Except for two common causes (outlined below), rough vacuum leaks are relatively rare. However, they can occur. The fitting connections between the Rough Pump and the Turbo Pump should be checked for tightness and integrity of o-rings (which can decay over time).
If the vacuum system has been running satisfactorily, but a rather sudden drop or steady decline in rough vacuum is observed, the cause is typically Rough Pump oil contamination. Changing the pump oil will often return the roughing system to service.
Improper reconnection of fittings disconnected for other maintenance is another common cause of rough leaks. These should be checked for proper o-ring seating and tightness.