Water in compressor motor and head

[fireboat]

I have a 2000 Discovery 2 that I just got with a sagging rear end. I talked to DiscoMike who gave me some suggestions to track the problem down (much thanks!). However, I was surprised to find that the real problem was a lot of water inside the compressor head and motor. Apparently, water entered through the breather tube that goes to a canister on the frame rail. The canister and tube looked intact without any cracks. What would cause water to get in here? Is there some sort of check valve? I took the motor and compressor apart but it was totally full and had been for some time. I will see if I can dry it out and get it working. If not I will go to roverland parts. But I don't want this to happen again which is why I am trying to figure why it happened in the first place. Thank you.