I think sometimes some dealers just don't want to work on an old car with a real time consuming problem. They probably would rather work on 5 cars, charging labor and parts, making a profit on both, than work on 1 car. Dealers, like most retail businesses, make their money on volume. I doubt the dealer would actually do the valve job themselves. They would probably send it out, so they don't make as much on labor and really only profit from the valve train parts. My father had an A/C problem and dealer he took it to just kept replacing parts with no fix. They eventually just gave up. He took it to an independant shop and they found out, using the MB chassis manual on CD, that there was a fried resistor in the climate control panel. Dealers rather "turn them and burn them" than fiddle with an "old" car.
1994 MB E320 Coupe (199,500 mi)
1992 MB 190E 2.6 (173,000 mi)
1983 Porsche 911SC (148,000 mi)
2004 Volvo V70 2.5T (198,000 mi)
President of CT/Westchester Section