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GS750 master cylinder and caliper rebuild kits

H

hannibal

Forum Mentor
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
 
4) The mounting flanges are different. I noticed this when buying, but didn't think the calipers themselves would be different (78 on left, 77 on right)
caliper mounting flange.jpg


As for the rebuild part numbers, I have 59100-45813 and can confirm it includes the larger 43mm piston. My other options are 59100-45810 and 59100-45830. Anyone know the differences between the two and which one fits this 78 caliper with 38mm piston?
 
As for the front master cylinders, I pulled both the 77 and 78 apart to compare the internals. I found this thread that was very helpful in determine which rebuild kit to order: http://www.thegsresources.com/_forum/showthread.php?210413-What-part-Front-MC-KIT-1977-GS750-Help. Based on what he said, I believe the 77 MC uses 59600-45821, the 78 uses 59600-33810, and the dual disc bikes with 5/8" MC use 59600-45811. I can't reach into the bore to measure, but both pistons are just under 14mm. Does this agree with what you guys have seen?


First pic: 77 MC internals
77 front MC.jpg
 
So I decided to call Rocky Mountain since they were the only online parts store with the both caliper rebuild kits in stock. The parts guy confirmed that 59100-45810 was replaced by 59100-45813 and is for the larger piston calipers. 59100-45833 is the kit for the smaller piston calipers.

Will post once I have the kit in my hand and can confirm 59100-45833 fits the smaller piston calipers...
 
59100-45833 (which supersedes-45830) is the rebuild kit for the caliper with 38mm pistons.

I was able to remove the seized piston from the 77 caliper that came with the bike. After soaking in Berrymans, brake cleaner, then vinegar I tried boiling it in water. Pulled it out the water and took a big pliers to it. It started to turn but quickly became stuck. After 3 more 15 minute trips to the pot, the piston came out.

I'll be able to use what I believe is the true caliper for single disc 750s. I still can't find anything to say the 77-79 single disc bikes came with anything besides the 43 mm calipers. I'm guessing lots of the dual disc calipers found their way onto single disc bikes over time.

As for the MCs, I ordered the -33810 kit and it looks like what came out of my 77 MC. I ordered the -45821 kit so we'll see how that looks.
 
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