For some background the tranny has been acting up, once it's warm suddenly not engaging in any gear, but if the engine is shut off for a few minutes it engages first, but as soon as it tries to go to second it slips and won't engage any gear again. Already had to creep it back, shutting it off every half mile a few times now. Let it sit overnight and it's back to running through the gears just fine, until it heats up. Did some searching and it seems like in a 126 you can get to the B2 piston without dropping the tranny. Any input would be great...

Update... been reading, and have B2 symptoms except for reverse doesn't engage, any other opinions on tranny failures?
BTW one thing I did was switch vacuum connectors on the 3-way and 2-way coming off the bleed valve, they were cracked and leaky giving low vacuum to the tranny all the time. I'm ready to switch them back and testing it, but whats the long term effect of allowing unfiltered air to be sucked into the vacuum pump via leaky fittings?

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'84 300SD veg-oil bliss
'83 300SD veg-oil beater

That does not sound like a B2B piston problem. One of the characteristics of the B2B failure is the trans continues to shirft into reverse. Also the B2B usually goes breaks, and does not get better with a cool down.
Here is a photo of a broken B2B.
http://ac.unpatented.com/pics/trans/brokenb2b.jpg
The B2B can be lowered easily enough to get to the right side and access the B2B piston. Need to remove the support crossmember. Here is a photo of the B2B in the trans. It is located behind the big round cover on the right side. Just push the cover in and remove the circlip and then pull the cover and the piston out.
http://ac.unpatented.com/pics/trans/oldtransmission.jpg
This is the band behind the B2B. It should feel springy, the B2B pushed in to this band and it tightens like an oil filter wrench around a drum to engage the tranny. I have a screwdriver here pusing against the band.
http://ac.unpatented.com/pics/trans/transmissionband.jpg

Thanks for the pic links. It' failing quicker and quicker, this time could barely roll around a parking lot. Think it's worth pulling it to check it?

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'84 300SD veg-oil bliss
'83 300SD veg-oil beater

Took me about an hour to pull it out. If the B2B is broke you will know it right away, it will be in pieces. Takes about the same to reinstall. Most of it is easy work.
Most of the work to pull the B2B would need to be done to get the trans out anyway, sounds like you are headed that way as it is.
You might try pulling the trans pan and looking for metal shards. I am not a trans mechanic, but seems to me to me you either have a valve body problem, or a trans pump that is failing. The torque convertor sometimes starts to break up and it takes out the pump. That is a common failure on these transmissions.

Thanks for giving me some stuff to look at. Exactly the kind of info I needed...

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'84 300SD veg-oil bliss
'83 300SD veg-oil beater

Well, pulled the B2, didn't come out in pieces, but isn't exactly springy either. How strong is the spring action supposed to be? It does now engage reverse but not forward.

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'84 300SD veg-oil bliss
'83 300SD veg-oil beater

Bump--- does the B2 piston have another failure mode from just being in pieces, or can it just bind?

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'84 300SD veg-oil bliss
'83 300SD veg-oil beater