This is a VERY IMPORTANT step!
Motion Pro - Damping Rod Tool
Failure to do this step, will only cost you another set of seals down the road!
You left out one important step Charlie. Pumping the oil out of the cartridge. The piston inside the cartridge is filled with oil. You should place the cartridge on a folded towel and pump that puppy until all the oil is removed. If you miss this step, the fork will be overfilled with the addition of 512 ml of oil.
By increasing the oil level in the fork it leaves less air space. This air space is compressed as the fork travels through its stroke. As the air pressure is increased this acts like an overload spring and gives the spring more resistance as it nears the bottom of the stroke. Makes the fork act like it has a progressive spring. If this air pressure is to great it will blow the seal out prematurely.
So if you are going to use this method to replace the seal it's very important to remove ALL the oil from the cartridge!
Motion Pro - Damping Rod Tool
Failure to do this step, will only cost you another set of seals down the road!