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

[Eric_Shea]

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?

The biggest part of this equation is realizing it's a 50 year old non-boosted brake system. Mom's minivan and your pinky toe can do better by today's standards.

Depending upon the front struts, you could easily go up to say... 911RS braking power but, I don't think that would do what you want it to either. (IMG:style_emoticons/default/huh.gif)

If you daily a 914 (which most of us don't do), it becomes "normal". If you daily a 2019 ______, that becomes normal and a 914 tends to feel bad.

Aside from a boosted circuit, this is a difficult (yet extremely common) question to answer with all the variables. My answer comes from living with 914 brake systems for 42 years and dealing with this question for almost 20 years. I think they're not only adequate but "extremely good" by 1970-76 standards.

[Superhawk996]

(IMG:style_emoticons/default/agree.gif)

People have become used to overboosted brakes and power steering. Very few people recall driving an early 70s Chevelle without power steering, drum brakes, and without power brake boosters.

Even when domestics finally went with discs up front and power assisted brakes, they were terribly overboosted and difficult to modulate vs a 914.

@bkrantz

500N (112 lbs) is the force to be applied to the brake pedal during the failed power portion of FMVSS 135 testing which would be conducted for a power assisted brake system with a failed brake booster. There are other tests within FMVSS 135 that allow up to 1000N (225 lbs) of brake pedal force. Any human in reasonable physical condition should be able to obtain these pedal forces.

At 100 kph the vehicle must stop in less than 168m (551 ft) with a failed booster and 500N of pedal force.

That is a bare minimum accepted performance that you should be able easily beat.

I don’t have any easy recipe to set up a cheap foot operated load cell so that you can directly measure these pedal forces but I outline them so you have some sense of what is considered normal pedal forces that can easily be achieved and the stopping distance that must be met by any new production vehicle.

[bkrantz]

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)

My attempt to lock the brakes is only an extreme test. When I tried swapping in some Textar pads, I could not lock up any wheel.

[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?

The biggest part of this equation is realizing it's a 50 year old non-boosted brake system. Mom's minivan and your pinky toe can do better by today's standards.

Depending upon the front struts, you could easily go up to say... 911RS braking power but, I don't think that would do what you want it to either. (IMG:style_emoticons/default/huh.gif)

If you daily a 914 (which most of us don't do), it becomes "normal". If you daily a 2019 ______, that becomes normal and a 914 tends to feel bad.

Aside from a boosted circuit, this is a difficult (yet extremely common) question to answer with all the variables. My answer comes from living with 914 brake systems for 42 years and dealing with this question for almost 20 years. I think they're not only adequate but "extremely good" by 1970-76 standards.

Thanks, Eric. I have tried to account for my modern brake bias (no pun intended). I did have Mike M drive my car last summer, and he also thought the brakes should be better.

[bkrantz]

(IMG:style_emoticons/default/agree.gif)

People have become used to overboosted brakes and power steering. Very few people recall driving an early 70s Chevelle without power steering, drum brakes, and without power brake boosters.

Even when domestics finally went with discs up front and power assisted brakes, they were terribly overboosted and difficult to modulate vs a 914.

@bkrantz

500N (112 lbs) is the force to be applied to the brake pedal during the failed power portion of FMVSS 135 testing which would be conducted for a power assisted brake system with a failed brake booster. There are other tests within FMVSS 135 that allow up to 1000N (225 lbs) of brake pedal force. Any human in reasonable physical condition should be able to obtain these pedal forces.

At 100 kph the vehicle must stop in less than 168m (551 ft) with a failed booster and 500N of pedal force.

That is a bare minimum accepted performance that you should be able easily beat.

I don’t have any easy recipe to set up a cheap foot operated load cell so that you can directly measure these pedal forces but I outline them so you have some sense of what is considered normal pedal forces that can easily be achieved and the stopping distance that must be met by any new production vehicle.

Thanks. I certainly have gotten used to modern brakes. But at 220 lbs and 6-2, and fairly healthy, I am pretty sure I can apply a fair amount of foot pressure. And I have memories of 914s (my own and others) where we had to practice threshold braking to avoid lock-up. Once the snow is gone, and the roads get cleaned up, I can try again.

[930cabman]

(IMG:style_emoticons/default/agree.gif)

People have become used to overboosted brakes and power steering. Very few people recall driving an early 70s Chevelle without power steering, drum brakes, and without power brake boosters.

Even when domestics finally went with discs up front and power assisted brakes, they were terribly overboosted and difficult to modulate vs a 914.

@bkrantz

500N (112 lbs) is the force to be applied to the brake pedal during the failed power portion of FMVSS 135 testing which would be conducted for a power assisted brake system with a failed brake booster. There are other tests within FMVSS 135 that allow up to 1000N (225 lbs) of brake pedal force. Any human in reasonable physical condition should be able to obtain these pedal forces.

At 100 kph the vehicle must stop in less than 168m (551 ft) with a failed booster and 500N of pedal force.

That is a bare minimum accepted performance that you should be able easily beat.

I don’t have any easy recipe to set up a cheap foot operated load cell so that you can directly measure these pedal forces but I outline them so you have some sense of what is considered normal pedal forces that can easily be achieved and the stopping distance that must be met by any new production vehicle.

Every so often in the summer months I break out an original paint/never rusted 1970 Dodge W200, no power steering and she's not easy to parallel park.

[914werke]

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. (fire away)

Disagree. (IMG:style_emoticons/default/smile.gif) You mention in a later post that almost no one "dailys" a 914 anymore while id agree with that ..I find that Ill drive more examples, more often, than the average. (IMG:style_emoticons/default/driving.gif) & of that sample most have had braking work (by me) to bring them (back) to spec. IMO once calipers are refreshed lines replace decent pads applied & bled (typical) the MC action is easily distinguished. IMO the OE MC is more "progressive" in its "bite" which I prefer. (IMG:style_emoticons/default/shades.gif)