17mm vs 19mm master brake cylinder, pros and cons? Options

[914 RZ-1]

I need a new M/C. Stock is 17mm, but it seems everyone is selling 19mm. I'll get a firmer pedal and less stroke with the 19mm, correct? Is this good or bad or personal preference?

What are the advantages/disadvantages of both?

Thanks!
-Jeff
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[JeffBowlsby]

I need a new M/C. Stock is 17mm, but it seems everyone is selling 19mm. I'll get a firmer pedal and less stroke with the 19mm, correct? Is this good or bad or personal preference?

What are the advantages/disadvantages of both?

Thanks!
-Jeff
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(IMG:style_emoticons/default/blowup.gif) Incoming….

[mepstein]

19mm - moves more fluid, less pedal travel. Needs more force at the pedal.
17mm - moves less fluid for the same travel. Less force needed at the pedal.
It’s a ratio, you trade travel for pressure.

I personally like a soft, longer travel pedal. Easier for me to modulate and I get more force at the caliper with less force at the pedal.
Lots of people like firm pedal.

[Superhawk996]

I need a new M/C. Stock is 17mm, but it seems everyone is selling 19mm. I'll get a firmer pedal and less stroke with the 19mm, correct? Is this good or bad or personal preference?

What are the advantages/disadvantages of both?

Thanks!
-Jeff
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Correct - shorter stroke - more pedal force to achieve equivalent line pressure.

It comes down to personal preferences. I did brake engineering as my day job for four years. Tiny differences in force and travel can have big differences in perceived pedal feel. Some people are very sensitive to small changes while others lack the ability to detect large changes.

It basically comes down to whether you prefer force modulation or travel modulation keeping all other aspects of the system fixed and assuming that the basic operation of the system was properly sized and balanced.

There is a huge interaction with the compressibility and friction of the brake pads that are being used. There are also interactions with the type of flex lines being used as well as the stiffness of the brake calipers themselves.

Example: low friction, low compressibility pads with a 19mm master cylinder are going to leave some feeling like they have very poor brakes. They will feel as if they are pushing the pedal as hard as they can (but usually aren’t), and feel as if they are not stopping quickly enough for the pedal effort applied. This is especially so, now that virtually all of America has become used to the feel of power assisted brakes.

[914werke]

19mm - moves more fluid, less pedal travel. Needs more force at the pedal.
17mm - moves less fluid for the same travel. Less force needed at the pedal.
It’s a ratio, you trade travel for pressure.

I personally like a soft, longer travel pedal. Easier for me to modulate and I get more force at the caliper with less force at the pedal.
Lots of people like firm pedal.

(IMG:style_emoticons/default/agree.gif)

[mepstein]

Make sure you replace all your soft rubber lines if you don’t know or can’t remember when they were last changed. They will swell shut after a number of years. Cheap maintenance item.

[914Toy]

It also depends on which calipers you are using.
I have Baxter brakes in front and stock rear brakes with a 19 mm master cylinder. I find this the perfect match.

[914 RZ-1]

Thanks for the replies! I did search this site for info, BTW.

Perhaps I should have been more specific; in looking for 17mm, they don't seem to be readily available, or the bore size is not indicated. The research I did on this site had old info for links to suppliers that are no longer around. There does seem to be more 19mm M/C's available, so I thought I would ask if there were any major differences or issues.

My daily driver is a Chevy Silverado (power assisted, ABS), so I expect the brakes on my 914 to be different. I just want to make sure that if I use 19mm M/C I'm not damaging anything or causing any issues.

I'm searching part number 91435501200 (I have the warning contact).

I won't bore you with links to all the places that come up in a search that have them, but they are either N/A or not the right one (lots of 19mm replacement for stock 17mm, hence my original question), or I can't tell if they are the right one (different part number, usually for later cars).

URO makes one that is usually around $100 ±$20 or so. Is this one any good?

Hope that helps clarify things.
-Jeff
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[914 RZ-1]

Make sure you replace all your soft rubber lines if you don’t know or can’t remember when they were last changed. They will swell shut after a number of years. Cheap maintenance item.

Thanks! In the process of doing just that.

[914 RZ-1]

19mm - moves more fluid, less pedal travel. Needs more force at the pedal.
17mm - moves less fluid for the same travel. Less force needed at the pedal.
It’s a ratio, you trade travel for pressure.

I personally like a soft, longer travel pedal. Easier for me to modulate and I get more force at the caliper with less force at the pedal.
Lots of people like firm pedal.

Thanks, this is helpful. (IMG:style_emoticons/default/beerchug.gif)

[914 RZ-1]

It also depends on which calipers you are using.
I have Baxter brakes in front and stock rear brakes with a 19 mm master cylinder. I find this the perfect match.

Everything is stock. I guess I'll find out when I install everything and take a test drive! I'm not "used to" any particular brake feel on this car, so I think I can adapt to whatever it ends up being.

[914 RZ-1]

I need a new M/C. Stock is 17mm, but it seems everyone is selling 19mm. I'll get a firmer pedal and less stroke with the 19mm, correct? Is this good or bad or personal preference?

What are the advantages/disadvantages of both?

Thanks!
-Jeff
-----------

Correct - shorter stroke - more pedal force to achieve equivalent line pressure.

It comes down to personal preferences. I did brake engineering as my day job for four years. Tiny differences in force and travel can have big differences in perceived pedal feel. Some people are very sensitive to small changes while others lack the ability to detect large changes.

It basically comes down to whether you prefer force modulation or travel modulation keeping all other aspects of the system fixed and assuming that the basic operation of the system was properly sized and balanced.

There is a huge interaction with the compressibility and friction of the brake pads that are being used. There are also interactions with the type of flex lines being used as well as the stiffness of the brake calipers themselves.

Example: low friction, low compressibility pads with a 19mm master cylinder are going to leave some feeling like they have very poor brakes. They will feel as if they are pushing the pedal as hard as they can (but usually aren’t), and feel as if they are not stopping quickly enough for the pedal effort applied. This is especially so, now that virtually all of America has become used to the feel of power assisted brakes.

Thanks, this was all helpful and interesting.

I'm not sure what I like, TBH, LOL. I just want to be able to stop! I'll have to pay attention more when I'm braking and see.

[Superhawk996]

17mm m/c are indeed hard to come by. NOS pops up occasionally and they aren’t cheap when they do.

Highly recommend ATE. There are issues with some of our favorite vendors less expensive master cylinders ranging from weeping Welch plugs to using o-rings where they should be using lip seals.

[76-914]

I installed one of Mark's 19mm when the 17mm went south and I honestly haven't noticed the difference. Everything I read said I would need to press harder but I don't notice the difference. (IMG:style_emoticons/default/confused24.gif)

[Cairo94507]

Just because, I have a 23mm master cylinder, 914-6 GT rears calipers with drilled and slotted rotors, 88 Carrera front calipers with 24mm drilled and slotted rotors, no proportioning valve, rubber lines at all 4 wheels and I love the way my car brakes. It is a firm pedal and easily modulated. (IMG:style_emoticons/default/beerchug.gif)

[mepstein]

PMB sells Porterfield brake pads. I’ve been really happy with the way they grab the rotors. Just make sure to bed any pads per the video on the PMB site. Especially important if you do pads and rotors at the same time.

I agree , the 19mm MC is much easier to find and usually more reasonably priced. I’ve driven both 17 and 19mm equipped cars quite a bit (my own) and you get used to whatever you have. No bad choices between the two.

I was always very picky about the brakes on my road and mountain bikes. When I sold bikes at the shop, people liked a firm feel during the parking lot test drive. I liked a very soft feel with lots of travel but maximum braking. Do a 3-4 hour race and you realize you want maximum breaking with minimal effort. Also, reaching your hands and fingers out further to pull on the lever will fatigue them faster than when your hand is almost in a fist.
So how does this relate to a brake pedal?
Your legs are strongest and fatigue less when they are almost fully extended. A brake pedal with little travel will have you pushing when your knee is bent. More travel will straighten your leg. Also, the more pressure you need to apply, the harder it is to modulate the pressure. Our nervous system differentiates light touch better than heavy exertion.
None of this makes a big difference between the 17-19mm debate. It’s just something to think about if you want to be particular about setting up your car.

[Eric_Shea]

In a "real world" application, the feel between a 17mm and a 19mm is (as @76-914 mentions) basically indistinguishable. I'm certain it can be measured but you "really" won't noticed it. There will be no leg fatigue whatsoever. I'll buy you a protein shake the next time I see you if you get leg fatigue pressing your brake pedal. (IMG:style_emoticons/default/cheer.gif)

This 17 vs. 19 is just a non-issue that gets way overblown via the Itnerweb thingy.

The real issue (as the OP is finding) is there are no more 17's being made. None of the suppliers carry them. And so goes the 17... so should threads like this. (IMG:style_emoticons/default/biggrin.gif)

I've also found that a "harder" pedal gives me the best modulation. The feedback and feel in the foot is extremely precise with a harder pedal. YMMV (IMG:style_emoticons/default/wink.gif)

(fire away)

[bkrantz]

At the risk of high-jacking (sorry) what can be done to maximize braking power (real and perception) with the stock system?

As part of my build, I completely overhauled the brakes, with 4 calipers and rear pressure valve rebuilt by PMB, all new lines and hoses, new Porterfield pads, new disks, and a new ATE 19mm MC. I did lots (LOTS! of bleeding) and had a very firm pedal. And I carefully did the bedding procedure that PMB recommends. But the brake power always seemed lacking. I could get one or both front wheels to lock up (barely) if I really stood on the pedal.

@Eric_Shea , and others, any comments?

[76-914]

IMHO, I try to avoid locking up the brakes. You can get bigger and/or softer tires. Hard stops aren't my thing but it is nice to know you can stop quickly. Are you sure your brakes are bedded in? You can do the bedding in process again if in doubt. (IMG:style_emoticons/default/beerchug.gif)

[mgp4591]

[quote name='Eric_Shea' date='Mar 13 2023, 12:22 PM' post='3064794']
In a "real world" application, the feel between a 17mm and a 19mm is (as @76-914 mentions) basically indistinguishable. I'm certain it can be measured but you "really" won't noticed it. There will be no leg fatigue whatsoever. I'll buy you a protein shake the next time I see you if you get leg fatigue pressing your brake pedal. (IMG:style_emoticons/default/cheer.gif)

The bottom line on this subject is, just buy your 19mm m/c from PMB and you'll be happy! (shameless plug...) (IMG:style_emoticons/default/shades.gif)