Jamescoop

04 CTT ~110k miles

Let car sit for a week over xmas, came back and brake pedal would travel all the way to the floor, fairly easily. (might have done it before but I had not noticed...it was also cold in the 20's during this time). No codes.

This summer, I had cracked plastic vacuum booster lines that I replaced with some rubber vacuum line. I have not checked for more cracks -- when this happens, it typically throws a code which I do not have.

Today I bled brakes all around, looks very good (clean fluid as I changed brakes a year ago or so - no bubbles).

Pedal can still be pressed to the floor when car is on, but pumps up and seems to lock when car is off.

How could I test for booster and/or master cylinder problems? Thank you in advance!

PartsGuyGT

If it were a bad booster you would have a very stiff pedal. Sounds more like a bad master cylinder since you don't see anything leaking. They leak internally causing the pedal to go to the floor.

ScootCherHienie

As mentioned before, the power brake booster failing will result in the brakes and pedal working fine, but you'll have to apply 5 to 20 times more pressure on the brake pedal to get the stop accomplished. If the pedal goes to the floor, here are the pertinent troubleshooting symptoms to consider:

1) Goes to the floor very quickly - this would indicate a total seal failure in the master cylinder (not even possible on many vehicles) OR it could indicate a fair amount of air in the brake system. If the brake fluid in your reservoir is below the minimum indicator on the "tank" the system could "suck air" into the brake lines. Brake fluid should be changed every couple of years anyway, so changing the brake fluid should (or bleeding the air out of the brake lines if the fluid is relatively fresh) would resolve this problem. If the master cylinder has a massive failure that prevents it from delivering pressure to the calipers, the master cylinder would have to be replaced. Also, air in the brake lines will eventually "pump up" if you push the pedal to the floor, let it come back up, then press the brake pedal down again. If it gets progressively "stiffer" each time you press it to the floor and pump it again, chances are you got air in the brake lines and the chances are this came from air in the brake lines. It can take as many as 3 or 4 pumps of the brake pedal for it to get firm again if there is air in one or more brake lines.

2) If you repeatedly press the brake pedal and it does not "come back" and get firm, you are either leaking brake fluid out onto the ground or you have a massive failure in the master cylinder (have never seen this happen in real life though).

3) A master cylinder with wear will generally mean the brake pedal goes to the floor SLOWLY as you press on the brake pedal. So sitting still, apply the brakes, if the pedal feels firm initially, but the pedal "sinks" to the floor slowly, that's normal wear in a master cylinder and it has to be rebuilt or replaced. I've seen a BMW X5 master cylinder with this type of wear after 80,000 miles, but our old Acura went 180,000 miles before the master cylinder was obviously worn. Those are the extremes I've personally experienced in master cylinder "life".

Warehouse33.net

I would also say a bad brake master cylinder. It could be leaking into the booster then burning the brake fluid thru the intake manifold.


-Jason

Jamescoop

So I replaced the Master Cylinder yesterday...pedal is still feeling spongy and slowly goes to floor...I made sure to bleed it very well (filled up and did a bunch of small pumps until I saw it coming out of the hard lines into the cylinder, ltightened and let sit overnight. Then I bled the lines on the cylinder, then went around twice bleeding the calipers both outside and inside nipples-- did I miss anything?)...

Could it be the pressure sensor? Is it supposed to go to the floor eventually? It doesnt feel like any difference...problem with vacuum lines perhaps? No evidence of old one leaking into the booster.

Im disappointed...replacing the master cylinder wasnt the easiest...although now wouldnt take much time at all. For whatever reason, Porsche put the most inconvenient hard line around the top of the master cylinder, that doesnt budge at all. And the other hydraulic lines are a total PITA to remove/install....zero flex in a tight spot where you need some wiggle room. I took some photos so if I'll try and post a DIY install once this is fixed.

ScootCherHienie

What did the fluid level in the master cylinder reservoir look like before you removed the old master cylinder? If the fluid level was very low, it could be leaking through the booster and getting sucked into the engine and burned, as someone mentioned already mentioned. Another possibility is the braking system modulator that pumps the brakes individually if you engage the ABS system. We had one of these go out on an Acura and it was a $2500 part, no telling what Porsche's price is... could be less since the Acura was 1996 and they may have figured out ways to make them more reliable over the 10+ years since the Acura was designed compared to the Porsche. If you are NOT loosing fluid and the pedal still sinks slowly under steady pressure... the brake fluid has to be going SOMEWHERE... the only 3 possibilities are: leaking somewhere in the engine compartment; leaking at 1 or more calipers or around 1 or more pistons in one or more calipers; leaking past the booster and getting sucked into the engine and burned. All of these would result in the brake fluid level in the master cylinder reservoir going down over time. The ONE way you could have a leak is if the brake fluid leaks past the seals in the master cylinder and pressure you TRY to get to the brake calipers leaks around the bad seals in the master cylinder and the brake fluid just goes back into the master cylinder reservoir. For this last one to be possible, they would have had to send you a leaky rebuilt master cylinder. You wouldn't think that would be very likely, but it could happen. Aside from these 4 things, nothing else comes to mind re. a slowly sinking brake pedal.

Jamescoop

Fluid was OK level-wise.

Drove around today, took to local Porsche Shop (not dealer). Their Cayenne guy was out, but the other tech agreed something was off. Suggested going to the dealership and having "Joe" take a look at it, as he drives a Cayenne as well. So tomorrow, ill try that and hopefully something more will come of it...

ABS will not engage. 50mph-5mph hard brake in the rain....

trendy996

These cars don't use vacuum from the intake manifold. Therefore it can't get into the intake manifold. The bleed process is inside out of the caliper from rear pass, rear driver, front pass, to front driver. I would also use a vacuum or pressure assisted tool to bleed the fluid. Not the old pump and crack method. You may have a small pin hole in a rubber line or a small leak somewhere in the lines letting air into the system giving you these conditions.

PartsGuyGT

Aside from this ^^^ maybe there's an air bubble in the ABS. I don't know if the Porsche's are like this, but some of the Audis I've worked on you have to cycle the ABS with the scan tool in order to bleed out the air. Anyone with Durametric every tried this? Just a thought...

Jamescoop

I had bled them properly, through an extra liter of fluid as well, so dont think there is air...however this is with pump and crack method so maybe theres something there. Ill get a tool for next time I change the pads. I dont ever see any fluid, nor any drop in fluid level, so dont see how it could be a hole int he hose? unless the hole just lets air in....idk how to check for this. Would a bleeder screw go bad? There are replacements available on ECS

Got on snow and have cycled ABS quite a bit over the past couple months. No change.

I think my rotors are warped as it shakes when coming to a stop from high speed, have to replace that and new pads then ill look back in to this. The warping problem was there since last summer. I keep having issues here and there so i've become of the nature "wait till it brakes" as opposed to preventative, unless its a complimentary fix.

ScootCherHienie

There has been a lot of discussion about whether what we have been calling warped rotors is just corrosion on the discs from the car being parked wet... where the pads "stop" on the rotor, you get a rust pattern on the rotor where the pad was sitting and when you drive the car, you feel some shudder. Eventually, it wears off. This happened to be at least 5 or 6 times in the 140,000 miles I owned a BMW. One time dealer measured discs for run-out (non-flatness) and didn't find any run-out, yet they agreed that the rotors felt warped. I decided not to do anything at that point and just drove the car. After 5000-8000 miles, what felt like warped rotors was gone without replacing the rotors. Not saying you absolutely DO NOT have warped rotors, I just think the liklihood is a lot lower than most of us assume... the liklihood of the rotors being warped is lower than we think.

The 4 wheels have to have rubber hoses in the brake lines to allow for suspension travel on all four wheels and to allow for steering the front wheels. Perhaps something is going bad with those hoses. Have someone pump up the brake pedal and maintain pressure while you look at each of the 4 rubber hoses near each of the 4 calipers to see if any of them are bulging alarmingly. You would hope Porsche would use aircraft-grade hoses with braided steel reinforcement so the rubber hoses can't weaken and get stretchy in their old age. If the hoses are expanding under the pressure of pumped-up brakes, they should all be replaced.

Jamescoop

Great idea about the hoses -- will try next time. Semes to be working OK right now, though still spongy it stops well, even pulling trailer. .

Definately have warped rotors. They were cheap, and when slowing from highway speed the steering wheel shakes like crazy. Has happened last 5k miles...not a lot of freeway driving so its OK at the moment, Im trying to see if I can warranty them.

knfeparty

FWIW I had similar issues and replacing the master cylinder made no difference. REALLY thorough bleeding did. Use a rubber mallet to whack the calipers. Use a pressure bleeder. On Gen 1 there is no way to activate ABS with the computer for a bleed, so put the car on 4 jack stands and just put it in drive (obviously be careful when you do this and jack it up well) and the ABS will go nuts trying to use PSM. This is the best way to thoroughly cycle the ABS pump to get air out.

v10rick

The CTT brake booster does use vacuum from the intake manifold. The external electric vacuum pump is a backup to the manifold vacuum. There are 3 check valves in the vacuum circuit to make this work as it should. Now its not.

Could be the electric vacuum pump is kaput.

PTEC

This is correct. OP, are you getting a brake booster warning on the dash? If you had a leak in the vacuum system the pressure sensor would catch it and throw a warning on the dash.