I just completed a complete Power Steering System replacement on a 1998 ML320 with 237k miles. I am the original owner and have the entire service history since new.
The work was done mainly because the original steering rack was leaking at the driver's side inner tie rod boot. This project included a ZF Factory rebuilt steering rack (rack and pinion unit), new MB OEM hi pressure and return hoses with new O-rings, outer tie rods left and right, ZF Factory rebuilt power steering pump, O-ring and OEM reservoir (filter). I have flushed about 5quarts of new PS fluid through the system. When I button everything back up, top up the fluid and run it, the PS fluid is frothy with what seems to be air entrainment.
Any recommendations appreciated on what to look for next.
Thanks,
Ken

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2014 E250 Bluetec 4-Matic
2014 GL350 Bluetec 4-Matic
1996 Chevy Tahoe 4 x 4
2009 ML350 (sold)
2005 SLK350 (sold)
1999 E300 Turbodiesel (sold)
1998 ML320 (sold)
1991 300D 2.5 Turbodiesel (sold)

How long have you run it after replacing? It is generally hard to get all the air to purge out of the system quickly. I always turn the wheel stop to stop and shut off the engine with the P/S reservoir cap off. I do this over and over until no more froth shows up. The froth is aerated fluid from the pump - as it works air out, it is mixed with fluid. There is a lot of waiting upon purging. OR, you can modify another "spare" power steering cap and pull a vacuum with a mighty vac to pull a low vacuum on the system while you initially run it. You may get some pump whine while pulling a vacuum, but after it is purged and the vacuum is let off, it returns to normal...FWIW.

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Thanks,
Mark in NC

"Spark plugs?...We don't need no stinking spark plugs!"
1985 300SD "Der Silberne Schlitten" 420,000 mi


Wish these were diesel:
2003 Ford Club Wagon 130,000 mi