The first step to the piston cover O-ring removal process is to put the transmission in park, set the parking brake, chock the wheels, lift the car as high as you need, then put a stand underneath.
Clean the transmission very well. Compressed air is useful. The slightest piece of dirt can cause mechanical problems if it gets inside, not to mention it can get caught in the sealing surfaces.
The control pressure rod clamp will be in the way of the B1 piston, so completely remove the nut and bolt and then slide off the clamp and set it out of the way.
Loosen the bolt holding the dipstick tube to the intake manifold. Put a container underneath the transmission and remove the drain plug at the dipstick banjo bolt. Let the fluids drain out. Also remove the plug in the torque converter to let it drain (you'll have to turn the engine over until the plug is at the bottom, either by using a socket on the crank, or on the power steering pump pulley bolt [some advise against using the pulley bolt]). Once drained, remove the pan and continue to let the fluid drip out into the container.
The exhaust pipe clamp was in the way, so I loosened it and slid it out of the way.
Now the tool can be bolted to the transmission. Make sure to put the socket through the new O-ring before compressing the piston, otherwise you won't be able to get it in.
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1967 W110 Universal Wagon, Euro, Turbo Diesel, Tail Fins, 4 Speed Manual Column Shift, A/C
1980 W116 300SD Turbo Diesel, DB479 Walnut Brown, Sunroof, Highly Optioned, 350,000+ Miles