To detect very small leaks, especially Down stream of the source, apply the gas via a disposable plastic pipette tip to a limited area and wait to see if you get a response. Patience is the key to good testing. However gaskets of different material will respond differently to solvent. Metal gaskets such as Copper or Gold will show a positive gauge response while elastimer seals such as Viton may swell and show a slow negative gauge response especially if the leak is small. The closer the leak is to the gauge the faster the response will be.
Leak testing can be a very frustrating task.
Patience and perseverance are your allies. Such a shotgun approach often makes matters worse. It is much easier to avoid leaks on mass spectrometers than to find them. There are several easy and relatively inexpensive things that you can do to prevent the majority of leaks. I generally reuse copper gaskets on Conflat type flanges only once.