1974 750 GT Brake Question

[Brid]

OK -- I know, not a Monster question -- but, I figure someone here can hopefully help out? Got a 74 roundcase 750 GT, with a barely operable (single disc) front brake. It has an SS braided line, and we just rebuilt the Brembo F08 caliper and PS15 master cylinder - also fitting new pads. In recent months, we have been doing a lot of Laverda work - and all the 1200/1000's have  PS15 rear master cylinders and F08 calipers -- AND, the brakes just DO NOT work (a well documented Laverda malady) Can anyone please verify the (factory fitted) correct front master cylinder size for the bike? Fit a PS12 to the Laverda's - and you immediately have a good brake.
Cheers,
Brid.

[Speeddog]

I cannot confirm OE fitment for a 74 roundcase GT.
My Falloon book appears to confirm the F08 caliper with a very small picture.

I've run a single P4/34 caliper with a 15mm goldline master, and I'll be charitable and descripe it's feel as wooden, and lever effort high.
Same caliper with a 13mm big-pivot master was much better, lever effort fine but a bit too soft for my tastes.

I can't imagine the P08/PS15 working well at all, as it's got ~ 2/3 of the piston area of the P4/34, so would be even more wooden and higher effort.

Even the P08 with a 12mm master will be about the same piston ratio as a P4/34 with a 15mm master.

[Brid]

Thanks for the input Speeddog. Right now, we have an engineer designing/fabricating a 12mm (which is actually 12.7mm!) sleeve for the PS15 cylinders, as the same customer has 4 Laverda's (3 1200's and a 1000) that would like a rear brake - that actually does something (Also owns  72 and 74 750 GT's). We managed to find a PS12 (as fitted to the 500 Laverda -- AND, Ducati Pantah 500/600!!) and fitted it to the Laverda Mirage 1200TS. It was quite the revelation, as it brought the brake to life - enough that he can now quite easily lock the rear wheel.
As for the GT front brake - as much as I have been able to find is a supplier showing a 74 exploded diagram - and the repair kit says PS15. It came stock with a single disc, and drum rear, and I have suggested adding a second disc - which the PS15 would handle just fine. However, he wants to keep it as stock as possible. If I can find anyone that knows what as originally on there - we'd at least be happy enough. I have a modern PS12 ordered (rectangular reservoir) but know he's not happy, it not being OEM fitment. The puzzle goes on!