This is the look of new brakes and struts on a Porsche . . .
Struts need to be changed when they blow out. When that happens, the side of the strut will be black with dirty oil, and the vehicle will bounce like a pogo stick over bumps. All too often people look at struts during other service, and they say "These look okay" if they are dry.
But are struts really okay, just because they're not leaking?
In this car, the struts were not visibly failed. However, the car was 8 years old, and the struts had over 60,000 miles on them. In the mind of this particular owner - a driver enthusiast - that was reason enough to take the strut loose for inspection.
That proved to be a good call. When compressed and extended on the workbench, there was a night and day difference in the damping power of the old struts versus the new replacements. The old struts hardly had any damping left, it seemed. And when we installed them that feeling translated into much firmer handling. The car once again drove like new. It just goes to show, a quick visual check often fails to tell the whole story.
What about brakes? Can they be checked visually?
In the "before" photo above, you can see a ridge along the outer circumference of the brake rotor. That visible ridge is a sign the rotor is at or near the wear limit. How do I know? Simple . . . modern cars don't have a huge margin for allowable wear. But the time the wear is visible at the rotor edge, you are near the end. If you want to know more precisely, you can gauge the rotor and look up the thickness in the workshop manual, but this rule of thumb will serve you well most of the time.
There is also a good rule of thumb for pads. Most European carmakers say pads should be changed when the friction material is worn to the same thickness as the metal backing plate. You can't see that in this photo, but if you'd crawled into the monitor and looked sideways you'd surely have seen the worn pads.