914 Master Cylinder - question? Options

[TRP]

Hey Gang!

So, if you've been following my build notes/thread on the forum here, you may know that I was waffling back and forth on changing my master cylinder. The brake pedal never felt right from the day I picked it up on August 7th of this year. The pedal felt weak and I'd need to pump the brakes a few times to get a firm-ish pedal. Even with that, it never felt confidence inspiring

I tried bleeding the system 2 times with my motive power bleeder, and it got a little better but still not good enough. I had a few serious pucker moments in traffic where I though I was going to eat the car in front of me. (IMG:style_emoticons/default/barf.gif)

I was on the fence between trying to bleed them again or just swapping out the entire MC. Well, today I bit the bullet and did the swap. Everything went very easily because I had the tank out and everything already taken apart for the fuel line swap, sway bar install, and steering rack bellow replacement. In for a penny, in for a pound... right?

One thing I noticed when I pulled the existing MC out of the car, they previous owner didn't install the little silver washers into the inlet bores of the MC. He just had the rubber grommets/metal lines jammed in there. Could this have been part of the issue I was having with a weak pedal?

I installed the new 19mm MC with the silver washers in there and I am working on getting the system all blead nicely. So far everything is going well, I have already blead out about half of a big bottle of DOT4. I'm letting everything settle a bit and I'll go for one or two more rounds before I have my wife help with the final foot pump/open the bleeders dance.

Just wondering what you guys think about the necessity of those silver washers.

Thanks
Ted

[mepstein]

100% absolutely necessary. Brake fluid can leak without the washers. People leave them out for a couple reasons.
They don’t know any better.
It’s already hard to put in the rubber grommets.
They weren’t included with the new mc
Etc, etc.
Doesn’t matter, you must use them.

[TRP]

Thank you! They are in there on my new install. I feel better having done the swap.

I just hope I can get the brakes blead properly so that I can have some confidence in driving and stopping this thing. (IMG:style_emoticons/default/biggrin.gif)

I followed this thread: and I see where they are mentioned by more than one person. Glad I put them in. (IMG:style_emoticons/default/smile.gif)

Now when I first hit the pedal, it's high and firm. I can actually push hard and keep applying pressure and the pedal will bleed down a bit. I can't find any leaks, but I'm still concerned. Maybe I still have air in there somewhere?

(IMG:style_emoticons/default/dry.gif)

[mepstein]

Maybe. I usually have my guys bleed the brakes, I go out and bed the pads and then they bleed them again. My feeling is the hard stops (down to 5mph, not completely stopped) works the air out of the nooks and crannies. It also heats up the whole system, expanding the air and giving you more air volume to bleed out.

Once the system is right, you will know it. Firm pedal, positive brake feel, will lock up the wheels if you really get on it.

Also, you must adjust the rear pad clearance on a 914. Ignore it and the brakes will never feel right.

[steuspeed]

Going to 19mm is going to change things. Brake bias is going to add more braking to the rear. Not good. Pedal throw will be longer but firmer at the end. Ask AI for a detailed answer. I replaced mine with 17mm which are a bit harder to find and more expensive. Just because it's 911 spec, does not mean it's an upgrade.

[bdstone914]

Going to 19mm is going to change things. Brake bias is going to add more braking to the rear. Not good. Pedal throw will be longer but firmer at the end. Ask AI for a detailed answer. I replaced mine with 17mm which are a bit harder to find and more expensive. Just because it's 911 spec, does not mean it's an upgrade.

If you depended on AI for the answers you got wrong info.
Bigger master pushes more fluid and makes a firmer pedal as less travel is needed to move the same volume of fluid.
It does not change the bias. The relative piston sizes front and rear affect bias.
I have done several cars with 19mm masters and I find no issues.
Some like the softer original that has more pedal modulation. Maybe that is what you meant by bias.

[mepstein]

It’s almost impossible to find a 17 and 19 is a common replacement. A little more pressure is needed for the same stopping force but most people prefer the firmer pedal. Use some good pads (PMB sells porterfield pads) and you should be very happy with the brakes. Always change the rubber soft lines if you don’t know or can’t remember the last time they were replaced. They swell internally and reduce braking performance.

Like Bruce said, bias doesn’t change. Braking to the rear doesn’t increase in proportion to the front when you use a bigger MC. That’s not how physics works.

[Superhawk996]

Going to 19mm is going to change things. Brake bias is going to add more braking to the rear. Not good. Pedal throw will be longer but firmer at the end. Ask AI for a detailed answer. I replaced mine with 17mm which are a bit harder to find and more expensive. Just because it's 911 spec, does not mean it's an upgrade.

If you depended on AI for the answers you got wrong info.
Bigger master pushes more fluid and makes a firmer pedal as less travel is needed to move the same volume of fluid.
It does not change the bias. The relative piston sizes front and rear affect bias.
I have done several cars with 19mm masters and I find no issues.
Some like the softer original that has more pedal modulation. Maybe that is what you meant by bias.

We are so doomed by AI (IMG:style_emoticons/default/wacko.gif)

It scrapes the internet for erroneous information and then parrots it back as fact (IMG:style_emoticons/default/screwy.gif)