che - It's fairly easy to remove and disassemble your master cylinder to check the internal cylinder wall condition. If it's not pitted its worth ordering a rebuild kit, but should also include a light bore cleanup with a cylinder hone to remove any buildup/debris so the new kit will seal properly.
If the cylinder wall is pitted, you're looking at either sending it off for sleeving and rebuild or ordering a replacement. If you are on a budget, you can get a new master cylinder for your 1950 3100 from
Rock Auto # 35571075 for 1/2 the cost Classic charges. Only real difference is the Classic cylinder comes with an original style metal vented fill cap, while the Rock Auto part has a plastic vented fill cap. You can always take your existing metal fill cap and transfer it to the Rock Auto cylinder if you prefer a more original look.
