I have examined both master cylinders for the 56 F-100 and F-250. The F-250 will not swap out with the F-100 because the bolt patters on the 250 are further apart. Both cylinders appear to take the same rebuild kit. I checked in the 56 Ford Manual and the diagram does not show where a flat round rubber washer goes in the kit. the washer is rubber, 3/16" thick, 1 1/4" in diameter and the hole in the center is 5/8" in diameter. The Ford Manual shows the kits internal parts from left to right....valve, spring, cup (rubber), piston, stop plate (metal retaining washer) and snap ring. There are no diagrams with the rebuild kit. Does anyone have an idea where this washer is installed?

 

Is it for the fill plug?

 

It is the first part to go back in. Here are a couple of old threads about the extra rubber washer

https://www.ford-trucks.com/forums/7...ant-place.html

https://www.ford-trucks.com/forums/1...r-rebuild.html

 

Thanks Craig. If I get what is being said in those articles, the washer is the first to go back in. It also mentions the possibility of this being an "extra part" for "P" series which I assume is parcel delivery vans. It sounds like the kits may have extra parts for multiple applications and the washer in question could be an extra part. But then a majority of the posts talk about the washer being the first part to be installed in the cylinder. I do know the master cylinder on my F-250 is longer than the cylinder on my F-100. I also know the washer in question came out of the master cylinder off my F-250. I wonder if the washer may have something to do with the different lengths of the two cylinders and the travel of the pistion? Thanks again for pointing me to that thread.