Isaacsracing

Hi 968 racers and experts,

In my 968, the brake pedal is long and mushy. I don't mind the pedal in my street car, but the brake pedal in the race car goes so deep that heel and toe is a challenge. I'd like a shorter brake pedal with as few changes as possible. I've spoken with the engineers at Wilwood and have done a lot of reading. I'm waiting for an email response from ATE classic division. I'd like to finish the car and this is one of the last important pieces to figure out.
Here are my thoughts. Please let me know if any of this makes sense or not.

My calipers are M030 front and rear, but the master is the stock master cylinder. That means that the pistons in the calipers are bigger than stock and the piston in the master cylinder is unchanged, therefore the pedal required more travel to push the large piston in the caliper. Just for information, I'm using steel braided brake lines from Lindsey racing, PF11 pads, and the original vacuum booster.

M030 Pistons sizes that are in the car are:
Fronts 44mm and 36mm - four pistons at each wheel. Total area: 5076 sqmm
Rears 30mm and 28mm pistons - also four pistons at each wheel Total area 2645 sqmm

I can't find the stock piston sizes right now, but I think the stock sizes are:
Fronts 40mm and 36mm - four pistons at each wheel. Total area: 4549 sqmm
Rears (same I think) 30mm and 30mm pistons - also four pistons at each wheel Total area 2645 sqmm

If the above is correct, the M030 Fronts have 11.6% more area and the rears have the same area. That front difference accounts for the longer pedal.

The brake master cylinder has a stepped cylinder size to create braking bias. A 23.81mm bored piston for the line to the front brakes and a 20.64mm piston for the rear brakes. Focusing on the front only, that's 445 sqmm of piston area.

Here's my idea. I'm thinking about getting a 1995 BMW M3 master cylinder. It is also stepped with a 25.4mm piston for the front brakes and a 20.64mm piston for the rear brakes. The piston for the front here has an area of 506 sqmm. That is an increase in the area of 13.9%

So in summary:
I have calipers with piston area larger than stock by 11.6% and I would be putting in a master cylinder that increases pushing area by 13.8% from the stock master cylinder. I think that will both shorten the brake pedal and provide a firm feel.

Am I thinking about this the right way? I'm praying that the BMW master will bolt in, but I don't have a way of knowing until I order one. I do know that the Porsche lines from the master both use M10x1 and the BMW has one M10x1 and one M12x1, so I will need an adaptor for that.

What problems am I not recognizing? I'm wondering why I can't find people who have done this online.

chudson

I don't have time to elaborate right now but we can talk on the phone later.

First, there is no M030 rear caliper - it simply doesn't exist. All the 968 use the same rear caliper. Also, the factory engineered the system to accept the larger front calipers with no noticeable change in master cylinder pedal stroke or feel. 2 of my cars have the 030 front calipers as does a friends car that I both setup and occasionally drive (track car). All of these brake pedals are rock hard / very little movement and the only change that I made to accommodate heel/toe was to take the majority of the play out of the master cylinder adjustment rod (which raises the pedal) and to lower the accelerator pedal a bit. Are there any cracks and/or significant flex in your firewall? Again, I'll elaborate when we talk.

Cliff

Isaacsracing

Hi Cliff,
Glad you chimed in. thank you. I haven't had the booster out to see the firewall, but I can certainly dig into that and also adjust the rod. I think Linsey or soeone sells a firewall panel to stiffen it up. I can start there.
I had thought that about rear calipers that the M030 package came with the exact same calipers as the front.

Jay Wellwood

Pretty good read here on the construction of the brake booster and how it works.

https://rennlist.com/forums/924-931-...-teardown.html

KEVIN ANDERSON

That is a very good post Jay. Something I never even thought about and I sell the 951, 944S2, 968 993 Turbo Big Brake Kits. You would think that if the braking system was topped off with fresh fluid, no bubbles or anything, that any pressure on the brake pedal would move the pistons because it is completely topped off and even if the pistons have a larger diameter, they are still backed up with nothing but brake fluid so they would still move with a nice, short, firm pedal travel. I'm just speculating here. Maybe your master cylinder is not operating at 100% or there is a slight leak in the brake booster system and pressure is not making it fully to the pistons. I'm just as curious as you are. Thank you! Kevin Anderson

Isaacsracing

I agree that if the system is not leaking that fluid should move fluid, but the engineers at Wilwood have explained that the ratio of the surface area of the master cylinder to the sum of the surface areas of the caliper pistons create the amount of pedal motion, pressure required and feel of the pedal. As an example, given the caliper pistons are a fixed item, a larger diameter master will require a shorter stroke to move the same amount fluid, thus giving a shorter pedal that requires more force and results in less feel. If you go too big with the master cylinder, the extreme case is it becomes an on/off switch and may require serious leg strength to create real friction.
My next move (after I align and corner balance, which is the current project) will be to remove the booster, check the firewall and try to adjust the pedal. The current master is a year old and I don't believe that any lines are leaking. As I don't mind the general feel of the pedal, if I can move the pressure area of the pedal stroke up, I'll be happy. If not, I'm going to try a master that is one size larger.
Ate classic sent back an email with this to offer. They are pretty tight with their words (maybe they speak little English?). This master that they are proposing is from the 1990's and was used in the Mercedes S class, but only seems to be available outside the US right now. I'm still thinking the BMW M3 1995 master. Maybe I won't need that route at all.

thomasmryan

I have a 95 and 96 S320 if you need exterior dimensions of the master cylinder. the local MB dealer gets odd things for me all the time.... wholesale.

Charlie

Your front brakes are the same size as the 944 Turbo S and 89 Turbo, what master cylinder do these cars have? To see is your firewall is cracked just watch the firewall and have someone press the brake and clutch pedal. If it is cracked you will see it move. I have had plenty of experience with cracked 968 firewalls, both of my 968s had this problem.

Turbodan

how about increase the thickness of the pedal by adding a plate screwed into it?

Eric_k

I think you may have some other issue going on as Porsche used the same master cylinder for brakes this size on the 944 TurboS and 968 MO30.

That said, this is the BMW master cylinder you want to look at:
BMW 750il brake master cylinder part number 34 31 1 156 643

It worked great for me with Big Reds on the front. The aft brake line mounts on the outer side of the master cylinder, but other than that it bolts right on.
I have since gone to a Tilton Pedal box and even bigger brakes.

Isaacsracing

Thank you everyone. That's all good information. Keep it coming. I'll start work figuring out what's up in a few days.
Is there any possibility that the ABS system is somehow interfering. I really don't know how that box in the fender works and I'm not sure if the ABS in this car works at all as it has never engaged while I've driven the car. There is a procedure to check the ABS in the shop manual, but I haven't gone down that road yet.