the poor vaccum in condenser

A couple days ago we had problem related to poor vaccum in steam turbine condenser.
The vaccum was on the level of 81-82 kPa. The normal level is around 90 kPa. Low vaccum creates the unit load limit that is so important keeps it as designed.
Before the vaccum drop there had been a turbine trip. The machine was taken in service again but trough the some moment the LP bypass got pressurized and the LP bypass pipeline shook.  The turbine was operated under stable condition but the vaccum did not come to previous, required level.
All vaccum pumps were taken into service but it did not help, in paralel we started to search for the reason.
The fitters found the manhole withe air ingress. The manhole is located on the LP bypass pipe just before the connection of it with condenser shell. It was sealed, the screws tightened (some of them were loose). The vaccum is improved now almost 90 kPa.