The Parker 51 is a great pen but it has a lot of parts plus the connection points of the body and filling system are threaded. This can make repair difficult in contrast to other vintage pens. I would not recommend a Parker Vacumatic repair be your first.  The Vacumatic filling system is a difficult task without the proper tools and you will run the risk of damaging the pen if attempting without. I will not be detailing the disassembly of pens, there are plenty of videos that can show you that. With the Penatomy series of posts the idea is to dissect common filling systems to show the component parts that make up our favourite writing instruments.