We had a leak from the B2 cover of my 722.315 transmission (1985 300D) and, when we had the B2 cover off, we saw we had the old-style piston, We replaced the B2 piston with the new style (1072700432) and at the same time replaced the outer O-ring (005 997 70 48), plastic bushing (126 277 08 50) and lip seal (006 997 73 47). With the job done, the transmission feels no different (it felt fine before) but we have a terrible leak from the B2 cover now....much worse than before.

• I am sure we got the rod in the B2 brake band, but I am wondering if I should have replaced the rod itself. Is the length of the rod the same between the old-style B2 piston and the new style?
• Am I correct in assuming the problem has to be an issue with the outer O-ring seal against the B2 cover?
• How does the cover seal against the ring? Does the piston push against anything in any way to help with the seal or is it completely unrelated?
• I'm pretty sure we got the lip seal oriented correctly. As we recall, one side is an open groove and the other side is flat. Which side goes toward the inside of the transmission? Is there any chance this could be related to the leak? What symptoms would we see if we got this wrong?
• I'm quite convinced I am seeing fluid collecting at the bottom of the B2 cover with the car stationary and running, but I am wondering if there is any chance that the leak could be from somewhere else and collecting on the cover? Looking at transmission diagrams I see only a vent at the top of the transmission above the cover but maybe I'm not considering something? The leak exists even when the transmission fluid is so low that it won't even register on the dip stick (yes I wish I didn't know that) so I know it's not a matter of over-filling.

Thanks in advance for your help/advice.

-John

I think the o-ring was not seated properly in the slot and you buggered it up, i.e. it is cut into half when the cover was pushed in. I did the same before. Redo the job with a new o-ring. Put some vaseline jelly in the slot to hold the o-ring, then push the cover back in slowly to make sure nothing is in the way.

Good luck.

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Not MBZ nor A/C trained professional but a die-hard DIY and green engineer. Use the info at your own peril. Picked up 2 Infractions because of disagreements. NOW reversed.

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You sure the leak is not coming from the secondary pump? Or the kick down solinoid?

The Cover+ O-ring + O-ring Bore seal independently from the B2 Piston itself.
The O-ring can be a pain to get into the groove. When I did mine I spent a lot of time messing with it and having it pop out.
The only way the B2 Piston helps to seal the cover is the seperate B2 Piston Spring pushes the Cover back against the Snap Ring.

I don't know if it is possible to get the Cover on backwards.

See pic. Sorry the pic is not so good but what I have learn over the year assembling things is that if you push that sharp machined edge unevenly over an O-ring there is a chance to cut a piece out of the O-ring.
Not shown in the Pic is that I took a fine file and round off that edge and then took some 500grit sand paper to it to smooth it and I coated the O-ring and that edge witha generic STP before I insgalled it.

You also want to inspect that flat area from that shap edge the arrow points to downward to be sure it is not knicked or gouged. That nicking and gouging can happen when you are trying to remove that Snape Ring.

I don't know how much a new cover would cost but if it is knicked or gouged degrease the area with Brake Cleaner and fill it in with JB Weld Epoxy and after it is cured sand it down flush with the other metal so the O-ring has a flat area to seal on.

Concerning the thrust Pin. As far as I can tell from what I have read you do not need to change the Thrust Pin due to a change in the B2 Piston but there is different sized Thrust Pins to sort of compensate for the B2 Brake Band and Drum wear.
However, in order to take the measuerment to find the correct size you need to remove the Valve Body; at least that is the way it appears in the ASTG Transmission Book.

Without taking the Measuerment with the Old Thrust Pin and going from there it would be hit or miss to change the size/length of the thrust pin.

I believe there is a seal on the Dipstick Tube that can leak but do not remember how close it is to the Cover.

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__________________
84 300D, 82 Volvo 244Gl Diesel

I am going through same thing now. I can not get both hands up there to put the o ring into place. I did learn it is best to remove the VB to access the pin into the band.
From your description, I would guess that the pin missed its target and the o ring slipped out of its groove.

Looks like I am going to drop the tranny to get the o ring in!

Good luck.

Tom

I'm not sure how to make sense of this all. We took the B2 cover off and found the o-ring happily seated perfectly in it's groove. Nonetheless we replaced it with a new one (it was annoying but not SO bad to put in it's groove) and gingerly put the cover and circlip back on. Result: B2 cover leaked terribly when car running with transmission in drive but not moving. Rinse, repeat, rinse, repeat. Always the o-ring was seated perfectly and always the cover leaked badly.

The never-answered question in our minds was which side of the piston is supposed to get pressurized: the inside facing into the transmission housing or the outside against the o-ring and cover. What we wondered is if the o-ring/cover is supposed to handle high pressures or not. If not, then perhaps the problem lay before for the o-ring and cover? Earlier in this job we replaced the lip seal and plastic bushing.

In any case, as a last resort, we removed the B2 piston, stared at things, confirmed that the lip seal and bushing looked correct to our untrained eyes, and tried to make sense of it all. We then put the B2 back having changed nothing. When we put it all back together...this time...no leak (or perhaps a very very small leak --- at this point I can live with that.) What changed? I have no idea.

When we had the B2 out, I noticed that a portion of its shaft has already dulled from wear. This seems a little odd. I'm wondering if this is the result of dirt perhaps that got sandwiched in the bore when we were working on this? This is a new piston --- maybe only several hundred miles of use...

BTW it does not appear necessary to drop the pan to ensure that the B2 piston rod goes inside the B2 brake band.