Let me throw my 2 cents worth in the hat. Im in the process of overhauling a tranny on my 87 420sel. I can only speak about that tranny. Not sure about yours but I suspect it is similar to mine.

First off, there is no such thing as a B2 piston pump. There is a B2 piston and there is an primary and auxillary oil pump. On my transmission the B2 piston and auxillary oil pump can be changed without removing the tranny. You might try that first. But I would think that a full overhaul is probably not far off.

Dont run the tranny untill you have a catostrophic problem since that usually destroys the hard parts which are very expensive to replace. The soft or friction parts slowly wear out and you dont want to wait until metal meets metal. No way to know for sure unless you do a full tear down of the tranny. I had a sudden and catostrophic failure on my 91 420sel at 170k. Prior to that day no problems. You are living on borrowed time. Do the right thing.

I have found that the dealer offers a factory rebuilt unit at a good price. If you have someone rebuild it, make sure that are an expert on MB trannys. Or if you feel adventurous, DIY. My hands smell like ATF as we speak. Join the club.

Let me know what you find out.

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I got too many cars!! Insurance eats me alive. Dave

78 Corvette Stingray - 3k
82 242 Turbo Volvo - Manual - 270k
86 300e 5 speed manual - 210k
87 420sel - 240k
89 560sl - 78k
91 420sel - 205k
91 560sel - 85k
94 GMC Suburban - 90k
97 Harley Davidson Heritage Softail - 25k
00 GMC Silverado 1 ton 30k