Definately sounds like the B2. Mine failed suddenly like that as well. It's a bit of a PITA (to do it right) if you don't pull the tranny. I did it not long ago and these were the parts I needed:

107 270 04 32 B2 Piston
005 997 70 48 B2 seal ring
126 277 08 50 plastic bushing for updated B2 piston
006 997 73 47 seal that holds in the bushing
126 270 44 77 K1 Spring Kit (not necessary, but was an updated part so I replaced it)

Was about $200 in parts. This will give you an idea of whats involved: B2 piston replacment The only reason I recommend removing the transmission is because you need to remove/replace the piston seal to replace the metal bushing with the new plastic one. It's difficult to do with the tranny in the car without damaging the bore. Unless you were/are a mechanic or are mechanically inclined and have alot of patience, you could very well damage the bore. That being said, I did replace it without removing the tranny and damaging the bore, but I'm an ex-mechanic

Just a couple of tips:
Make sure all the aluminum shards/chips are cleaned from the bore and the drain at the bottom of the bore. If those shards travel further into the valve body, it can hang things up (another reason to pull the tranny, easier to make sure it is all cleaned out)
Trans-gel is your friend. You can buy it at a NAPA store. Makes the assembly process a breeze.
Contrary to what is said in the B2 replacement link, replace the piston bore cover O-ring as well. It is easy enough to put the new one in, especially with Trans-gel. Not worth having a leaking tranny after all that work.

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