Alright. Yeah, disassembly is required for that seal. Too bad. Make sure to clean the housing off really well and get you a very clean place to work on it. DO NOT use a cloth rag on the internal parts. Not even to set the pieces on. Get you a roll of blue shop "paper" towels. Yes, that pin can be driven all the way through. Make sure when you split the two housings that the plungers stay in their bores. Some people will say that this isn't important. I disagree. I have had many piston style pumps apart, and that is like rule #2. (Rule #1 is there is no such thing as being too clean when working on these.) As added advice, there is no reason to remove the opposite side pin. That seal can be changed without taking it out. As long as you keep everything clean, don't wash the internal parts. Just clean the case gasket face and put the internals back in. If cleaning is necessary, lube with hy-tran on reassembly. If you want read through this write up in the technical section.
Hydro's: How they work.