Not sure if it is necessary for you to upgrade but if you decide to Cityracer has a great dual diaphragm brake booster that is plug and play as well a master cylinder for FZJ80 that is a nice upgrade. I recently upgraded mine because my booster failed. (PO was running stock booster, MC with 4 wheels discs). I love this combo.
post #29 in ”The Iron Yak” build thread
When I installed front discs on my 1973 40 I left the stock booster in place and it worked fine. Even after I replaced the rear drums with discs I retained it, and it worked fine. However, recently it began making some end-of-life noises, so I replaced it with the City Racer dual-diaphragm unit mentioned above. There was a noticeable improvement in pedal firmness.
The stock booster and master cylinder will work, though you will probably need a proportioning valve for the rear. I ran mine for years that way, but as has been said, the City Racer booster and a non-ABS FZJ80 master cylinder made a huge improvement after I installed them last year.
If your current MC and booster are working well now, I wager they will continue to do so with the rear disc brakes added.
I added front Toyota discs 16 years ago, put the rear discs on 10 years ago, all with the original booster and '74 master (rebuilt a few times though (the master, not the booster)).
As an aside, I was never able to really lock up all 4 tires (now 37s) with the discs like I could the the old drums (with 35s). don't know if it's the mis-match of the MC and the discs or the tire size. I've heard that the bore of the 80 series MC is different from the 55 (and 40), forget if it's bigger or smaller, but it made sense at the time. Stroke too.
And, recently, I've noticed the signs that say my MC is getting old (fluid in one reservoir is slightly rising, other slightly falling) so i bought a new 80 series MC from NAPA, figured I should put a new booster in too, ordered City Racer's. Should be here any day now. Counting my blessings that I got 46 years out of the original booster, will be retiring it with honors.
So, to make a long story short (too late, I know), no, you do not need to upgrade your booster and MC to handle the addition of rear discs, but, soon, you will want to and should plan on it. YMMV