Have you tried the Lucas Power Steering Stop Leak? I'm not sure if it will eliminate the symptoms of this problem but it's supposed to and it did stop a leak on my 87 300D. It may be worth a try. If it doesn't work you can always replace it with something else. The most important thing is to keep the fluid at the proper level and replace it regularly. If you do that you don't need to flush. Just disconnect the low pressure hose and let the fluid drain or suck it out with a MityVac.

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2004 VW Jetta TDI (manual)

Past MB's: '96 E300D, '83 240D, '82 300D, '87 300D, '87 420SEL