Hi all,

I've got a brake issue and am hoping some from the community may have a solution.

I have a standard '07 C2 car. I have previously bled my brake fluid before and ran Motul RBF660 for two track days. The brakes were fine, distances were short and no fade and ABS worked, but after event #2 I was searching for a little more pedal modulation - and something that would stay a little stiffer longer.

So I replaced the lines and master brake cylinder with a 997.1 GT3 unit based on feedback I've read here and other forums about the positive experience with the unit. The install went fine. I used a flare nut wrench for the lines and of course sprayed my eyes with a little brake fluid as I pried that master cylinder off (hot hint; wear safety glasses!). I re-bled the brakes using Autozone Dot 4 fluid with a helper inside the car and a motive power bleeder canister. I drove around the block with a spongy pedal and went back a couple of days later and re-bled as usual. I got what I believe to be most/all of the air bubbles out and my pedal feels like it used to.

So tonight I go for a drive, cautious of the brakes, they are working fine at regular speeds and feel normal up until I run some emergency braking around 40-50 mph. They are hitting a wall where no matter how much force I apply to the pedal the car won't increase brake pressure and it won't kick in the ABS. It's like I'm only getting up to 60% braking threshold then the pedal is rock stiff and stays at 60% until I come to a complete stop. I hear a vacuum back pressure/sucking sound as I let off the brakes but I can't recall if that is normal or not.

Ideas?

I'm wondering if there's a vacuum issue, more air in the lines, a booster/master cylinder mismatch, ABS PIWIS engagement procedure, etc...

I'll be putting the car up on jack stands tomorrow and doing another bleed.

 

Brake systems are designed so that the orifice sizes in the master cylinder and number and diameter of the caliper pistons complement each other. The GT3 has much larger pistons in the calipers and also 6 piston calipers that are designed to work with the GT3 master cylinder. If the brake pedal feels firm when you go to use it that means you don't have any air left in the system and I believe by moving to the GT3 master cylinder and keeping the stock, non-GT3 brake calipers, you've created a mismatch in the braking system and why you have poor braking performance. You probably need to go back to the original master cylinder or to GT3 Calipers and rotors.

 

Couple thoughts.....

1. Did you bleed the booster/master cylinder after you bled all 4 corners?
2. If your car is a manual, did you bleed the clutch?
3. Why use Autozone fluil when you had Motul 660 in there? The Motul is far superior.
4. With a Motive unit you don't need a helper.
5. Are you sure you didn't run the brake fluid resoviour dry when bleeding?
6. What order did you bleed in? RR, LR. RF and LF?
7. And Pete above has a very good point.

You have air in there somewhere or you do have a mis-match.

 

rock hard pedal and you hear vacuum leak........go over your work around the booster imho

 

If the fluid reservoir ran dry, you'll need a Durametric to finish the job as you have to go into the ABS pump in the actuations menu to cycle the pump while bleeding each of the 4 wheels with a Motive pressure bleeder. I made that mistake once when trying to use the Motive in Dry bleed mode, and it took me a couple hours and 3-4 more bottles of fluid to get the brakes back. Now every spring, I put 4-5 bottles of fluid directly into the Motive and do all 5 P-cars one right after the other as I have all of them running Motul RBF600 now.

 

If this is the case, this would be a bone head move on my part!

1. No, I'm not familiar with this procedure - please describe it!
2. I have not.
3. I have two bottles of Motul RBF600 ready to go once the system is working correctly. The Autozone fluid is placeholder for now.
4. I read in the PelicanParts book it helps to depress the brake pedal (held) until the valve is tightened with the Motive - I noticed more air come out when done that way vs Motive alone.
5. Positive - the initial swap I put in a little bit of fluid into the two chambers then ran the Motive wet with 1/2 gallon of the autozone fluid. Once saturated albeit with it air, I let it sit for a couple of days and then ran the Motive dry filling the reservoir as needed to prevent starvation.
6. Yessir!
7.

I tugged on fittings last night but will go into further detail today. I'll look up the torque values again and re-affirm everything is buttoned as it should be.

Durametric Pro or Enthusiast? From what I read of others people experience with the GT3 M/C I don't recall them having to cycle the ABS system in the bleed but I suppose it wouldn't hurt to get that fluid out of there, and the durametric is a useful tool to have anyways...

Thanks all who've commented so far!

 

I meant the ABS pump. Once you activate it you use the bleeder screws on the RF to flush it. I have the enthusiast Durametric and it has that function.

 

Because racecar . . . that's what happens when we try to improve on perfection!

 

Solid - thanks for the tip, I didn't know the RF valve was the one to use. Ordering the cable tonight and will cycle ABS next week. Although the shop manual suggests it need only be cycled if I'm replacing the 'hydraulic unit' which looks to be the ABS module in the diagrams. Either way, I think it's a good idea to get the fluid out and replace.

I'm going to do the clutch bleed today and another bleed run through each caliper to see if there's any more air in the system + check the fittings into the booster for leaks and re-check torque values (17 ft lb booster and ~19/36 ft lb m/c). I've got a buddy coming by so he can cycle the pedal(s) and hold it down while I torque them shut.

I see there is a vac line that run from the engine to the booster, maybe something is unplugged or incorrect causing a leak which isn't giving me full depression? I'll see what I can find from the PET/schematics that show. I wish a clean service manual or Bentley manual were available for our car!

Shot in the dark here but figure it's worthwhile mentioning; I've got my headunit out of the car and on my workbench as I install a Tranzit Blu dongle. I'm going to take a look and make sure nothing has been damaged but I see the lateral-acceleration sensor/rotation-rate sensor is located behind the console... If it was damaged in any way, could this have an effect on the ABS module not working? And on that note; would any damage to speed sensors cause the unit to "fail safe" and not function properly? I imagine I'd get some sort of dash light if those were the case though...

I had disconnected my battery before performing work, on start up after work completed I got PSM failure notification but it went away within 50 feet of driving. Figured this was standard operating procedure.

 

Hah, hardly! There's always room for improvement. Look at the race cars! They're constantly getting better.

 

​​​​​​Oh? You planning to run LeMans next year?! 😁

 

Depends, I might run F1 next year

 

Problem solved.

I hope this can be of some use for people searching for: stiff brake pedal problems no-abs can't lock up brake pedal issue etc.

So I swapped the master cylinder to a GT3 unit, brake lines to SS units and new brake fluid. I had a stiff brake pedal and wasn't getting full travel. I rebled brakes, no luck. I re-torqued all nuts and bolts per manual, still nothing. I checked the brake pedal under the dash, looks normal. With the car back up on rear jack stands I decided to swap out the motor mounts*, too.

Last night I bled my rear brakes again and then did the clutch after the driver side rear, had pops manipulate the clutch pedal - wow that fluid was ripe and ready to come out! I cycled another 1L of fluid through the system using the Porsche guidelines; 60 seconds, 90 seconds, etc. For anyone wondering it's an 11 mm nut on the clutch bleeder valve. It's a very tight space in there; I ended up cutting my wrench in half to get it to fit comfortably.

Went out for a drive - yikes, it was 10x worse than before! I could barely press the brake pedal whatsoever; backed out of the driveway and went right back into the garage. But the clutch felt great. Then it hits me, the Aha! moment right before I fall asleep; I've had trouble with a vac. line cracking before in the engine compartment* maybe my motor mount swap (air box out/in) changed something. That would explain why I'm getting no booster assistance and why the pedal would be so stiff. So I check it today during lunch and sure enough, it's off entirely. Wow wow wow, yes! This is why I love wrenching on the car, it's a great big puzzle! I slip it back on with a bit of tape around the cracked ends for a temporary hold and I go for a drive around the neighborhood; perfect brakes. ABS engages, they have great modulation and stop the car predictably every time. Then I went to the nearby empty church parking lot and had a few goes at it; works great and I can only imagine it'll feel great coming down from higher speeds. Woo hoo!

So there it is, a stiff brake pedal could be a disconnect or crack from the vac line in the engine bay behind the air intake snorkel. I was so obsessed with the front trunk space booster unit, master cylinder and ABS module I blanked on an obvious potential problem source. It's a formed plastic hose piece (extends from a metal/aluminum pipe) behind the air intake snorkel towards the top of the engine compartment to the right of the coolant overflow tank. It connects to a 2/3 hose coupler. When I replaced my coolant overflow tank I saw this thing was starting to crack towards the sleeve end. I looked up this piece a while back and it's sold as a unit (3 pieces) for around $200 from Porsche. I may have even asked here about it, was told it's for the brake booster and a common break item. I'm going to explore some options to maintain it better or just order the whole piece all together next week. I suspect I may have moved something with a enough force (snorkel, air box, throttle body, etc.) that may have shifted the hose partially off the coupler after my first go which was causing the pre-mature stiffness I originally posted about. Or maybe a crack grew enough to not allow for a full vac seal.

As for the GT3 master cylinder 'upgrade' - I think my perception on the whole braking experience now is a bit tarnished because of the riddle, so take it with a pound of salt, but I believe it becomes a slightly stiffer pedal to modulate sooner now than it was before. Could be just in my head but here's how I perceive the differences:

Stock: press on brake pedal; by ~70% depression pedal stiffens to a perceived threshold (solid pedal) that can be modulated with more precision for braking force application.

GT3 MC: press on brake pedal; by ~40% depression pedal stiffens to a perceived threshold (solid pedal) that can be modulated with more precision for braking force application.

I did do the SS brake lines simultaneously but I don't believe they'd contribute a significant difference in brake feel for cold stops or in the way I think it does. They should help at higher heat points with repeated stops, though. At least in theory...

Would that make sense from a common-sense stand point? Assuming the GT3 master cylinder has a little more capacity in the chamber or if the diaphragm takes a little more volume to push, then sure - by keeping the same booster application in place; the ratio of force/fluid compression would be slightly changed. Just like when there is no booster and all fluid - it's rock solid hard. So pushing that ratio to slightly more fluid than air should stiffen the pedal a bit, and more sooner. Makes sense in my head at least.

 

Awesome, glad you got to the bottom of it and thanks for sharing the fix.

SS lines definitely help pedal firmness as all of the fluid force you're creating with the pedal has nowhere to do but to the caliper piston since the lines can't expand out. Rubber lines can expand out a little under the internal pressure and that's what can give the less solid pedal feel. I change the lines in all my cars to braided stainless when I buy them for this purpose. The other advantage is that they're teflon lined which doesn't react with the brake fluid and cause the line to swell. Rubber lines actually swell internally over time, narrowing the passageway that the fluid passes through, reducing braking effectiveness.

I created a great clutch slave bleeding rig using an 11mm pass through socket, silicone hose and collecting bag. Zero mess and the ratchet gives you the ability to loosen and then tighten the bleed screw without ever removing it from the fitting.

 

Thanks Pete and thanks again for the suggestions.

That's a sweet socket setup, definitely saves some headache and knuckache.