Brake issue, opinion wanted

My 2003 911 C4S is currently at the shop because it has brake fade. The shop recommended that I order a replacement brake master cylinder, which as of today has been installed but unfortunately the brake fading is still there. Their next recommendation is that I replace the ABS part 996 355 955 CX (aka 996 355 955 53, if I'm correct). The shop does suspect that the solenoids may be sticking. This part is pretty pricey so I'd like to make sure it's not anything else before spending the money on buying one or getting it rebuilt (if that's possible?). Also worth mentioning that I am getting a PSM failure. I'm just recently one year into Porsche ownership so forgive me if this is an obvious fix or if it has been answered elsewhere, from what I could tell it doesn't look like it has. Also, there is no bubbles/air in my brake system. Any help or insight would be greatly appreciated, thanks!

Street or track driving? What brake fluid and last change? What pads and rotors are you using? Have your calipers ever been rebuilt?

Just street driving, nothing too intense. Brakes fade under normal driving circumstances. Aftermarket pads I would assume, and the rotors are drilled if I remember correctly. I got the car last year so I'm not sure when the calipers were last rebuilt. Thank you for the reply

When you say fade, what do you mean? Grip at first and then no grip after awhile. A few more details would be helpful.

I can ask the shop on Monday exactly what they mean by fade. Originally the car was just taken to the shop for an oil change and that's when the shop noticed the fade as they were pulling the car in. Based on my experience with motorcycles I would assume that the brake pedal initially gets stiff when pressed then softens as pressing continues. Sorry I don't have as much details, like I said I'm fairly new so I'm still working on what questions need asked. Thanks again

You trust the shop?... Since you personally don't notice anything wrong with it, I would ask them to leave the brake system alone and hand the car over back to you. They're nitwits if they deem something wrong with the brakes but cannot pinpoint what it is. ABS unit?! They don't know how a brake system works and what symptoms are caused by what defect. If anything, then they should check how many brake pedal actuations the vacuum in the brake booster lasts and yields a stiff/hard pedal once it's been depleted. Then check if the vacuum line from behind throttle plate to brake booster is airtight and then check the brake booster if it holds vacuum.

With what you have said so far, and if it was my 996, I would find a new shop asap that specializes in Porsches. In particular, one that knows their way around the M96/M97. Dont know where you are located, but i bet many of us can provide recommendations.

Your shop is fleecing you, brake "fade" doesn't warrant anything to do with the master cylinder or ABS pump. Find a new shop ASAP. If your brake pedal is consistently firm, and you can lock the tires at approximately the same amount of pedal travel each time, that is good enough for a street car.

Dude, google "what is brake fade" and be ready for a surprise

The 996 brakes are pretty damn good straight from the factory. The brake system and its components are extremely durable as well. Sure, after 2 decades of use, you may possibly need to rebuild the calipers. But this will depend quite a bit on usage and the weather conditions you drive in. Easy to check their condition. The brake pads you use also have a big influence on grip, stopping power and distance. More aggressive pads create more dust, but will stop you sooner regardless of the rotors. You will probably notice that if you wash the car or after a rainstorm and the car sits for a few days, you will see surface rust on the rotors which is normal and no big deal. It will wear off after you use the brakes a few times and may mute the brake response before it goes away. Old brake fluid doesnt help either. Especially if it has developed moisture in it. When it gets hot, such as on the track, the brakes heat up and you get air bubbles. The brakes get spongey and less responsive. Its also known as boiling the brakes. So its good to run fresh brake fluid that is high quality. I have a 996.2 without PSM. But i understand guys with PSM have problems when running the wrong diameter tires, as well as abs issues under heavy braking. They also have issues with running a 997 master cylinder. But someone else will have to explain what they have experienced since I dont have PSM. Refreshing brake lines are also something that will eventually need to be done. Be good to find a Porsche Indy with experience that knows the car.

I suspect we have some communications or interpretation issues here. The shop noticed the issue as they were moving the car on their lot. My guess is that the brake pedal "fades" to the floor when pressed. This would indicate a master cylinder leak in most cases. Reading codes might have uncovered the ABS unit being bad though, so the shop still has some splainin' to do. I've used Module Masters for ABS module repair. Very satisified.

Curious as to the current mileage..and the shop has had your car since 5/8/25 for brake fade ?

Brake fade while pulling car around in lot? Not likely. if as you said later it's just that they thought the pedal was traveling too far I would think new pads and a good flush is all that's warranted.

Hey C606….sorry you are going through this ordeal which happens often when entering the Pcar world but only have experience with general mechanics; so don’t feel bad. They told you they felt brake fade so my first thought is had you experienced any difference in your brakes before taking it to them? You just trusted that they said you needed to swap out the master and it sounds like there was not a real explanation of why. No report of codes etc. I just changed a brake booster out on my 04 c4S that started with aP0507 dtc and the leak was hop scotched down to being the booster. This job required removing everything in front of it including the master cylinder…full drain of fluid and “install in reverse “ as usually the case. When this job is over after all the bleeding then codes have to be erased or you get PSM/ABS light…you ge5 fault code# that indicate a pressure sensor failurep4380( may not be correct) and also get given the 99635575559 part number that appear# to be part of the $5000 ABS PUMP. I couldn’t clear the codes with my Durametric at first and got the ABS/PSM warning lights and my clutch locked up…car locked up…undrivable …The second time I reset the computer by touching battery leads together. Afte4 that I was able to clear the faults for ABS PRESSURE SERSOR. I was able to drive the car after this …however Im still getting ABS/PSM warning but car hasn’t locked up. Im going to acces the brake pedal switch.. maybe exchange it and see if that will help clear the code. If not I may seek a Porsche scan tool and clear the codes with it and see if it makes a difference. im saying all this to say that when doing the master cylinder replacement(which still hasn’t been proven to be needed and they learned on your car… this shop did not return the computer back to correct settings. Maybe that is your issue …if it is indeed ABS hardware there are 2 sensor in the ABS PUMP that could be in need of a check. I,m still trying to determine if my pump is bad cause like yo7 say…an expensive part. Good luck

As long as C606 can/does not explain (no judgement towards you!) what the shop means, when they state 'brake fade', we're all just speculating and probably cause more confusion than enlightenment, I'm afraid.

Hey all just posting an update, thanks so much for all the input and opinions all of them have been great to hear, and have been helping educate me more. I agree that it would be best if I go ahead and get it out of the current shop it's at. Like @hardtailer and @GC996 mentioned the shop may not know what is going on (if anything at all) even though they do specialize in German vehicles. My plan is to get it out of the shop Monday and see what the brake feels like. If there is an issue, I'll be sure to post up an update with more descriptive symptoms, and in the hopeful case that nothing is wrong I'll update on that as well. Thanks again!

Got in the car and the pedal was super squishy and needed to be pumped up. I limped it to the street pumping the brakes and parked it so I can have it hauled on my car hauler either home or some other shop in northern Utah. The mechanic of the shop said the brakes were leaking "internally" and that's why there is not any fluid leaking anywhere on the ground. If anyone has opinions or ideas on what to do next I'm all ears. Thanks!

Yea leaking internaly is not a thing and not how brakes work. I would put $100 on that there is air on the abs system and the unit needs ro be cycled with a piwis or durametric. A simple pressure blead may also do it with a bunch of hard stops and abs acuation on a gravel road. If there r no leaks its air in the system.

Quotes source post: 20169939

+1 this

Quotes source post: 20169785

Master cylinders effectively can leak internally when the seals go. They can bleed off the pressure behind the bellow on the shaft.

Quotes source post: 20170094

Yea but they replaced the mc and still no joy.

Do exactly what DJ recommended. If I were you,,, bring it to a qualified shop that knows how to bleed brakes. A correct pressure bleed, maybe including abs, should take care of your problems more than likely.

Quotes source post: 20170189

They might have put a bad MC in? or the wrong one? But yes get it to a decent shop, brakes are sort of important lol

Quotes source post: 20169939

I'll try tracking down one of those tools to get air out of the abs, thank you and everyone else for the comments, I'll keep the thread updated as I figure things out.

Quotes source post: 20169939

Hooked up the tool, and did a few bleeds with it and it still is the same unfortunately. There was a few instances when the tool was hooked up that the pedal would go stiff but not consistently. Not getting any error codes either. Very strange.

U wont get an error for just air in the system. U may have to pressure blead all the lines again. Activating the abs with a scanner will force the air out of the modual but u still need to get it out of the lines and calipers.

I'd have to agree that you should do a full, thorough bleed of the brake lines and calipers. I had "fade" meaning the pedal would slowly move towards the floor. It had air in the lines. simple fix.

Figured it out. I think the shop didn't bleed the inner bleeders on the calipers. When I bled them lots of air came out and now it's working great. Thank you everyone for the help!

You have proven what a bunch of ignorant nitwits they are in that shop you mentioned in your first post. Their shoddy work is a danger to their customers and to the general public in the streets. Glad you found and fixed it!