[pure&simple]

I understand in theory that a larger master cylinder should require less pedal travel and more pedal pressure vs. a smaller one to achieve the same amount of braking, and am thinking that might help reduce the mushy pedal feel that I've experienced at track days. Does anyone have experience using a larger master cylinder or know of the challenges involved in getting this to work?

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[bob blackwell]

I haven't found a need for a larger master cylinder on "ol blue" either on the track or in daily use. I have replaced my front brakes with the M030 equivalent from the 1987 928S4 and the master cylinder is still adequate to the task. If you have a "mushy pedal" it is most likely air in the brake lines, or the flexible rubber lines deteriorating from moisture and just old age. Try bleeding them first and if that does not cure the problem try replacing all of the rubber lines (there are two up under the body in addition to the ones out at the brake calipers) then bleed them thoroughly again and see if that does not fix your problem. Good luck, Bob Blackwell.

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[pure&simple]

Thanks, Bob! Unfortunately the pedal is soft even when I bleed using the two person "pressure bleeder + foot on the brake pedal" method. The only thing I have not tried is bleeding, going out and braking hard to activate ABS, then bleeding again. I have stainless lines to the calipers, but I didn't know about the two rubber lines up under the body... do you know if stainless replacements for those are available? If not, I suppose new rubber lines would be next best.

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[jfrahm]

Your current MC might be bad, foot bleeding can be hard on them, or anytime the pedal goes to the floor. The piston travels past the comonly used area and grunge is forced through the seals.

You might also have air in your ABS controller.

Sometimes bleeding after holding pressure over time is helpful. Disable the brake lights and wedge the pedal down overnight (I use a couple blocks of wood and the seat.) In the AM, open one bleeder, then close it, then let the pedal up and wedge it down again. Pop another bleeder in the PM, and so on.
The theory is that the bubbles get smaller and move more easily under pressure.

Follow up with a normal bleeding procedure as some bubbles might collect in the calipers while others move up to the MC.

Another trick is to bleed at banjo bolts to find air that is trapped in funny places. Perhaps near the ABS pump. Be ready to clean up the brake fluid.

The Hammer probably can activate the ABS pump also, good for proper bleeding.

Good luck,
-Joel.

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[pure&simple]

Had the MC replaced today with a new OEM part. Pedal is slightly firmer, but could just be due to the brake bleed that was required. I am told it is not possible on our cars to activate the ABS with the Bosch Hammer - apparently this only works on certain 911s. If I notice any improvement at the Driver's Ed event at NJMP 4/29-30 I'll post here.

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[Jens Ove]

Was there air in the brake system, or is the longer pedal travel a result of a larger caliper piston/cylinder upgrade?

I assume that you like to upgrade your brake master cylinder to make your brakes better suitable for track days. If so - have you studied the setup of the true and only race version of 968?

The 968 Turbo RS has a brake master cylinder different fron the other 968 models.

According to Porsche's part list, it uses part number 968.355.011.00. Ordinary 968, CS and Turbo S use 951.355.011.01.

I have no specification for this master cylinder, but I'll guess that it was more suitable for racing than the standard type.

As only 4 Turbo RS was build, I doubt that it will be easy, if possible, to get that part. Maybe Porsche can give some specification for the differ of that type of master cylinder compared to the standard. And maybe someone with experience from driving an original Turbo RS can tell how the pedal is there.



At http://www.968forums.com/topic/7521-fron...s-at-rear/ I've specified the calipers used for original Turbo S and Turbo RS. The caliper piston sizes should match the master cylinder, or you will get wrong pedal travel. It's all about moving a specified volume of brake fluid.

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[Cloud9...68]

Is it possible that what you're feeling is actually flex in your firewall? This is a very common problem in these cars, and it tends to move where the firewall intersects the left inner fender. You might want to have someone press hard on the brake while you're looking at this area for signs of excessive movement. Good luck.

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[AJG]

An extreme measure I'm about to suggest would interest racers mostly I would imagine. Ready? Okay have completely new calipers fabricated that employ a double piston arrangement. By that I mean pistons in line but sealed in separate chambers from one another. This instantly doubles any force applied to the system. Not cheap, but different!



Andy

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[pure&simple]

Thanks for the info!



>> Is it possible that what you're feeling is actually flex in your firewall?

I hope not... I have Flash's firewall brace (version 1).

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