Brembo P8 pistons are 38mm diameter.
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Yes, my 1979 G5 V1000 has 38 mm pistons.
I measured across the caliper pistons on the 1977 T 3 and they weren’t even 38 mm. More like 33 mm.
They came fitted to the bike/combination when I bought it. The handlebar O/S/F one was solid and ineffective.
just finished fitting and bleeding the 4 pot one matched to the 14 mm Nissin master and took it for a blast. Took a bit of spacing to centre the caliper. Probably easier to do without the complication of leading link forks. I can now lock up the front wheel with two fingers at 50mph on a wet road.
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Hi Godrey. Glad you got it sorted out in the end. They are a definate ‘upgrade’ and a relatively easy one at that. Even looks ok the ‘wrong’ way round!!! Nik
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Yes definitely. Thanks for your input.
Just finished fitting the N/S front 4 pot Brembo. Took apart the other side too as I wasn’t happy about the lower caliper mounting nut, as it was running too close to the rotating brake disc, because I’d reduced the shim width to centralise the new calipers. I cut them both out and welded in new nuts within the mounting plate rather than where they were on the back of the plates.
The original foot operated master cylinder works both linked brakes ok but is close to its limit because of long pedal travel. I believe a slightly larger bored cylinder would be optimal.
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Just admiration for your persistence and skills in getting this job sorted! Excellent. What a win! 
The replacement Linked M/ Cylinder from Brembo is a PS16, whereas the original Linked M/ Cyl was a PS15. Might give you a slightly better ‘pedal’
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Eventually got a day to fit the slightly bigger new PS16 Brembo rear master cylinder. The original one gave a good brake but even correctly bled and adjusted allowed the pedal to hit the foot board if you tried. Because its all a bit space compromised under the cover I had to figure out on how to site the master cylinder by making up a joining offset lower mounting bracket and a longer operating rod. This to also allow the existing brake pipe to reach its new destination and get a decent angle for the operating rod to be efficient. The new longer operating rod needed threaded at each end to accept pivot pieces at the m/c and foot brake. I used a remote Nissin brake fluid reservoir of about 50 ml capacity with a long hose. I mounted this off existing frame holes to sit alongside the rear mudguard and under the rear of the seat. The existing brake line to the linked compensator did fit. The M/C operating arm and brake activation is very sensitive to being even slightly on so required some return springs. The flasher units got moved a couple of times until I was happy they were insulated from vibrations and any interference from moving parts.
Reverse bled the new to me front 4 pot brake caliper from the nipple, and then did the shorter run of the rear brake to splitter and m/c. Bled first time and gives very good brakes. The downside is how quickly the sidecar now comes around as it doesn’t have the space for an additional brake 
Because I wasn’t sure of success I made sure that nothing was altered that couldn’t be reversed to put back to how it was.
The dearest part was the new master cylinder at just under £150.
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