Bvm air rams.
 

I have just been prepping my 20 year old BVM F4.

One air ram was leaking with air passing past the piston feeding down air into the up circuit.

Once this ram was identified, ( it will be replaced with new) I cut it open to examine.

To my surprise it was not a poor O ring but the internal walls were very rusty and rust particles were caught up in the ring and compromising the seal.

So, it might help you guys avoid this problem by using a water trap in your pump, and injecting some BVM O ring lube to, hopefully, prevent this corrosion.