I got the transmission back on the mower today and it seems to work good now. The problem had nothing to do with the dump valve pin. I could tell from the wear marks on one of the surfaces on the center block that I didn't flatten it well enough. The block I used to flatten it had a low spot in the middle so I ended up making it lower towards the center where all the pressure is. I found a better technique for surfacing the center block by first flattening the motor block on a surface plate with sandpaper, then I glued sand paper onto the motor block surface that I flattened using a medium strength spray adhesive and put the splined shaft through it and into the center block the way it goes together when assembled. That held it in place so I could just rotate the motor block and flatten the surface of the center block. I covered the surface with marker to measure the progress. The other side looked okay so I didn't do it but I think the same technique could be used on the other block as well by removing the shaft from the case and doing the same process. The glue is easily cleaned off with Wd40 or PB blaster. I used 400 grit paper and finished with 600 grit. If I were to do it again I think I would polish the surfaces with finer paper. I'll have the owner let me know how it holds up and update if there are any issues with the rebuild. If it holds up then that saves $400 in parts.