Gas Topped up Three Times: Hidden Leak at the Service Valve

• –Repeat cooling-loss history — three top-ups in under a year — matched a slow, steady leak pattern rather than a sudden failure.
• –Wider circuit checks on pipe joints, flare nuts, and indoor coil connections showed no detectable leak.
• –Bubble test and electronic sniffer both detected activity at the service-port valve core on the outdoor unit.
• –The leak path was small but consistent — enough to drain system pressure over two to three months.
• –Valve core showed wear and slight deformation from age and repeated service-port access during previous top-ups.

We explained that the leak was at the valve core — a small, replaceable component inside the service port — not at the indoor coil or compressor. We replaced the worn core with a new brass-seated core matched to the port size. We then performed a nitrogen pressure hold test at 28 bar for thirty minutes to confirm the system was fully sealed before recharging. The pressure gauge held steady with no drop, confirming the leak path was closed. We recharged the system with the correct weight of R410A refrigerant and verified cooling performance. Temperature differential at the indoor coil and subcooling at the outdoor unit both checked normal. No other circuit repairs were needed — the rest of the system had tested clean during the initial leak check.

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

Why the same top-up keeps failing — and how to find the hidden leak.

• –A leak at the service-port valve core is small — often less than a gram per day — and easy to miss during standard checks that focus on pipe joints and flare nuts. But it steadily drains pressure over months. That is enough to cause the same top-up cycle repeatedly while the wider circuit tests clean.
• –Each top-up works because pressure is restored, but the leak path stays open. The gas escapes again through the same point, and cooling fades on the same schedule. If the interval between top-ups is consistent — roughly the same number of weeks each time — that regularity is a diagnostic clue. It points to a fixed leak rate rather than an intermittent or worsening fault.
• –Focused checks around the service port — separate from the wider circuit — are needed to catch a valve-core leak that broader inspections miss. This means capping the gauge hose, applying bubble solution directly to the valve core face, and holding an electronic sniffer at the port for at least thirty seconds. A quick pass with the sniffer along the pipe run will not catch it.
• –Repeated gauge connections during previous top-ups can accelerate valve-core wear. Each connection compresses and releases the spring-loaded core. Over nine years and multiple service events, the brass sealing surface degrades. The very act of topping up can worsen the leak over time, creating a self-reinforcing cycle.