Looking for B2 piston on a 126...

[SoyDriver Matt]

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?