If your vacuum pump will not reach its ultimate vacuum, the most common causes — roughly in order of likelihood — are a system leak, contaminated or incorrect oil, condensable vapours, a blockage or undersized pipework, worn vanes or seals, an open gas ballast, or a faulty gauge. The first step is always to work out whether the problem is the pump or the system it’s connected to.
First: is it the pump or the system?
Isolate the pump from your chamber and pipework and measure the ultimate vacuum directly at the inlet (blanked off). If the pump reaches its rated figure — for example around 2×10-3 mbar for a two-stage rotary vane pump such as the Edwards E2S — the fault is in your chamber, seals or pipework. If it doesn’t, the pump itself needs attention. This one test saves hours of chasing the wrong thing.
1. Air leaks (the most common cause)
Even a pinhole leak lets atmosphere in faster than the pump can remove it, so the system never reaches target. Check every joint, fitting and O-ring, tighten flanges evenly, and leak-test with a proprietary leak-detection spray or a helium leak detector. Leaks are the number-one reason a system won’t pull down.
2. Contaminated or incorrect oil
On oil-sealed rotary vane pumps, degraded, wrong-grade or wrong-quantity oil is a leading cause of poor ultimate vacuum — it lets vapour back-stream and ruins the seal. If the oil is dark, cloudy or milky, change it and use the manufacturer-approved grade. See our guide on how often to change vacuum pump oil.
3. Condensable vapours
Water or solvent vapour condenses inside the pump, contaminating the oil and blocking the gas path. Open the gas ballast to help the pump clear vapour, and fit a cold trap or condenser at the inlet for wet processes such as freeze drying.
4. Blockages or undersized pipework
Narrow, long or kinked connecting lines and clogged inlet filters throttle the effective pumping speed. Use short, wide, smooth pipework matched to the pump’s inlet, and check any filters or traps aren’t blocked.
5. Worn vanes, seals or a tired pump
Vanes, shaft seals and bearings wear over time and the pump gradually loses its ultimate vacuum. If oil, leaks and vapour are all ruled out, the pump is likely due for a service or reconditioning — a strip-down, new wear parts and a test rebuild will usually restore performance for a fraction of the cost of new.
6. Gas ballast left open
The gas ballast intentionally limits ultimate vacuum. If it’s been left open after a wet run, the pump will plateau well short of its rating — close it once the vapour has cleared.
7. A faulty or misplaced vacuum gauge
Sometimes the pump is fine and the gauge is wrong. Check the gauge is calibrated, of the right type for the pressure range, and fitted close to the point you actually care about.
Pump still won’t pull down?
Our engineers diagnose, repair and recondition vacuum pumps of every make from our North Walsham workshop. Email [email protected] or request a quote.
Frequently asked questions
How do I know if the problem is the pump or the system?
Isolate the pump, blank off the inlet and measure the ultimate vacuum directly. If the pump reaches its rated figure, the fault is in your chamber, seals or pipework; if not, the pump itself needs service.
What is gas ballast and when should I use it?
Gas ballast admits a small amount of air during compression to stop vapours condensing inside the pump. Open it when handling water or solvent vapour, and close it once the vapour has cleared to reach the deepest vacuum.
Can a vacuum pump that won’t reach vacuum be repaired?
Usually, yes. If the fault is wear-related (vanes, seals, bearings) the pump can be serviced or reconditioned and returned to its rated ultimate vacuum, typically for far less than the cost of a new pump.
Written by the Girovac technical team. Girovac Ltd has supplied and serviced industrial and laboratory vacuum equipment from its North Walsham workshop since 1983. Last updated: July 2026.