brake hydraulics problem?

This is on a 99 C2 with 90K miles. I only drive the car for weekend autox, no daily use. A few weeks ago the "brake wear" warning light came on, but would go away if I turn off the ignition and restart, but the light would come back on after a dozen miles or so. Brake fluid reservoir is full, front pads have about 7-8mm left, rear pads have about 4-5mm left. So I ordered all new pads as well as rotors since the rotors looked visibly worn. This past weekend, after a 60mile drive on the highway, the brakes were dragging after a stop, I stalled the car a few times as a result. But the car tracked straight so both sides must be dragging the same amount. I noticed the driver front and passenger rear wheels seemed hotter than normal. The dragging was intermittent and I autoxed the whole day with the drag coming and going. When the drag happens, the brake pedal felt much harder, as if I already pressed the brakes. When I got home I pulled the driver front caliper off and noticed that it was extremely difficult to retract the pistons, as if the brake fluid cannot flow back to the master cylinder. Pushing in one piston caused the neighbouring piston to come out, but the pistons on the other side of the calipers didn't move. Is this normal? Last night I pulled the right front caliper out and it had the same problem. The only way I could retract the pistons was to open the bleeder valves. The fluid came out nice and clean, I don't know how old it was but certainly older than 2 yrs since that's how long I've had the car. After putting in new pads and rotors in the front, and make sure the wheels were rotating freely (piston retracted), I pumped the brake paddle a few times, and both front wheels have significant drag, I could rotate them by hand but they certainly wouldn't spin freely. I haven't touched the rear brakes yet, will do so this weekend. Question: What is the likely problem?

Time for a caliper rebuild with new seals and pistons. The pistons are probably corroded in the calipers and not retracting.

I was hoping for something simple like that (piston rebuild). But when I loosen the bleeder nipples, the pistons can retract very easily, that leads me to think the problem is not the calipers but upstream, may be the ABS or the master cylinder. I need a way to diagnose the problem.

It could be the master cylinder, but I don't know a way to test it without removal and operating the master cylinder piston to see if it springs back smoothly. Doubt it is the ABS system.

The brakes are vacuum assist, too, right? Maybe something to maybe check there as well. A misbehaving leak could cause the master cylinder to apply pressure in that way. Only way I can think to check is to pump the brakes with the car off a bunch of time to remove all vacuum assist.

I would also inspect the brake lines themselves. Age and general road wear can cause brake line deterioration. I have seen rubber and plastic lines collapse in on themselves due to age, preventing fluid from flowing freely in and out of the caliper (dragging or freezing the caliper at times). I haven't had this issue on any P car I have owned, but it happened to me on two other higher milage daily drivers I have owned.

Could be water intrusion into the brake booster. There's a TSB for '99 if you search.

It could be the pistons just corroded around the top edge and as you have new pads it pushes the pistons right in to the corroded part otherwise it may be you have a problem with fluid bleeding back into the system. If you go onto design911 then click full website at the bottom, on the left of the main screen it says parts diagrams, then go brakes then you get exploded diagrams for each area of brakes, you can have a look at things, I use it a lot just to see what I'm dealing with. Also I did notice when you click on disc brake front axle to go into the diagram underneath you get a full parts list with part numbers and prices, very useful!!! There is a brake booster modification kit listed at the top of the list, could this be your answer?? Good Luck

Sorry the kit is listed under the master cylinder diagram so click that then go down to the list!

As said Nein_11 you should take a look at your brake lines. The rubber hoses are 19 year old and should be replaced whatever. There is a video on YouTube describing a problem similar to yours, fixed by replacing the rubber hoses. You Tube $80 for 4 Ebay: https://www.ebay.com/itm/Porsche-911-Boxster-ATE-OEM-Set-of-4-Brake-Hydraulic-Hoses-for-Front-Rear-Side/361205950163?ssPageName=STRK%3AMEBIDX%3AIT&_trksid=p2057872.m2749.l2649 Good luck

With the car sitting cold for a few days, the rear brakes are stopping and releasing properly when the brake pedal is pressed and released. The front brakes are both sticky and not quite releasing (this is with new pads and rotors, but it was like that with old rotors and old pads). So I suspect the front brake rubber hoses are bad. The rear brake hoses dont have a metal spring outer jacket, is that normal, or an indication that they were replaced in the past and can explain why they seem OK. Now I have an additional problem of having two broken shims embedded in the top pistons of the rear caliper, I need to figure out how to remove them without removing the pistons. I�ll update more as I work on it more. Thanks to everyoneś suggestion so far.

After replacing all the brake pads and rotors, as well as the front brake hoses, the problem remains. It seems like the brakes dont drag when cold, and drag more as the brakes get hotter. When I parked the carwith the brakes dragging, the drag goes away after a few hours of cooling, only to come back after a few miles of city driving. Whatś next? I�ll need to bring a jack with me on a drive, wait till the are nice and hot and dragging, then jack up the car to tell which brake is dragging, and repeat a couple times to see it is always the same wheel(s).

Just curious what brake fluid are you using?

pentosin dot 4. Today I played around some more with the brakes. they definitely don't drag when cold, and drag when hot. When the brakes are dragging, I pulled into my driveway (very slightly sloped up), and the car rolled back a few inches as I was getting out, that was enough to un-seize the calipers and all the wheels where rotating freely. Then I pumped the brakes couple of times, and the rears were rotating quite freely with a tiny amount of drag, but the front were hard to rotate. Wait for half an hour or so and the fronts were freely rotating again. Any ideas?

Brake fluid absorbs moisture like crazy. Old brake systems on domestic cars had cast iron master cylinders and sealed reservoirs and were less likely to absorb moisture, hence could go longer without service. Modern brake systems have aluminum master cylinders and plastic reservoirs that are vented and require service flush every 2 years. No telling how your car has been serviced but should have had about 10 brake fluid flushes. You could very well have a stopped up ABS. I would use a PST2 or PIWIS to activate ABS while flushing to see if it would fix the issue. You may still have to replace the ABS.

I don�t know how long the old brake fluid was in there since I own the car only for two years, but the drained fluid looked very much like the new fluid that I put in, there was no sediment or dark colour coming out when I bleed the brakes yesterday. I used up two litres of fluid, so I assume the system was thoroughly flushed yesterday. I worry that the ABS may have a problem though, since that would be expensive and beyond my abiliites to fix. But I know the ABS itself worked since I autox it every couple of weeks and triggered the ABS frequently, I marvelled how well it worked since it allowed me to brake really late nd still turn the car at the same time. If the actual ABS functions well, should I still supsect it ? Or can I just try simpler things like rebuilding the front calipers? What I don�t understand is why the problem only shows up when the brakes are hot, but works fine when the brakes are not hot.

Popular Mechanics says: �The first thing to consider is that a caliper piston might be dragging, causing friction that cooks the brake fluid and results in high brake-line pressure, locking that corner. ... If all your brakes are locking up simultaneously, the problem is likely with your master�

If your brake pedal is not fully returning*, the pads won't retract, the brakes will drag more as the fluid heats up and expands with no place to go (master cylinder blocking the feed hole from reservoir). Make sure that your brake pedal is returning to the stop. If the pedal is totally free then I think you may need to replace the master. *This can be caused by water/debris contamination in the vacuum assist.as Ahsai mentioned more prevalent in 1999.

I wasn�t able to find the TSB mentioned by Ahsai about the water contamination problem. Can you briefly explain what it is? My car at least in the past two years have not seen water other than the very occasional car wash, and it is a california car all its life, but there may still be water problems. I took the front calipers out tonight, it does look like they were worked on a few times in their life, all the nuts for the hydraulic tubes are slightly rounded, and the clear-coat is flaking off. When disconnected, I can easily push all the pistons all the way in. I haven�t pulled out the pistons to inspect yet, it is too late at night to fire up the air compressor. Iĺl pop the pistons tomorrow. One strange thing is that when I disconnected the brake hoses in the front, I expected brake fluid to gush out. Instead there was nothing coming out, and only a few drops dripped on the ground after a few minutes. Is that a concern?

Does your brake peddle return all the way back up when you let off?

When the brakes are hot and the fronts are binding, crack the lines at the master and see if this releases the pressure on the fronts, if it does the problems in the master, if not repeat the same and crack the bleaders if the pressure releases the problems in the hoses, if not the pistons are binding.

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my thought also. Cup blocking the refill hole. https://rennlist.com/forums/996-foru...ky-brakes.html https://rennlist.com/forums/996-foru...hemselves.html You Tube From https://www.renntech.org/forums/topi...g-brake-pedal/ You should join it is well worth it. Sorry, like Creekman I was thinking about the clutch pedal. There's no spring on the brake pedal as far as I know. Still, water intrusion in brake booster is a well known problem for '99 cars. There's an updated booster and linkage. The water fills up the diaphragm of the booster, which makes it not returning to the rest position even after you take your foot off the pedal. Edited March 11, 2014 by Ahsai ... update on the sticky brake pedal. My master cylinder and booster were fine. The sticking was caused by worn bushings in the pedal assembly. I hope this helps others in the future. Thanks to everyone for the advice.

Though the brake lines degrade over time and may clog or have buildup within the line to cause back pressure etc, because both the fronts are equally affected, it seems that the issue may be higher up in the system. The brake booster may be getting stuck or may be leaking that is leaving residual pressure as the system gets hotter which would explain a majority the current situation.

I managed to pop off three of the four pistons off one of the calipers with an air compressor. I couldn�t get the 4th one out. The three that are out look pristine, even the outer dust seals look perfect after cleaning off the brake dust. I haven�t pulled out the inner square profile rubber rings yet, but the bores and the rings look perfectly fine. Don�t know if the 4th piston is also perfect, or is the one causing problems. I don�t know if I should bother rebuilding them since the calipers may be better than my rebuild job. Unfortunately I didn�t check if the brake pedal returns fully before I pulled out the calipers, so I can�t check until the calipers are back in. So far the only insights I have is that the rear brakes seem to be fine.

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Ofcourse you should "rebuild" them. If the pistons and bores are perfect then the rebuild will only be cleaning and replaceing the o-rings and dust boots. The square o-rings can harden with excess heat and time, the o-ring actually flexes under pressure and when it relaxes it with draws the piston in a small amount to lessen drag, so if your old ones are hardened the new ones will help.

You�ve convinced me to rebuild, new softer rubber o-ring may help the pistons to retract. But now I�m suspecing the problem to be upstream. BTW, I popped off the 4th piston, it too is pristine except for the scratches I made when trying to pry it off with screw drivers :-(

A quick update: I rebuilt both front calipers by replacing all the piston seals and dust boots. There is a bit of improvement. When cold the car acts normal with no drag. When fully warmed up there is a slight drag as noticeable when you reach an almost full stop, the car does a full stop by itself. I just did a 8 mile drive of mostly city, speeds from 20mph to 50mph. Afterwards the front rotor temps were 100C, the front hubs at 110C, the brake calipers at about 60C. The rear rotors were about 55C, the rear hubs also around 50C so the parking brakes were not dragging, and ambient temp was 20C. A summary of what were replaced: all four rotors, all brake pads, borh front brake hoses, and rebuilt both front brake calipers. I guess I have to check out the master cylinder next. The brake pedal seems to return to rest position after braking but I can�t tell for sure with my foot, and I can�t see the pedal when I�m braking. Will get a friend to look at the brake pedal while I brake to see if it returns to position. I

When you feel dragging just use your right foot to lift up the brake pedal and see if it makes a difference. If the dragging goes away, it's likely the brake booster.

I�ll do that next time I drive duncan. I removed the plastic panel covering the master cylinder. It looks like the master cylinder has two lines coming out. One goes straight to the ABS unit, the other goes through a 2" long black metal cylinder before going also into the ABS unit. The ABS unit has two lines coming out into the driver side fender well. Can I assume the line from master cylinder directly to ABS is the front brake circuit, the other line that goes through the black cylinder is the rear circuit and the black cylinder is some sort of pressure regulator thingy? The two lines coming out either go to each diagonal brakes, or one goes to the front brakes, one goes to the rear brakes. Either way I don�t see how it can prevent a wheel from locking, Shouldn�t there be 4 lines coming out of the ABS to each individual brake so that each brake can be unlocked individually in a skid of when hitting an ice patch? I need a lesson on our ABS.

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Loosening the brake reservoir cap seems to alleviate the brake drag, but I don�t know if it was just placebo. Can the cap be faulty? I know it is vented but looking at it I don�t see how it vents hence my experiment with loosening the cap to improve venting. This afternoon I jacked up one side of the car in my driveway with the engine running after a few miles of driving, and hand rotated both the front and the rear wheels without too much resistance. Then I stepped on and released the brake, and there was a lot more resistance when I hand rotated the wheels afterward. I lifted the brake pedal by a couple of mm, and the wheels were much easier to rotate. So basically the brake pedal returns to the stop, but all four brakes are dragging, If I force the pedal up 1/8 of an inch or so, and the brake drag mostly disappears until the next time I brake. From reading various posts, it seems like the vacuum booster is the likely suspect. There is no signs of water damage to the booster, the two long T45 bolts holding down the booster are nice and shinny without any sign of corrosion. There is no sign of water damage near the booster/pedal/horn area (it is a california car). Can the master cylinder be the problem? Replacing the master cylinder isn�t difficult nor expensive, but removing the booster is a lot trickier since it may not come out without first removing the ABS unit. Whether the problem is in the master cylinder or in the vacuum booster, why does the problem not manifest itself when the brakes are cold? It isn�t that the MC or booster get heated by the fluid inside the calipers. Can the problem be as simple as the adjustment between the booster and the pedal being off? Perhaps even with the pedal at rest, the master cylinder is still being pushed in a very tiny amount to cause a slight drag? Should I go ahead and rebuild the rear calipers just in case :-)?

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I don�t see how the rear calipers are the problem. You rebuilt the fronts and still have the problem, so i would expect that if you rebuilt the rear, the problem would still exist.

You have first posted of this problem on July 6th and asked for help from an internet forum. Many have weighed in and offered advice. You still have the problem. Perhaps you should consider taking it to a professional brake specialist for evaluation. Horror of horror I know, but even a Porsche dealer has some very experienced technicians. Brakes and brake systems are something you really need to have work perfectly.

If you are determined to repair this issue your self, I suggest you get a quality Digital pressure gauge with a range of 3000 psi and an accuracy of 1 psi and install it at your caliper for diagnosis. You can get fittings to install at the bleeder valve or at the flex hose.

I think i would have just bit the bullet and got a new booster and 996 master or maybe the 997 master. I have seen threads on the upgrade? U have already replaced the most of the system not that many more things that can be wrong.

I suspected the problem is either in the master cylinder or the brake booster after watching this video last night: https://theshopmag.com/videos/traini...od-depth-gauge Tonight I pulled out the master cylinder and brake booster, and measured the clearance between the master cylinder and the booster push rod with a vernier caliper. I got a clearance of -1.1mm. This is most likely my brake dragging problem. I have no idea how it got that way, and I don�t know if it is the master cylinder being off, or the brake booster being off. Both are in very good condition with no corrosion. The foam air filter in the back of the brake booster is disintegrating from age, but there is no sign of water damage or rust in the booster, the internal parts are nice and shiny. The proper thing to do is to shorten the booster push rod, but it doesn�t look adjustible.

A new vacuum booster from TRW arrived this weekend, and I measured the push rod, and it is recessed 1.5mm more than the old booster., that gives the need clearance to avoid constantly pushing the master cylinder piston. After putting it in, the brake drag problem seems to be gone. I drove around a couple of hours and the front wheels were hot (50-55C) and the rotors were about 60C, so I think the problem is fixed. Unfortunately the one of the hard brakelines from the ABS to the fender has a slight weep. I didn�t tighten the brake lines to the ABS too much since I was worried about stripping the aluminium block, I tightened the said line by about 1/4 turn, we�ll see if that fixes the weeping. I need to bleed the brakes again since they now feel a little spongy, may be I didn�t bleed properly or that weep was causing the spongy brakes. At least there is light in the end of the tunnel! Thanks for all those suggesting the replacement of the booster, I should have listened to you guys and wouldn�t have needlessly replaced so much stuff. Live and learn.

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If your car is a manual, you will likely want to bleed the clutch slave too as part of your bleeding process. You've had the system open a bunch, so best to bleed it from all possible areas. There is a good writeup on RennTech and Pelican.

Meh its probably not a bad thing that u rehabed the rest of the system.

Glad you've found and fixed your problem. If the abs hydraulic lines have been opened, I've read that you'll need PIWIS to cycle the abs valves while bleeding. I think Durametric can do that too but not sure.

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I hear you, even though I didn�t notice anything funny with the clutch, the clutch slave cylinder was last replaced 14K miles ago in 2012, and the previous ownerś receipt says the fluid was replaced with Motul. It is now six years later so I should do a fluid change this summer since I don�t want to mix Pentosin with the old Motul. Back in January I changed out the LWFW with the OEM DMF, and can vaguely remember that removing the clutch slave cylinder from the tranny was a major pain. I hope bleeding it doesn�t require dropping the tranny again!

You can bleed the slave from underneath without removing it. It has a bleed nipple like a brake caliper. It's tight, but you can do it.

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Removing the right rear wheel makes it much easier. Still sucks, but not nearly as much as replacing it.

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It turned out to be pretty easy to bleed the clutch hydraulics, but I cheated by using a pressure bleeder. The car is working fine now, ready for a workout at this weekend's autox. BTW. I noticed some similarity between the 996.1 ABS unit and my miata's ABS unit. miata 996.1