I decided to go the whole hog and change out the master cylinder to better suit the new setup. The new item is from a M3 with MK60 ABS pump which has a 25.4mm front and 22.2mm rear bore. Side benefit of the later model master is the ports for pressure sensors, will integrate them into my custom electronics for monitoring at some point.
The swap out will change the (calculated) front bias from 63.4% to 60.5% and should firm up the pedal feel.
It's all completed now and the brakes certainly have a very solid feel so pretty happy with the result for sure and was well worth the effort, seat of the pants test suggests braking performance is improved but could be related to how solid it feels.
Now for the story in pictures ... first up was the booster and master sourced from Ebay / https://mpartsworldwide.com/ at a pretty reasonable cost even with the shipping.
Removal of the 318 item and a side by side comparison reveals that the M3 uses two M12x1.0 ports and the 318 item is one M12 and one M10. I sourced an adapter from Ebay (and unfortunately their international shipping program ugghhh) which is USA made and adapts M12 to M10 bubble flare directly on the master.
The other issue is the reservoir from the M3 was filthy and I could not remove the filter successfully to clean it. Ordered a replacement reservoir along with fresh grommets.
My alternate plan was to use the reservoir from the 318 but it turns out the retaining screw on the M3 item inserts from the other side making this not possible without machining, who would have thought !!
While having both units out I also compared all of the critical dimensions, and successfully confirmed they were a like for like fitment 👍
Disassembly of the M3 master for a full clean, based on what i saw in the reservoir. Can not source replacement seals unfortunately so re-assembled.
That's really about it, reassembled everything and bled the system ... happy days !!