I'm starting to see the early 251's come in with sagging suspensions. The most common fault I've found is leaking air springs. Try spraying soapy water into the folded section of the spring and you should be able to spot the leak. I've had a few with very slow leaks that I removed and inflated with compressed air so that the spring is fully extended. You don't need to inflate it to 100 PSI, just enough to extend it. I've found that they normally leak at the folded/creased section. Good luck.