Highly recommend ordering the charge relief ball and spring since the 1/4 ball is plastic. HG 70403. It wears heavily. You need to pull charge pump anyways.
Two o-rings will need to be at least replaced.
HG 51232
HG 9004101-1340
Though other seals needs to checked while you are at it.
Also this a job might be better done with pump off as pistons and springs will tend fall out of the cylinder. However it can be done with it still in place if you can keep the cylinder, pistons and springs in place.
As StarTech says this can be done in place, which is what I did. The key to keeping the master cylinder in place is to not get in a hurry as you lift the cap off the body after the retaining bolts have been removed. The piston springs will push the cap up off the body after the bolts are removed (push the cap down and feel it spring back up to get a feel for that) With a free hand, feel under the cap for the cylinder block as you retract the cap with your other hand. If it continues to come out, stop and push everything back in place. The 20w50 oil is very sticky/tacky and the the entire cylinder (and valve plate) will come out as one piece if you aren’t careful here. Which is exactly what happened to me, and only by sheer luck did I get the master cylinder back in place without pulling the pump off the mower.
Lastly, the valve plate(which is between cylinder and cap) is stenciled “up” to make sure it goes on correctly. IOW, I think it can be installed upside down if you don’t pay attention.
