Birchmeier backpacks are the best backpack sprayers available and will last many years with proper use and maintenance. One of the parts that wears out on the Birchmeier Backpack Sprayer is the lid gasket. This is the large o-ring in the lid that prevent chemical from dripping down your back.
The lid gasket should be replaced once a year or when it fails. How will you know when it fails? Your Birchmeier is covered in chemical and your back is wet.
This photo shows a Birchmeier lid gasket that is clearly past its prime. Don't live with a leaking backpack, replace this gasket. It is a good idea to keep an extra one in the office so you have it when you need it. Purchase authentic Birchmeier replacement parts.