Hi all, I'm looking for changing my brake master cylinder from S1 to S3, and have some custom stainless brake lines fabricated instead of changing fittings.
I've found that S3 master cylinder has 10x1mm and 12x1mm fittings, but don't know the size of Brake proportioning valve fittings on S1. 3/8 x 24? Anyone has an idea? Thank you!

 

I decided to use the S3 reservoir... the cap will hit the hood brace. I just cut a small section out of it so it would clear.

Most new MC's come with plastic adapters so you can continue to use the S1 reservoir...

Sorry but have no idea of the size you are seeking... I found there was only the one brake line fitting that had to be changed - adapted.

 

Thank you Roger for the feedback, so S3 hood braces are different: will take care of this before slamming closed the hood!
I'd like to use S3 reservoir as mine S1 is cracked and I prefer the cleaner look (and less tubing for further possible leaks...)

 

The hood barely hits the cap, but it causes brake fluid to leak out the rubber seals at the bottom....

This ruins the paint underneath.. now I get to sand and repaint that fenderwell... I too chose to use the later reservoir for on the road parts replacement in the future...
easier to find S3 brake stuff than S1 brake parts and it is much cheaper.