Occasional Hard Brake Pedal......

Hi. Firstly an introduction. I have been a member for a long time, having owned a 964 which I sold a couple of years ago. I have had the 996 almost a year now and really loving it. It's an early C2 coupe with 80k on the clock. I changed the pads and discs all round about a month ago. The brakes are working fine and I am happy, however is have noted that since doing them under hard acceleration followed by immediate braking and a gear change results in a hard brake and clutch pedal, similar to if the engine wasn't running. i checked the fluid level and it was way above the max mark, following fitting new pads so if have drained off to just below the max level mark and I would say the issue has improved, however it does still happen when I accelerate hard up steep hills then brake at the top. The fluid was changed in February as part of the service. I have done the usual servo (booster) checks (pumping pedal when engine off and pedal getting harder, then starting the car with my foot on the brake and it softens) and it is functioning as I would expect. just gone for a 40min drive around the lanes and it was all fine, got to my usual short hill, accelerated fully in third, got to the top, braked and changed to second and the clutch and brake pedal were hard but still worked then after a split second softened. From various searches I think I am right in saying that the 996 uses manifold pressure for the servo, as do most cars, but the 997 used a separate pump. From what I have described above, is this an idiosyncrasy of the 996? I cannot believe it is, because I cannot find any other post on various forms about this and its only started since I changed the discs and pads. Could it be a loose connection on the abs sensors? The strange thing is the clutch going hard as well. From the parts diagrams it looks like the clutch and brake share the same spindle. Would the power spring cause an issue? As the clutch and brakes use the same fluid there is a link, but surly the power spring assists the clutch, not the servo (booster)? Its really got me confused. Any thoughts greatly appreciated. Thanks

No, my car does not do this, and I'm not familiar with this description. I'm having a hard time imagining what it's like - but I don't want to encourage you to record a video of your brakes acting up for obvious reasons. If you were braking just as you crest the hill, I suppose it's possible that your ABS is firing and you're feeling that, but the ABS pulsing sensation is obvious and it sounds like you know what you're doing - so that seems unlikely. I would suspect the brake booster, but you say this only happened since changing the pads and rotors, so that leaves me puzzled. You mentioned that when this happens, it's hard to push the brakes, similar to when the car is off. By that you mean after you've pumped the brakes a couple of times and there's no vacuum left to assist the braking? If so, it sounds like a vacuum issue - but I'm still not sure how this would be the result of a pad/rotor change.

It's certainly not like a normal abs pedal feel. Best description is the pedal feels like you have pumped it two or three times with the engine off. But it's only for a split second then it right back to normal. You don't even ease off the brakes, just keep pushing and the soften. Even pulled the foot mat out in case it was that getting snagged on the pedal and tried different shoes! But it's still happens.

I think I know your problem: The steering wheel is on the wrong side! ;-) You mentioned that the fluid level was way above the max mark. How did this happen? Did you do the brake job or somebody else? You lowered it and it got better, but has issues going up steep hills. I would try lowering the level to mid-way between high and low and try again. Once the brakes are worn down quite a bit, the reservoir might a little more fluid, but that's not going to be for many miles. I have never heard of a similar issue to yours before.

I suspect you have a leak somewhere in your booster or its vacuum line. There's a brake booster vacuum hose connection at the left intake manifold that leaks sometimes due to the hardened o-ring. Rapid acceleration will reduce vacuum becuase throttle plate is open thus less vacuum in the intake manifold for the brake booster. When you step on the brake, you're off the gas and that allows vacuum to build back up.

That's a couple of votes for vacuum. The description - nailing the brakes after an extended period of full throttle and realizing you have no power assist - sounds terrifying. I do not think I would like my 996 to start exhibiting this "Surprise, no brakes bro!" behavior.

Thanks for the replies, even yours Paul , thought someone would mention the old steering wheel joke! Tonight I have traced the vacuum line form the manifold to booster and it all appears sound. With the engine running I sprayed it with some soapy water, wiggled and jiggled it but could not get any bubbles to appear. I have investigated the power spring on the clutch and that, though the early type appears ok and I have checked for play etc in the clutch and brake peddle assemblies and found nothing. I have drained a little more fluid out, 20ml so it is now sitting about three quarters of the way between low and high. I have not had a chance to drive the car again, so will see what it is like tomorrow. I have also made a booking at my local indie to drive the car on Thursday morning. He is fully booked for work until the 9th may, but least he can spare half an hour for a test drive. I just need to get him to experience the fault though...........

Having said and done all that, it won't bubble will it, because it's a vacuum! Need to rethink that, maybe if I disconnect the manifold end I can pressurise it then repeat with soapy water. The connection at the manifold end is identical to a push fit plumbing fitting, can easily connect an air supply to that.

Mine had similar symptoms. Pretty sure it will wind up being the booster.

Heh. I was wondering where we were going when you said you were watching vacuum lines waiting for bubbles to form. If it makes you feel any better, I'm the idiot who installed a new oil filter on top of the seal for the previous filter - and blew two quarts of high-pressure oil all over my garage as soon as the pressure hit 4+ bars... Good luck with actually testing for leaks. If your compressor has an output pressure regulator, crank it all the way down so you don't run too many PSI through the system. Also, congratulations not being the type of owner who would add, "Have I hurt the car with the drop or two of soapy water I might have introduced?"

Spray carb cleaner around the suspected hose with the vacuum leak with the engine running, if you have a leak, you will hear a change in idle

OMG soapy water everywhere....... I like the carb cleaner idea. Will give that a go tonight. Thanks

I have tried the carb cleaner and had no joy which any changes in engine note or speed. Today I had an appointment with my local independent who service the car. I described the issues to the tech who then drove the car. He had never experienced this exact issue and couldn't detect any fault and felt all was working as it should. The only positive suggestion, was he had experience of early 996 having water collect in the bottom of the booster, and this sloshing around could explain the issues as the problem occurs whilst going up hill. He said that if the battery tray blocks and fills, water can collect around the bellows and run into the servo under curtain conditions. I am going to pop the vacuum line over the weekend and see if I can draw out any water from the servo with a syringe and plastic tube. Not convinced at all myself, but it's worth a go. Will report back. Thanks all.

Yes, your mechanic is correct about the water intrusion problem in the brake booster for MkI 996s. The usual symptoms though is the brake applies itself and keeps dragging so you have to lift the brake pedal UP with your foot. If you can hear water sloshing, definitely worth checking. You can check if you drain holes around the battery and firewall area if they are plugged.

Yep, it's the booster.