Dust grabs dust, if you are in a really fine dusty enviroment you want to see a fine layer of dust on the outside. Most machines have a filter minder that checks the amount of vacuum. You also want to look at the other end where it seals in the housing if you see streaks on the rubber seal you have a leak.
Cat sure hasn't changed much in some of its parts that rad cap design has been used since the early 70s. The pre-cleaner bowls are the same and you are right very brittle especially when one is 30 years old.
How can a dirty primary air filter be more efficient than a clean one? As the filter becomes increasingly clogged up with dirt, less air is able to pass through it, making it less efficient.
