Ok it's an 06 3500 ctd and in the last few weeks I have lost my power steering cap 5 times and at $15 that is getting me. The cooler is clean, the cap goes on tight, the locks are still good and it's hard to get off. but it seems to blow off on trips that use the highway. it never happened before. And the dealer says there is nothing that could make that happen. Can someone explain???? I bought a new pump and reservoir but haven't installed it just yet. It would make me to have it pop off of the new one too.

 

Lots of info on this one in the forums here. A lot of people do the mod that takes just a little it of material off where the vent is (making it actually vent). I did it myself as soon as i bought mine and it solved the pop that you get when you pull the cap off the power steering res. The short is the vent isn't designed very well and will trap pressure inside the system. The long is a nice law of physics that states that pressure increases as temperature increases and volume does not change.

Check this thread
Link

I did the same as post #6 and mine has been fine since

 

There was a Recall on my 2010 that replaced the cap because of pressure build up.