Brake pedal stops normally then slowly fades to floor. Master cylinder? Booster? - 986 Forum - The Community for Porsche Boxster & Cayman Owners

I took my '01 S to an autocross this weekend. It was close to 100, so hot, but not outrageously so. When leaving the autocross, I stopped at a red light. I noted the braking power was pretty normal. But as I sat at the light, the pedal very slowly lowered to the floor. Something like 6 seconds to go from stopping to lowering to the floor. If I released the brake, it would once again stop in the normal position and then slowly lower (as long as I maintained steady pressure on the pedal). Fluid was replaced with Bosch fluid a month or two ago and bled. I'm pretty sure those symptoms aren't brake fluid boiling symptoms. I get normal brake feel and travel initially. So, I'm guessing either master cylinder, bad brake booster or vacuum leak. The engine seems to run normally. No idle issues, etc. Pedal stiffness is normal. So I'm ruling out booster and vacuum leak. However, I have read some threads out there on brake boosters having some water in them. Those symptoms are a little like mine, but not identical. If it's not that, then the only thing left is the master cylinder as my remaining suspect. That said, it's possible I haven't considered something. Anyone else out there have this problem?

I had that issue after I installed stainless brake lines, turns out I had a small leak in one of my back brake lines where it connected to the hard lines. I didn't discover it until the brake fluid low light came on, right as I was about to start my autocross runs at Daytona. That was just me, when I was searching the "brake pedal lowering" issue I seemed like I saw a lot of Cayman folks had it, but I stopped searching once I resolved my particular issue. Steve

Hmm, I would think that a brake fluid leak would allow some air to get in the line. If air was in the line, the brakes would feel squishy. I guess you had a leak where fluid would leak out but air wouldn't leak back in? Did you happen to notice brake fluid leaking out of the fitting?

I did notice the fluid leaking when I got home, it must have been a very small leak, but it must have flung off the inner wheel barrel during driving, so small, but enough to drop the brake fluid in the master cylinder over a 2-3 day period including the drive to Daytona (3 hours). After I tightened it up and did a bleed, I can't recall if an unusual amount of bubbles came out or not. The only symptom I really noticed was a firm pedal on application, but it would slowly sink down if I kept pressing it hard after I came to a stop. Reading the various Cayman threads had me convinced it was all in my head

Keep an eye on brake fluid level. My bet is you have a bad seal...either in the slave or master. If you see you're down on brake fluid but don't see a leak, its possible that the fluid is collecting in the brake booster. I've not experienced this (yet) with my Box but did with my Alfa Spider. No issue with air getting into the lines but I managed to wait long enough to repair that the fluid ruined my brake booster.

The first thing I would check would be the bleed valves on the calipers, make sure that they are tight. Most likely you'll have to fix(rebuild/replace) the master cylinder. Your symptoms are what normally happens when the master cylinder goes bad.

I think you have a bad brake master cylinder.

Checked all the brake line connections. Also checked the bleed valves. No signs of fluid leaking. I pumped on the brakes for 10 minutes and found no loss in fluid level in the reservoir. So, I don't think fluid is leaking out anywhere. Given the comments above and no loss in fluid that I can find, I'll replace the master cylinder to see if that fixes it.

If this was my car and the fluid wasn't low, I'd order a new master cylinder without blinking.

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+ 1 and... Perhaps you may want to buy a power bleeder as well, it will make the bleeding for the brakes (and clutch) a breeze Black Label European Power Bleeder Kit - PelicanParts.com

Installed new Master cylinder. Bled the brakes with a power bleeder (I got one a few years back). Bled the clutch. Still doing the same thing. Normal brake feel with full ABS kicking on when I stomp hard. Plenty of room between lockup and floor. Then slowly the brake pedal lowers to the floor. Checked all the connection points and cannot find a leak. Fluid level in the reservoir isn't changing. Guess I'll put on a new brake booster since I can't think of anything else. If someone has another suggestion, let me know...

This really has me stumped. The master cylinder is the only place where fluid can leak out of the pressurized part of the system without the fluid leaving the system and eventually leaking to the ground. When the brake isn't activated the reservoir and the brake lines are connected. As soon as you press the pedal the connection to the reservoir is sealed from the brake lines by the piston in the master cylinder. All the the booster does is to assist the master cylinder in pressurizing the brake lines by using vacuum. Any fluid leaking past the piston in the master cylinder will end up behind the piston, which is now connected to the reservoir.

I think I got it. You have a pinched brake line that barely lets fluid pass. When you initially press the pedal the brake lines that aren't pinched activate the brakes they go to. When you keep applying pressure you slowly push brake fluid through the pinched line. Here's what I suggest you try. Try pumping the brakes with each push including the part where your pedal just keeps moving towards the floor. If the second or third push doesn't allow the pedal to go beyond the initial movement I believe my theory has been validated. If you indeed have a pinched line you can locate which one is pinched by removing the brake pads on each wheel in turn and see if its easy to push the pistons into the calipers. The brake that has the pinched line will be difficult to push the pistons into the calipers. Hope I am right.

Rubber brake hoses IMO are a 10 year wear item.

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Have you verified the integrity of the 4 brake caliper rubber hoses with a helper pushing the break pedal? A worn out hose could be 'ballooning' under pressure, but they will return to their collapsed diameter when you relieve the pedal.. just a thought .

As it turns out, I had a new set of steel braided brake lines ready to install. I meant to install them during the master cylinder replacement a few days ago. After I installed the new master cylinder, I got excited, bled the system and then realized I forgot to install the brake lines. I was too lazy to crack open the hydraulic system again, so I put them on a shelf in the garage. After several suggestions to replace the brake lines above, I decided to get unlazy and install them. Did that this afternoon and bled the system. Still the same problem...seems to be completely unfazed by the new master cylinder or the new brake lines. Certainly, I've now got 8 new fittings that maybe I didn't get tightened correctly that might cause a small leak. I can't see fluid leak and I tightened the fittings until I started to get nervous that I was going to stretch the threads. I used flare wrenches on both fittings, so I could get them pretty darned tight. New booster will be in tomorrow. I'll install that. In the interim, is there any way the clutch hydraulics can affect the brakes? For example, is it possible that a bad slave cylinder would feed back into the brakes?

An update on my ongoing brake fight. I've still got the exact same problem. Here's what I've done to date: 1) Replaced the master cylinder. Twice. That's right. I'm on my 3rd master cylinder. All do the exact same thing. So, unless I'm the unluckiest person on the planet when it comes to Master Cylinders (and they're all broken out of the box), then it's not the MC. 2) New brake booster didn't fix it. Yea, I didn't think it would, but I was grasping at straws. 3) Replaced all brake lines with braided steel. Same issue. The lines aren't pinched nor are they 'ballooning'. Not the brake lines. 4) There are no leaks. I've checked all the connections repeatedly. Thinking I might not be looking at the right spot, I sat in the idling car and pumped the brakes for about 15 minutes. That's about 45 swings of the pedal to the floor. When bleeding the brakes, 3-4 pedal pumps will take the reservoir from max to close to min. After 45 pumps, it was visually in the same spot (I put a pencil mark there). So, no leaking. 5) At this point, I've now purged gallons of brake fluid through the system with a motiva bleeder. And I'm not kidding. I think I'm up to 3-4 gallons. I'm using Pentosin Super DOT 4 because it's the cheapest DOT 4 fluid at the auto parts store. When I'm going through gallons of fluid, I can't afford Porsche OEM. In any event, I can't imagine there are any bubbles left in the line. Even if there were, the brakes aren't spongy. They're firm...but slowly lower to the floor. So, pretty sure it's not purge issues. So, I'm moving on and am going to start replacing parts one-by-one downstream from the MC. The next two up are the proportioning valve (pressure regulator 986 355 305 00 according to the parts catalog) and then the ABS. If anyone has ideas or suggestions, certainly send them my way.

vacuum leak.

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I've thought about a vacuum leak, but don't think that's it. When I replaced the booster, I played around with the vacuum quite a bit thinking maybe I did have a leak. To simulate a huge vacuum leak, I just left the vacuum off the booster. Hard to get a bigger leak than that. I still had the same problem, except now without the booster functioning correctly I had to push much harder. Since the behavior was identical with a huge vacuum leak to the booster (well except now I was having a small hernia pushing the brakes), I figured it's not a vacuum leak. That slowly lowering to the floor part makes me think I have fluid flow. Since it's not leaking out, it's flowing internally. Somehow.

You never answered when I asked you whether pumping the brake after it has bottoms out changes the behavior?