Leak Detection Methods for Pressure Leaks ( not Vacuum):

1. SNOOP - this is essentially soapy water; NOT PREFERABLE, as it may be pulled into vacuum. If used, clean area throughly with DI water afterwards before pulling vacuum.
2. Helium Leak Testing ( sniffer) - DO NOT USE, glass in PMT's are very permeable to He, which will then ruin them. 
3. Hydrogen Leak Testing (sniffer) - PREFERABLE, uses 5% H2/95% N2  nonflammable mix test gas. Sniff as with He using appropriate equipment.
4. Gas Bag - PREFERABLE Wrap plastic bag material very loosely around suspect joint and seal tightly; watch for inflation