I have a 77 GS750 and I bought it with a trashed brake system. I pulled off the front MC because it was broken where the lever attaches. I pulled the front caliper to rebuild it and discovered it was seized. I just wanted to share what I've learned in trying to find replacement parts and their associated rebuild kits. I bought a front MC and caliper from a 78 GS750 thinking the parts would be identical, but both parts are different from the 77 parts.

The 77 caliper has a larger 43mm piston, while the 78 calipers has a 38mm piston. I have not been able to confirm that the 78 GS750 (single disc) actually came with the smaller 38mm piston, but the member I bought it from says the caliper came from a 78 single disc bike. Previously, I thought the smaller 38mm calipers only came on dual disc bike. I've seen several calipers on ebay that supposedly came from 78 single disc bikes and they look just like the one I bought on this forum. As best as I can tell, the differences are: 1) the 78 smaller piston caliper from 1978 has markings that read "Japan" and "Tokico" and a small "A" below the right bolt hole. The 1977 caliper is bare except for a small "C" below the right bolt hole.

78 caliper on the left, 77 caliper on the right
big vs small caliper 03.jpg