DC021

Hey everyone.

We’ve lived with our new to us 981 Base for a bit now and have spent some time making it “right for us” - to include the slightly upsized front calipers and rotors from an 981S. One thing we haven’t been able to achieve is proper brake pedal feel. I’ve bled the ABS unit twice and ensured there are no trapped air bubbles in the lines.

Don’t get me wrong, it’s probably perfectly fine for most, but it still feels over-boosted and lacks modulation to me.

I think the next step would be retrofitting an m/c out of the 991 GT3 / 981 GT4. Before I go down that route, is there anything else I should try? Is there a specific p/n that’s proven to work well?

DS2500 Pads - SRF Fluid - Goodridge Lines.

Thanks!

slivel

Perhaps I misunderstood your description of the problem, but "over-boosted and lacks modulation" means to me a hard pedal. By going to a M/C with more swept volume you would likely be exacerbating the problem as I understand it. In my track only 914 with Carerra brakes and a 19mm M/C the brakes were fine with new pads but with a little wear the pedal was long to engage and was soft. When I upgraded to Boxster brakes the 19mm M/C wasn't going to cut it. Ideally a 21mm would have been perfect but a Mercedes 23mm M/C was the only fitment that I could find for my car. The pedal was firm, engagement was predictably at the top of the stroke even with 50% pads and I always felt confident in my brakes. The cost was less modulation - not an on/off switch but not quite the perfect solution. I leaned to live with it. Good luck in your quest for a solution, but it sounds like something else going on here that a larger M/C won't fix.

okie981

After I added the larger Girodisc front/rear rotors to my 981 for track use, I noticed the same thing with the brakes being over-boosted and losing modulation feel. I've converted the car back to street life now with OEM rotor sizes and Sebro rotors. If I had it to do all over again I would have left the OEM sized rotors on and never installed the larger diameter Girodisc rotors. Even at Cota with the 3 big braking zones on really hot days I never experienced any brake fade with the OEM size rotors in a 25 minute HPDE. Racing might be a different story, but I have no experience with that. For reference, my top speed at entry of braking into T12 would be ~142 +/-1, per Vbox HD2 data. And I used Ferodo DS1.11 pads and Endless brake fluid from day 1 on my car for track use.

I've seen this product and heard good things about it from the ND Miata track rat crowd:

Variable Brake Booster Controller – MiataSpeed

Chrismalberts

I have a 2015 Base and I wouldn't describe the pedal on my car as overboosted or lacking modulation. In fact, compared to our other car (a 2019 GTI) the pedal feels very dead and with a lot of travel. I would add that I had front pads/rotors done at the local dealer perhaps 10k miles ago and the 40k service including brake fluid flush not that long before that. I don't mind the pedal being progressive but you do have to stand on the brakes to get action - they are not grabby at all.

I don't think your brakes should feel the way they do and shouldn't need an upgrade, certainly not for street use.

Just my 2c.

DC021

I guess what I’m feeling is that the brake pedal is too soft - requiring too little pedal force to actuate. There’s no issues with actual rate of deceleration with the braking system, it’s simply a feel thing for me.

Our legs can better differentiate the difference between 40lb and 60lb of pedal force, versus 20lb and 30lb, for example.


I’ll go ahead and order the 991 gt3 master cylinder as others have reported positive results from the swap.

Thanks all!

slivel

Guess I did misunderstand the description of the problem and if the pedal is soft and especially if the engagement is low on the travel, then I think you will benefit from the M/C upgrade.

okie981

My post was only with respect track use where the brakes are applied to threshold braking, just prior to ABS kicking in, and being able to modulate it around that point as the car slows and weight transfers during trail braking. For street driving, my brakes felt fine before and after the GiroDisc upsized rotors.

nb1234

I'd first suggest a simpler and cheaper option - consider changing pads to a different compound.

Overly grabby brakes (due to too pads with too high friction coefficient under initial "bite") with poor modulation on the street is actually one of my pet peeves. Some folks actually like this as they associate it with having "powerful" brakes. I hate it. I much prefer a firmer brake pedal with a wide modulation zone, and should definitely not be grabby when lightly applying brakes in street driving.

Assuming you're talking about a street application here, I'd suggest finding a set of ceramic compound pads. Ceramics tend to have a more gradual initial bite, good firm pedal feel and have the bonus of less dust. (Note, I wouldn't recommend ceramic pads for a track application. I use ceramic on the street, only, and swap to a track-only compound for my track days.)

DC021

Good suggestion, but the system feels incorrectly sized / overboosted on track also. Again, no knock against the Porsche engineers, the system was clearly tailored towards your average driver produced for the mass market, and perhaps I’ve grown too accustomed to my unboosted braking setups in race cars and the more appropriately sized GT4/GT3 boosted systems.

Parts were ~ $380 shipped, will report back when it’s in!

TEF

Not sure I would consider the addition of Girodisc rotors a disadvantage. In fact, the larger rotors should offer a mechanical advantage over the smaller OE rotors and their larger mass would absorb more heat during sustained braking. However, if one is using semi metallic pads, braking might feel abnormally grabby. As nb1234 suggests, peddle feel can be more a function of pads than rotor size. My BS has Girodiscs and ceramic pads on all four corners and braking is very linear. Since I don't track my car, I prefer ceramic pads because they produce little dust and are much easier on rotors (particularly important with replacement Girodiscs rotor rings costing about $800 a pair).

okie981

If you track your car and have OEM size rotors on it first then upsize to Girodisc, using the exact same pads before and after, you will notice what I referred to in my first post in this thread. The additional braking force gained by the larger diameter with a given pad friction force pushes the feel of the brakes to an over-boosted condition vs. same or under-boosted. It's more difficult to modulate the brakes at the threshold of tire grip and ABS activation, i.e. maximum stopping force. For street use, you typically only get near threshold braking in an emergency braking scenario, so there's no difference. The GiroDisc rotors look nice, fill up the wheel openings better compared to OEM, but the OEM size rotors have all the braking power you will ever need for HPDE events assuming you are using good pads and fluid.

Paul in San Jose

I absolutely agree with you - I have a 2015 Cayman S and a 2015 GT3, and the feel of the brakes on the former continues to annoy me, I haven't gotten used to it. My 987 was much better in this respect. I'm anxiously awaiting the results of your experiment! What part did you order, and did you get just the new master or also a new booster?

DC021

Thank you! I thought I was just being a weirdo with this one.

I ordered p/n 99135513081 and it should arrive this week!

DC021

Got a PM from Paul to update this thread.

The M/C is in and the news is good. The pedal effort has increased quite noticeably, resulting in more modulation, exactly what I am after!

Installation was straight forward, I bench bled the M/C the best I could and then installed it onto the vehicle, after which, I ran thru 2 ABS Bleed cycles before one final bleed with a pressure bleeder.

Squeaky

reviving this thread a bit. I’ve read the 981 Caymans have used 2 different MC designs. Depending on manufacturing date, you will need one or the other.

just wondering what vintage of cayman the part number you quoted will work with. Thanks!