For many technicians, the answer is no – which is bad. Vacuum pumps are supposed to suck. If you are using your compound gauge to tell you when you have a vacuum, you really don’t know what your vacuum pump can do because a compound gauge can’t tell you. The difference between a great vacuum and an awful vacuum just can’t be seen on a compound gauge. It is a little like trying to measure the distance of a millimeter using your car’s odometer. You need a micron vacuum gauge to check the vacuum your pump produces. It is not unusual for a vacuum pump to be in such bad shape that a vacuum gauge connected to it won't register anything. That is not because the gauge is broken, but because the pump does not suck.
The reason many vacuum pumps in the field do not produce anything close to their rated vacuum or capacity is that their oil is not changed often enough. How often should it be changed? Basically, whenever it gets dirty, it should be changed. You might be able to pull a vacuum on several small, clean systems before changing the oil, or you might need to change the oil in the middle of an evacuation on a particularly nasty system. Vacuum pump oil should be clear. If it has become cloudy or discolored, it needs changing. It is never wrong to change the oil. You normally have several hundred dollars in your vacuum pump even if you got it on sale. Leaving dirty oil in it really does not make economic sense because the crud in the oil is eating up your machine.
The oil not only lubricates the mechanical parts, it also provides the vacuum seal. When stuff is dissolved in the oil, the oil produces a vapor pressure from all the stuff in it – water, refrigerant, flush solution. The vacuum pump cannot pull down any lower than the vapor pressure of the oil in it. So if you have 10W30 instead of vacuum pump oil, or your 10W30 is full of water from all the systems you evacuated this past month, your vacuum pump likely does not suck. If you have a traditional rotary vane vacuum pump with the oil reservoir on the outside and a sight glass in the middle, you can usually remove that oil reservoir and clean it. We actually change our oil a lot at school – a whole lot more than usually happens in the field. But we still get a good layer of scum on the bottom. We dismantle our pumps at school at least once a year and clean out all the crud that has collected in the bottom of the oil reservoir. Just pouring oil through does not get the job done. So if your vacuum pump does not suck, try cleaning it and changing the oil.