Which master cylinder do I have?

[The1970's]

Hey guys, this should be a quick one here:

I bought this master cylinder a while back, when I got it it was froze up and I got it free and cleaned it up but it has now started leaking out the piston (no surprise there after hammering the piston out)

https://www.ebay.com/itm/1978-Suzuki-GS-1000-Brake-Master-Cylinder-With-Lever/162719712940?ssPageName=STRK%3AMEBIDX%3AIT&_trksid=p2057872.m2749.l2649

I'm putting it on my 1978 GS750 non E model so only one rotor. I'm looking to order the piston and seals for the master cylinder but can't really decode the models in the parts outlaw fiche. For my year they list both GS750EC and GS750B and both have different part numbers for the piston assembly. Any idea which I need to order?

[sharpy]

New one. Unless you put a stainless sleeve inside it and have them give you the piston and seals to suit.. The bore be most likely pitted, grooved or have a lip in it. Should be able to find a EXPERT brake person to do what i stated. but if not buy a new one or try a 2nd hand one and more than likely start the whole thing again.

[sam000lee]

I spent over a month trying to get my '77 GS750 brakes sorted out due to this confusion. Here's my thread from a few months back which might have some helpful information and also there are some links to other helpful stuff:

http://www.thegsresources.com/_forum/showthread.php?247082-Can-t-get-air-out-of-my-lines

The single disc on the GS750 needs a 14mm piston. The part #'s get a little confusing when talking about the early model 750s and it didn't help for me that a PO had installed a 15.8mm master cylinder on my bike. I ended up going with this aftermarket MC from 4into1.com and couldn't be happier, it totally ended the headache:

It looks appropriate and you can reuse your lever if you want the rubber end cap. Costs $45. Note there is no brake switch but you can get a hydrolic banjo switch that I think they also carry. Works great for me.

I also have a rebuild kit that ended up being the wrong size for my 15.8mm MC. I don't recall the part number but can check and could send it over to you for real cheap.



Also just worth mentioning - the letters B and C refer to the manufacturing year. B designates the 1977 models (made in 1976) and C designates the 1978 model (made in 1977). I was under the impression that the 1978 models all had the dual disc so maybe the GS750B part numbers are appropriate.

[The1970's]

I also have a rebuild kit that ended up being the wrong size for my 15.8mm MC. I don't recall the part number but can check and could send it over to you for real cheap.

Also just worth mentioning - the letters B and C refer to the manufacturing year. B designates the 1977 models (made in 1976) and C designates the 1978 model (made in 1977). I was under the impression that the 1978 models all had the dual disc so maybe the GS750B part numbers are appropriate.

So if I'm reading it right what you're saying is that the twin disc vs single disc master cylinders have a different piston size? When I had mine on there the brake lever felt WAY better than the wrong master cylinder someone had installed previously and the brakes worked great it was just leaking slightly. I think i'll go ahead and disassemble mine, measure the piston size and inspect the bore before I order any parts.

[The1970's]

I just spent some time reading and disassembling/measuring and I'm just going to go ahead and follow what Sam did and order the same MC and pressure switch. Hopefully it'll solve all my problems.

[sam000lee]

So if I'm reading it right what you're saying is that the twin disc vs single disc master cylinders have a different piston size? When I had mine on there the brake lever felt WAY better than the wrong master cylinder someone had installed previously and the brakes worked great it was just leaking slightly. I think i'll go ahead and disassemble mine, measure the piston size and inspect the bore before I order any parts.

In a sense...

What you are actually comparing is the ratio of total area of the pistons on the caliper to the ratio of the MC piston. Having a larger diameter MC piston will make the brakes MORE firm but you are actually exerting less force on the caliper piston. I found this helpful reading:

http://www.customfighters.com/forums/showthread.php?t=56704

In short with the GS750 caliper which, if I remember correctly, has a 49mm diameter piston - a 14mm MC piston puts you in the correct ratio range.

[The1970's]

In a sense...

What you are actually comparing is the ratio of total area of the pistons on the caliper to the ratio of the MC piston. Having a larger diameter MC piston will make the brakes MORE firm but you are actually exerting less force on the caliper piston. I found this helpful reading:

http://www.customfighters.com/forums/showthread.php?t=56704

In short with the GS750 caliper which, if I remember correctly, has a 49mm diameter piston - a 14mm MC piston puts you in the correct ratio range.

Thanks for the help. I also did some reading on the resource that 4 into 1 has on their website and it makes sense now. The factory one I got off ebay is the 5/8 bore so I ordered the 14mm. However, the bore does look nearly perfect on the factory one I have so if anyone needs one I'd be willing to let it go for cheap

[Chrisf]

Hey 70's. I'm looking for a mc for my 1980 GS1000G. Let me do some research, and if that mc will work u would be interested, provided you still have it. Does it have the rectangular reservoir?