during investigation into my lack of braking quality in my '74, i am looking at replacing the brake master cylinder. it is probably the original part, 30+ years old. i searched for prices and while on the pelican site, i saw that the 19mm is less expensive than the 17mm which leads me to the question: for an otherwise stock 914-4 brake system, is it "better", when replacing the MC to go with the 19 or 17mm? what is the current wisdom?

I think in reality it is a matter of practicality. Porsche parts department tells me they only stock the 19mm and the 17mm part is so much more expensive.
You do experience a change in braking physics. You will be used to it in a week or so and not know the difference... There is a Classic Thread in the Lapuwali Classics on this... I will let you read that.

For a street car the stock 17mm is fine. If you want to put a new one in though, I'd probably opt for the 19. I've got rebuild kits for the stock mc on hand, if your interested pm me.

I got the 19 because it was cheaper. It is a bit more effort than the 17 to push, but like said above you really won't notice. Now if you went to a 23mm that would be a different story...

this is a driver's preference issue. there is no better in this regard.

WARNING: if your brakes suck, there is something wrong!!! your rubber lines might be internally swollen causing reduced braking, requiring too much pedal pressure to stop.

19mm = firmer pedal, less travel, more foot presure required over shorter distance to lock em up, $ cheaper to buy because more were produced in 911 model

17mm = softer pedal, longer travel, less foot presure required over longer distance to lock em up, $ more expensive to buy because it's 914 use only, less were made

I got the 23 in mine. I often wonder what the pedal feel is like with a smaller master cylinder.

Really made me nervous when I first bought the car.

http://www.914world.com/bbs2/index.php?showtopic=55559

This is the link to the thread in the classics forum.

What Jim D. said.

At this point, get the 19mm. If you've been driving this thing every day you will probably notice a slight difference but, it won't be much.

The 19mm will give you a firmer pedal at the expense of more effort but, again, not so much more that you:

a ) Probably won't notice it
b ) If you do notice, you'll be use to it in a couple of minutes of driving
c ) You'll probably notice the firmer pedal feel more and enjoy it more.

My $0.04

yeah, I cannot lock up the brakes on this car. I thought that maybe a better brake pad compound might give better braking so I got a set of Porterfield R4S pads (I ordered them about 2 weeks before the group buy organized here) and last weekend i swapped out the new pads but noticed that 7 of the 8 pads are"pretty" thick while one is about half the thickness of the others. that made me wonder about the condition of the MC since it's likely original, 30+ yrs old. i don't think my conditions point to a MC but thought I'd ask anyway. problem is, i recently rebuilt the front calipers. my plan of action is to put the car out on a test drive when the weather and time is better and see what happens to braking power with new Porterfields.

That would make me wonder about the condition of the caliper (that piston specifically) that it came out of.

Sounds like that piston is not retracting properly

i have the same problem - less than ideal braking - so i put porterfields up front - some improvement - but i still don't have the ability to lock up my brakes. i am thinking i need a set of rebuilt front calipers from eric and new soft lines front and rear. but maybe a few more tries at bleeding first.

yes, and i was hoping I didn't botch the rebuild of the calipers so i kinda was assuming/hoping it was not the calipers.

It happened to me before. I remember one of my first caliper rebuilds on my 914-6. Rebuilt everything and it pulled to the right like crazy.

Went back in and rebuilt the calipers again.

Done.

Here's a coulple of basics I wouldn't miss on a caliper rebuild:

1. Hone the bore with a $16.00 brake cylinder hone (FLAPS)
2. Polish the pistons using a bench polishing wheel. Make sure they're like a mirror.
3. Clean and check you knock-back mechanisms and pins. This may be your problem.

Sometimes, when using air, the second piston can fly out of it's bore with such velocity that, if not protected properly, the knock-back pin in the opposite bore can be mis-aligned.

Check everything but, one worn pad cannot be created by a MC.

Twice as much as everyone else gets?

I replaced mine with a 19mm, but the real problem was the rubber lines, in the end. Start there, your MC may be just fine. On mine, the inner diameter of the line was less than half a pencil's. They were, I have to believe, the originals lines. Looked fine from the outside. I wish I had kept them to take a pic and post here.

What are the 19mm running these days? last time i bought one for my 75 was like $155.00. I'm sure they gone up since then.

low price of $120 for aftermarket MC from Zims in TX. pelican is about $260ish. GPR site was down when i searched. autohausaz had a good price.

I picked one up from Rock Auto for inexpensive... sub $100 mark. I just don't remember off the top of my head which 911 I picked to 'borrow' it from.

http://www.rockauto.com/catalog/x,make,PORSCHE,vtype,US

Be "very" careful with the aftermarket ones at this time. I've heard of 7% failure rate and know of a major retailer that stopped carrying them.

"Be very careful...?" what am I thiking? Let's tell it like it is; Don't buy them. I'm not an aftermarket parts hater but these just aren't ready yet. They just came on the market within the last year.

Yikes. Now you tell us. I hope I'm in the 93%... I guess if it fails and I don't tell you about it I won't be in the 7% you hear about. lol

Not that I'd be able to hide the effects of *that* part failing... hmmm... I better make sure that e-brake setup is fully functional.

I have 1987 911 front calipers, 1984 rear calipers and 19mm master.

This set up seems like it works. Been thinking about some new pads.

well I have NO problem locking up mine (911 front/rears) with a 23mm. Its a short distance on the pedal but very Q U I C K !!