I just rebuilt the clutch master and slave and now doing the brake master.

I noticed that the piston has two grooved recesses that appear to be for rubber seals however I only got one in the rebuild kit.
Attached is picture of the piston with the one seal in place.
Is the second groove such that it does not need a seal?

Hello Paul,
Likely that you've found your answer by now, here's the cutaway brake master drawing from the service manual. Can supply info the push rod forks if still needed too.

Thanks Greg. This explains the issue. I received the wrong rebuild kit. The kit I got only has the two smaller seals.

Paul

Manners told me this morning that the original master cylinder rebuild kit has been replaced with the one I have and the third seal is not required. So I will go with that.

Paul