down is most often connected to a large leak in the system or a significant leak in your hoses or gauges. Test the pump by itself first.
The number one mistake I see is technicians assuming their pump is bad when it's actually their equipment or the system they're working on that has the leak. A vacuum pump can't overcome a constant flow of air coming from a leak. The first step is to perform a "blank-off" test to verify the pump's health.
How to Perform a Blank-Off Test
- Disconnect everything from your vacuum pump.
- Connect a reliable micron gauge that you trust directly to the pump's inlet port.
- Turn the pump on.
A healthy, well-maintained pump with clean oil should pull down very quickly. A good pump will often pull down to below 100 microns, with many quality pumps reaching 25-50 microns in under a minute.
| Blank-Off Test Result | What It Means | Next Step |
|---|---|---|
| Pulls down below 100 microns | Your pump is healthy. | The problem is in your hoses, gauges, or the HVAC system. |
| Struggles to get below 500 microns | Your oil is likely wet or contaminated. | Change the vacuum pump oil and re-test. |
| Still won't pull down after oil change | Your pump has internal wear or damage. | The pump needs professional service or replacement. |
What causes lack of vacuum pressure?
Your pump passed its test with flying colors. But as soon as you connect it to the AC system, the gauge gets stuck at a high number or rises quickly during a decay test.
A lack of vacuum in the system is caused by two things: leaks or moisture. Leaks allow air back in, while moisture boils at low pressure, creating a vapor that registers as pressure on your micron gauge.
Now that you've proven your pump is good, you can focus on the system itself. This is where a little bit of science helps. You are fighting two distinct enemies.
The Hunt for Leaks
Leaks are physical openings that allow air from the outside to get pulled into your lineset. Even a microscopic hole can prevent you from reaching a deep vacuum. The most common leak points I've encountered are:
- Flare Fittings: These are notorious. Over-tightening can crack the flare, and under-tightening won't create a good seal.
- Schrader Valve Cores: The small rubber seals on these cores wear out easily and are a very frequent source of slow leaks. It's good practice to replace them on every major service.
- Solder/Braze Joints: A pinhole leak in a joint can be very difficult to spot visually but will absolutely prevent a system from holding a vacuum.
The Problem with Moisture
This is the hidden enemy that often fools technicians. Let's say you have a tiny amount of liquid water trapped in the system. As you pull a vacuum, the pressure drops, which lowers the boiling point of water. At deep vacuum levels, that water will begin to boil even at room temperature, turning into water vapor (a gas). Your micron gauge can't tell the difference between air and water vapor—it just reads the pressure of the gas. So, your gauge might stall at 1500 microns not because of a leak, but because you are reading the pressure of the water vapor as it boils off.
How long does an AC system need to hold a vacuum?
You've finally pulled the system down to 500 microns. How long do you wait, and what kind of pressure rise is acceptable? This is where the standing vacuum test comes in.
A tight, dry system should hold a vacuum with minimal rise. A common industry standard is to isolate the system and ensure the pressure does not rise above 1,000 microns after holding steady for at least 10-15 minutes.
The standing vacuum test (or decay test) is the final exam for your work. How the pressure behaves after you isolate the pump tells you everything you need to know.
Reading the Results of a Decay Test
- Achieve your target vacuum (e.g., 500 microns).
- Close the valve on your core removal tool or manifold to isolate the system from the pump.
- Turn off the pump and watch the micron gauge for 10-15 minutes.
| Pressure Behavior | Likely Cause |
|---|---|
| Rises slightly then holds steady below 1000 microns | PASS. Small amount of outgassing is normal. The system is tight and dry. |
| Rises quickly then levels off (e.g., at 1500-2000 microns) | Moisture. The pressure is stabilizing at the vapor pressure of water at that temperature. |
| Rises slowly and continuously, never stopping | Small Leak. Air is slowly seeping into the system. |
| Rises very quickly towards atmospheric pressure | Large Leak. There is a significant breach in the system. |
How do I know if my hoses or gauges are bad?
You've checked the pump, and you're pretty sure the system is tight. But you're still failing the decay test. Could the problem be hiding in your tools?
Yes, your hoses and manifold are common failure points. The rubber gaskets in hose ends wear out, and manifold valves can develop internal leaks. You must test your entire rig isolated from the HVAC system.
Before you spend hours leak-searching an AC unit, take 5 minutes to test your tools. This has saved me from countless headaches. The process is simple: configure your rig exactly as you would for a job, but instead of connecting to the AC unit, cap the ends of your service hoses.
- Connect your manifold, hoses, and micron gauge to your vacuum pump.
- Cap the service ends of the red and blue hoses.
- Open the manifold valves and turn on the pump.
- Pull a deep vacuum on your own equipment.
- Isolate the pump and perform a decay test on your rig.
If the pressure rises, the leak is in your tools. Check the small rubber gaskets in your hose fittings—they are cheap and easy to replace and are the most common culprit. If the gaskets are good, you may have a leak in the manifold itself, and it may be time for a new set of gauges.
Final Thoughts
A failed vacuum test is a puzzle. By methodically testing your pump first, then your rig, and finally the system, you can turn a frustrating mystery into a simple diagnostic process.
