Master cylinder rebuild kit

I have rebuilt master cylinder. The rebuild kit comes with a seal that seats in a groove on the primary stem between the other 2 seals. There was no seal there when I took master cylinder apart but the manual shows a seal and there was a seal in the kit. Should I leave it out or put it in?

If you look closely, is there any remains of a seal? A mark on the stem that shows there once was something there? Any indication it was once there?

Hi Rodney, and welcome.
I would trust the manual.


No sign of a seal being there and no sign of the master cylinder being rebuilt previously. The grooves mentioned are whats on the stem. No sign of seal remnants but very dirty from not being on the road for 12 years and the master cylinder leaking out. Other seals were solid and not dried out though. That is why I’m leaning to what I took out of it.

Without being sure what that seal does, I’d probably on the side of what was there with my own vehicle.

That said, it wouldn’t be shocking if it was supposed to be there the whole time and was left out at manufacturing. Additionally, Some kits and manuals just don’t cover all the permutations of what was made with split years, floor changes and overzealous engineers.

I know I just went full circle but, unless you know what that seal serves to do- it may just be best to try it both ways and see if there’s a difference. You know, at low speed, in the driveway, with the E brake at hand.

The manual that I have isnt a shop manial so it has more general information. That is why I’m leaning toward what was in there the master cylinder. Thanks for your insight.

Ill go with what was in the master cylinder until if and when i find out otherwise. It is a split year like you suggested. Thanks all for the input

It’s been a good while, but IIRC when I rebuilt my master cylinder there was some issue about an extra seal in there. Apparently some required it, others couldn’t take it if you wanted to, there was no groove in the piston for installing it.

I think the only thing you’d need to worry about is if there’s a port in the cylinder wall that you might obstruct. There is some slight possibility that you have a piston from one configuration and a cylinder from another and adding in the missing seal would ^&$#%^ it up, but I consider that highly unlikely.

If you could post photos of the parts as well as the diagram, perhaps I could make more sense of it.

Just rebuilt the master on my 77’

Turns out I had an extra seal as well.

For the sake of the poor soul who doesn’t document their rebuild, I took pictures of the arrangement before and after rebuilding- Here’s just after pulling the parts. Notice the arrangement and orientation of the cups, of which there are 2 different types.

Here’s after cleaning and new seals. Before reassembly.