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