Fortunately, the problem is usually quite controllable, and regular furnace inspections can usually keep those leaks completely under control.
Leaks most typically occur through some of the sealing-surfaces in the furnace, the most common leak-source being the O-ring seal-surface in the furnace door itself. As the door is opened and closed everyday, the light coating of vacuum-grease on the door seal and on the O-ring can pick up dust and dirt, which, if not properly removed regularly, might begin to initiate small holes/cracks in the O-ring seal, which can eventually open up enough to allow air to start to leak into the furnace during furnace operation. Additionally, seals between the furnace and the pumps, valves, pipe-connections, etc., all can become sources of potential air-leaks into the furnace.
Rarely is the furnace material itself at fault, but it cannot be overlooked. It is possible that some gas/air might get trapped between double-welds used in the initial building of the chamber, which could then very slowly outgas during subsequent furnace operation. Another potential source of difficult-to-find leaks might be any cast-metals that are used in the construction of furnace-flanges or other components that need to be vacuum-tight. Some aluminum castings used in flanges have apparently had this issue in the past.
As mentioned above, all vacuum furnaces will leak air into the vacuum chamber from the outside atmosphere in the factory during vacuum-brazing operations, and then the pressure inside the furnace will start to go back up towards atmospheric pressure. That is why it is a called the “leak-up” rate of the vacuum chamber, and must be monitored, recorded, and controlled on a regular basis.
Typically these leaks are very small. In fact, they are so small that they are measured in millionths of atmospheric pressure leaking into the furnace over a one-HOUR period! That’s right, only millionths of atmospheric-pressure per hour! Yes, the leaks are indeed tiny.
Important: All vacuum-brazing furnaces must be regularly checked for vacuum-leaks, and the “leak-up rate” determined and controlled!
There are a number of “dimensions” that folks use to specify this leak-up rate, but here in North America it is most typically measured in “Microns per hour” (often also referred to as “millitorrs per hour”, since the word “micron” and “millitorr” are used interchangeably for the same measurement).