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Old 08-22-2013, 05:48 PM   #1
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Brake master cylinder sizes

So I ran my first event in my Boxster last weekend. It rained on Saturday so not much dry track time. On Sunday I finally got a dry track. Everything went good. But there is one issue I might not be able to over come and this is going to sound crazy. The brakes work to good. I can't run the ABS in the class I run in SCCA. So braking is extremely hard to do without locking the brakes up. I am running Hawk DTC 70's in the front and HT-14's in the rear. I am use to having crappy brakes from when I raced Honda's and 914's. I really need a hard pedal that takes a fair amount of force to slow the car down. This fits my driving style. Now I am having to brake for a much longer time then I like to. I have read about guys swapping to a larger master cylinder on their 996's. I tried finding a list of sizes but came up short. Does anyone know of a large MC to make the pedal harder or have a better idea?
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Old 08-22-2013, 08:10 PM   #2
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A larger master cylinder will indeed make your pedal harder , but will also decrease the travel of it .
Have a look at the 997 gt3 one ... 997 355 910 30
Try searching on rennlist for posts by bill verberg , he ALLWAYS used to post all kinds of info on interchangeable brake components and gear ratios etc.
I'd look for you , but I have been banned many years
Maybe Have a word with porterfield etc as pads will also make a difference .

Last edited by Ian c; 08-22-2013 at 08:17 PM.
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Old 08-24-2013, 03:54 AM   #3
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Thanks for the info. I think the GT3 one might be too big. Looks like my car has a 23.8. Cars with PCCB have a 25.4. I think that is what I am going to try for now.
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Old 08-25-2013, 05:43 AM   #4
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The 997 gt3 is 27 iirc
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Old 08-25-2013, 09:36 AM   #5
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How about disconnecting the brake booster???
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Old 08-25-2013, 10:11 AM   #6
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Thought about that as well. I have not tried it yet but I have on other cars. Sometimes it becomes to hard.
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Old 08-25-2013, 03:20 PM   #7
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Got to be worth a try.
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Old 08-25-2013, 03:26 PM   #8
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I will try it.
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Old 08-26-2013, 05:04 PM   #9
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I tried running my 986S one time with ABS disabled, and found the same thing. From what I have found it seems that most manufacturers set up their cars with too much front brake bias, and let the ABS take care of it.

I thought about getting a brake proportioning "prop" valve to work on the front pressure supply line, but I never got around to trying it. You can buy them from Pegasus Racing, or probably lots of other places.

Please let us know if it works - I would love to get rid of the ABS.
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Old 08-26-2013, 06:54 PM   #10
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I have thought about removing the whole ABS unit. But I am afraid it might really mess things up. Might try it though. It looks like the the front brake circuit has a prop valve on it already.
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Old 08-26-2013, 10:36 PM   #11
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Quote:
Originally Posted by bam914 View Post
I have thought about removing the whole ABS unit. But I am afraid it might really mess things up. Might try it though. It looks like the the front brake circuit has a prop valve on it already.
I have a friend with a Boxster S that he AXs that uses no ABS unit and has instead a manual proportioning valve. If you like I'll get the details of what he has done.

I know from experience you can't just turn off the ABS and expect the rest of the system to work halfway decently - it doesn't. The brakes in this circumstance have way to little rear brake bias.
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Old 08-27-2013, 07:42 AM   #12
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That would be great if he doesn't mind sharing the info.
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Old 08-27-2013, 05:24 PM   #13
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Quote:
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That would be great if he doesn't mind sharing the info.
I'll second that - please fill us in.
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Old 08-30-2013, 03:58 AM   #14
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Lawrence told me about this thread yesterday, my setup It's nothing fancy. Just disconnected the whole abs and ran a proportioning valve to the front calipers to limit the work that they are doing hence change the brake bias.You can play with this set up till you get it to brake to your liking, in the end I would like to run a dual master cylinder set up but for now it works fine.
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Old 08-30-2013, 09:20 AM   #15
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A prop. valve to the Front sounds like a very bad idea. What if you have some brake issues and need to really press on the pedal, you won't have any extra line pressure available. The "proper" way is with a balance bar(dual masters), or replumb the system without the ABS, with a prop. valve on the REAR brakes. You could also try mixing brake pad compounds front-to-rear.

Last edited by stephen wilson; 08-30-2013 at 09:23 AM.
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Old 08-30-2013, 03:43 PM   #16
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Thanks for the info. Rebuilding the calipers this weekend so I will take the ABS unit out at the same time.

The front brakes already have a prop valve in them. See part #6. It is in the front brake line before the ABS pump.
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Old 08-30-2013, 04:57 PM   #17
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From that diagram you can't tell if it's in the F or R brake line. That's irrelevant though, since it's used in conjunction with the ABS system, once you remove the ABS from the equation, it's a whole different ball game.

I've never heard of reducing front brake pressure ( via a prop. valve) to adjust brake balance. You always want full pressure to the front, and control balance by reducing rear line pressure. To adjust overall brake effort, you can reduce or eliminate boost, change master or caliper piston sizes, or change the mechanical pedal ratio.
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Old 08-30-2013, 05:08 PM   #18
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"A" is the front brake line.

I will see what happens.
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Old 08-30-2013, 05:26 PM   #19
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That makes sense, and places the factory prop valve in the rear (B) line. You could just replace the factory one with an adjustable unit.
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Old 08-30-2013, 05:55 PM   #20
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The diagram is wrong. The prop valve is on the rear. The next page it tells where each one goes on the ABS unit.
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