Interesting about the three valves, courtesy of ROCKAUTO newsletter. cut/paste as follows,

"What about oil leaks? Shock absorbers and struts contain hydraulic oil. Truly ancient hydraulic seals made from materials now considered inferior might have “dried out” or otherwise lost their ability to seal over time, but Monroe shocks were made with modern seals even in the 1990s. These new/old Monroe shocks do not have any oil leaks. The mounting bushings also look identical to any more recently manufactured new bushings.

What about gas leaks? A “gas” shock or strut usually has nitrogen gas in the oil reservoir. A gas shock is not the same thing as an air shock. The nitrogen gas is primarily intended to help keep the oil from foaming when the shock cycles rapidly on washboard dirt roads or at high vehicle speeds. If the hydraulic oil foams, then the shock’s damping capacity is reduced. If the shock’s oil reservoir is not leaking oil then it probably is not leaking nitrogen gas either. One of the reasons nitrogen is now used to pump up tires is that nitrogen molecules are larger and less prone to leaking than oxygen and other components of air. Ambient “air” is 78% nitrogen, so there is no strong chemistry working to force the nitrogen out of the shock’s oil reservoir.

Even shocks that have been driven way beyond the 50,000-mile mark over many years usually do not fail because of oil or gas leaks. Modern shocks use multistage oil valves. Each valve is a calibrated metal disk that flexes to let oil flow through in response to different road conditions. The first stage valve opens in response to little bumps, the second stage valve opens in response to medium sized bumps and the third stage valve opens in response to big bumps. These valves are usually what wear out first. The metal valves fatigue and go limp after millions of openings. The first stage valve is the first to die because the vehicle is constantly exposed to little bumps and the valve is constantly cycling.

Multistage valves are one reason the old push-down-on-the-fender test for shocks is now almost useless. If the car keeps bouncing after a big push down on the fender, then that means the third stage, big-bump, valve is now worn out, the second stage valve has been worn out for awhile and the first stage valve has been worn out for a really long time. A shock that fails the fender-push test should have been replaced long ago. That shock absorber or strut has not been protecting the vehicle’s suspension parts and assisting with safe vehicle handling and braking.

Time will tell, but I bet I will continue to be very happy with the new, fifteen-year-old shocks on my thirty-year-old 300. We will see what my son, the handling expert, says the next time I let him behind the Chrysler’s wheel!

Tom Taylor,