Found this on the UK Volvo club site:
Quote:
How does a shop diagnose worn shock absorbers? ....
First, use a tape measure to check the distance from the floor to a common reference point on the body such as the upper lip of the fender well. If a difference in ride height is indicated, the shock absorber may have lost its gas charge, or the vehicle may have a sagging spring. When disconnected from the chassis, a shock absorber with a full gas charge will quickly extend the piston rod. In other failure scenarios, a shock absorber may stick up or down due to a broken piston assembly. Next, check for oil leakage around the shaft seal. If the oil leakage looks wet, or covers the length of the shock absorber body, the shocks should be replaced either in axle pairs or at all four wheels.
Rebound recovery tests, unfortunately, are a far more subjective type of shock absorber evaluation. During a rebound test, the suspension is cycled by placing the knee or hands on the front or rear bumper of the vehicle, bouncing the suspension and then observing the recovery rate. In theory, good shock absorbers should dampen spring bounce within one oscillation cycle. In reality, a technician should be very familiar with the suspension characteristics of the nameplate in question. Some auto manufacturers design soft suspensions; others design firm suspensions, depending upon model application.
If a chucking or knocking noise is detected during the rebound recovery test, at least one shock absorber may have a loose piston, a worn piston shaft bushing or worn mounting bushings. When the shock's rebound bumpers are worn or the tires show a mild cupping in the center of the tread , shock absorber fade may be occurring when the shock absorber fluid becomes overheated and loses viscosity during a long trip. When road testing, the most accurate observations are achieved by driving the vehicle over the same stretch of road. Worn shock absorbers will cause the vehicle to nose-dive during braking or roll during cornering maneuvers. Again, the feel of worn shock absorbers tends to be a subjective judgment that requires familiarity with the specific vehicle nameplate and application.
How Shock Absorbers Fail. How do shock absorbers fail? In most cases, the piston shaft seal fails and allows the fluid to leak out of the shock's oil reservoir. On high-mileage vehicles, the internal parts such as the piston seal or valving wear out, which alters the dampening characteristics of the shock absorber. Last, the nitrogen charge may leak from the shock absorber, which slightly lowers the suspension height and allows the fluid to foam during normal operating conditions. All of the above failures will, at some point in the shock absorber's service life, cause the shock absorber fluid to become overheated or foam during severe operating conditions or extended trips. Last, the piston itself can become disconnected from the shock absorber shaft. The shaft may then bend, which causes the shock absorber to bind at some point in its travel. Because some pistons are held onto the shaft by a threaded nut, tightening the piston shaft bushings or spinning the piston shaft with an impact tool may loosen the piston-retaining nut. The best alternative is to buy the appropriate tools needed to hold the piston shaft stationary while the retaining nut is being tightened.
The sled probably has gas-charged shocks and when the gas escapes, the shock no longer pushes up on the suspension, therefore that corner sags.
In my 33 years of owning a car, this is the first time I've ever encountered a shock failure. C'est la vie...
