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1979 P200E, 2017 GTS 300
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Location: San Rafael, CA
 
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1979 P200E, 2017 GTS 300
Joined: UTC
Posts: 38
Location: San Rafael, CA
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I upgraded the front brake caliper on my GTS300 to the 4-piston Frando. OEM front brake master cylinder isn't ideal in terms of extensive lever travel length, but fine for the time being. I'm wondering for others in this situation who have also upgraded to the 4-piston Frando or Brembo, what aftermarket brake master cylinder did you choose and what's your experience? If you could have done it all over again, would you have chosen a different master cylinder? In terms of radial master cylinders, Frando offers a 14mm and Brembo offers the trick RCS Corsa Corta in 15mm/17mm/19mm, or the standard RCS in 14mm/15mm/16mm/17mm/19mm. Also, available are OEM-looking axial direct drop-in replacements in 14mm and 16mm. Most importantly, I want to get the correct size piston. In this particular scenario, I wonder whether there is wide agreement that a certain size piston mm works best, or whether there is a small range of sizes that work best according to personal preference. Also, I wonder whether it is worth the effort and money to get a radial master instead of axial? It would mean trimming up my handlebar cover a bit, and perhaps needing a new brake line or adapter, and then properly routing / mounting everything, but not really that big of a deal. My GTS is pretty trick w/ full (quality) aftermarket suspension, entirely poly-mounted chassis / suspension, lightweight racing wheels, Pirelli Diablo Rosso tires, full Malossi 282 / V4, etc. Nothing feels better than trail breaking incredibly deep into turns. I just want to learn from other people's experiences, not shortchange myself, or make the same mistakes. Thanks.
@steelbytes avatar
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Veni, Vidi, Posti
2019 GTS300 Supertech E3 60,000km
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Veni, Vidi, Posti
@steelbytes avatar
2019 GTS300 Supertech E3 60,000km
Joined: UTC
Posts: 5710
Location: Batmania aka Melbourne, Australia
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[removed as I think I had misunderstood]
⚠️ Last edited by SteelBytes on UTC; edited 1 time
@waspmike avatar
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Ossessionato
LXV 150 3v ie. Midnight Blue (Sold) Now Honda Zoomer X
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@waspmike avatar
LXV 150 3v ie. Midnight Blue (Sold) Now Honda Zoomer X
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I did some surfing looking for high ratio levers, where the pivot point may be different. Then you wouldn't have to hack the bodywork. Sadly this morning I am running out of time.

If you can find a lever with a greater distance between the pivot and the contact point that would space the lever further from the bars and you can then adjust the hand distance which after market levers come with.

Of course it might look cooler to have a different cylinder sticking out through the plastic
OP
UTC

Member
1979 P200E, 2017 GTS 300
Joined: UTC
Posts: 38
Location: San Rafael, CA
 
Member
1979 P200E, 2017 GTS 300
Joined: UTC
Posts: 38
Location: San Rafael, CA
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Thanks waspmike for the reply. Like I mentioned in the original post, I do have the pretty painless option of purchasing (for relatively cheap) some axial OEM-looking direct drop-in replacement brake master cylinders in either 14mm or 16mm. (I believe OEM is somewhere in the 12mm range). When I reached out to a somewhat knowledgable salesperson who sells both, he recommended the 16mm over the 14mm. I find it interesting however that Frando's master cylinder is 14mm. Also when googling "Vespa GTS" and "Brembo RCS", units comes up that are 14mm as well. So I don't really know who or what to believe. That's why I'm reaching out here, to get the experience of more people who have already gone down this road. I could live w/ my stock master cylinder for a little while. The extended travel is a little psychologically nerve racking as the lever gets much closer to the handlebar (although there is still plenty of space and it'll never touch / bottom out). But the extended travel also makes for plenty of modulation and less effort, which is kinda nice. I'm also using my favorite 5.1 brake fluid (TBM) which has excellent viscosity coupled w/ no perceivable compressibility. Most of the other racing-oriented 5.1 fluids that everyone knows and loves (and I've experimented with a handful) also have excellent compressibility, but the viscosity is much lower and requires considerably more squeezing effort. Also, I've done a bang-up job bleeding my system (including the ABS unit), so whatever I'm experiencing at the lever doesn't have anything to do w/ inferior or aged fluid, or any potential air in the system. Although it must be acknowledged that, ABS, because of the extra plumbing, inherently doesn't feel as rock solid as a system without. I'm just looking to hear what others have done before I make any moves. At least I've already identified what's available on the market. And eventually I'll do something, because my situation, while adequate, can be greatly improved.
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LXV 150 3v ie. Midnight Blue (Sold) Now Honda Zoomer X
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@waspmike avatar
LXV 150 3v ie. Midnight Blue (Sold) Now Honda Zoomer X
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I think it can all be decided by calculator. I haven't done it before on a Vespa.

It is all volume vs. pressure.

The original master cylinder produced sufficient pressure and volume for two pistons. The effort on the lever is a constant as is the distance the pads need to travel.

The new system has four pistons which require less pressure for the same clamping force but more volume.

If we know the diameter of the stock caliper and master cylinder we can work it all out using the mathematics and physics that some think is/was a waste of time learning at school.
@greasy125 avatar
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Sergeant at Arms
Weird 80's Vespas & Cool Vintage Lambrettas
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@greasy125 avatar
Weird 80's Vespas & Cool Vintage Lambrettas
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Location: The state of insanity, SoCal
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I was researching stock MC piston sizes for another project awhile ago and it *seemed* as though the magic number was 13mm for the heng tong GT/GTS units and the grimeca ones were 12mm.

anyway, in my search I stumbled across this chart that lists MC piston sizes and caliper piston sizes, with a rating of force for each combo.

in short: big master cylinder= stiffer pull, more feedback, less travel
bigger (or more) caliper pistons= more force, but longer lever travel

the trick is selecting the components that will give you the desired effect.

from what I can decipher you're looking for more feedback and less travel. as a wild ass guess, I'd try a 14mm with an aftermarket adjustable lever.
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