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Gas topped up three times: hidden leak at the service valve

Cooling can keep fading after several gas top-ups when the leak is small and sits where routine checks skip. This unit lost gas three times in a year. The source turned out to be the valve core at the service port.

By Team Snowflake | Updated 28 Jul 2026

Case summary

Midea Wall-mounted9 years oldHDBYishun, Singapore

Concern
The worry was a serious leak in the indoor coil or compressor that would need an expensive part.
Found
Hidden leak at the service valve core
Key check
Focused leak checks around the service port and valve core after the wider circuit looked stable
Result
After the valve-core replacement and pressure recheck, cooling held steady and the repeat top-up cycle stopped.
R410a refrigerant canister connected to manifold gauges during system recharge

What we were told

After each gas top-up the unit cooled well for a while, then faded again. This had happened three times, with no clear answer for where the gas was going.

What we checked

When cooling fades on the same schedule after every top-up, the job shifts from adding gas to finding the leak. We reviewed the top-up history to gauge how fast the gas was escaping. We then worked along the refrigerant circuit in order, from the main pipe joints down to the smaller service-port points that a standard leak check often skips.

  1. Three top-ups in under a year, with cooling fading each time. That fit a slow, steady leak rather than a sudden failure.

  2. Checks on the main pipe joints, the pipe fittings, and the indoor coil connections found no leak.

  3. A bubble test and an electronic sniffer both picked up a leak at the service-port valve core on the outdoor unit.

  4. The leak was small but steady, enough to drain system pressure over two to three months.

  5. The valve core was worn and slightly deformed from age and from being opened on each past top-up.

What we found

The valve core inside the outdoor service port had worn and slightly deformed over years of use. Each time a technician connected gauges for a top-up, the core was pressed open and released. Over nine years and several top-ups, its sealing surface wore down just enough to let gas slip past. The leak was too small to find without focused testing at the port itself, which is why wider circuit checks on earlier visits had cleared the system. Gas escaped slowly through the worn seal, draining enough pressure over two to three months to cause clear cooling loss. Each top-up restored pressure and cooling, but the leak stayed open. That is the cycle that kept repeating.

What fixed it

The leak was at the valve core, a small replaceable part inside the service connection, not at the indoor coil or compressor. We fitted a matched new core, then ran a pressure hold test before recharging. The pressure held steady with no drop, which confirmed the leak was sealed. We recharged the system and checked that cooling was back to normal. No other circuit repairs were needed.

Outcome

After the valve-core replacement and pressure recheck, cooling held steady and the repeat top-up cycle stopped.

What this case teaches us

Repeat top-ups point to an unfound leak

  • If cooling fades again a few months after every top-up, the gas is escaping somewhere. A top-up only resets the clock.
  • The leak can hide at the same connection a technician opens to add gas. Wear there is easy to miss in a wider check.
  • Before paying for more gas, ask where the leak was traced. A sealed system holds pressure on a hold test before recharge.

Questions this case answers

Why does my aircon lose gas again a few months after every top-up?
Because refrigerant is not consumed. If cooling fades on a repeating schedule, gas is escaping and the top-up only resets the clock. Three refills inside a year in this Yishun flat fit a slow, steady loss rather than a sudden failure. The source was the valve core inside the outdoor service port, worn just enough to let gas slip past.
Where do small refrigerant leaks usually hide?
At connections rather than mid-pipe. Checks along the main joints, the pipe fittings and the indoor coil connections found nothing on this unit. The service port needed focused testing before it gave itself up, first with bubble solution and then with an electronic sniffer. The core inside had worn and slightly deformed with age and repeated use.
Does replacing the valve core actually stop the leak?
When the leak is confirmed at that point, yes. A matched new core went in, then a pressure hold test ran before any gas was added. The pressure held steady with no drop, which is the proof that the circuit is sealed. Only then was the system recharged. No indoor coil or compressor work entered the picture.
What should I ask before paying for another gas top-up?
Ask where the leak was traced and whether the system held a pressure test before recharging. A sealed circuit holds pressure and a leaking one does not, and that answer decides whether more gas is money worth spending. Without a located leak, the next refill buys the same short window as the last one.

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