Lost brake pressure. Need opinion.

rnln · 03-01-2011, 11:27 PM

I have not touched my car for around 1/2 year or maybe even longer. Today, I decided to take some times to change oil and get it back on the road. Oil change done, started the car to do some spins. First thing I noticed was the brake pedal was soft and hit the floor. I had to pump it several times to get some pressure, but soon I rev the engine, I lost pressure, pedal soft and hit the floor again. Pumped it several times and got it back again. Took a short drive (late, rarely see car on the road). I found that every time I hit the gas pedal, I lost brake pressure.

I remember I had this similar problem years ago with my first generation 240z but it was less pronounce. I only need to pump it twice to get full pressure. It was the brake booster. This booster is connecting to the engine with some hose and valve.

Now, it is as bad as 7 pumps to get full pressure. The level of pressure lost depends on how hard to rev the engine. If I drive as slow as 20-30 mph, I only need to pump 3 times (proximately). If I push to open the throttle a little more, I need to pump up to 5 or 7 times to get brake pressure back. And I Need to pump it slow. If I do it fast, pressure won’t be built up as quick as pumping it slow.

Is there such a brake booster on our 911? I don’t think it’s the brake cylinder is the problem. Any opinion?

DRACO A5OG · DRACO A5OG's Garage

Need to see if you have a leak somewhere, the booster is that black round thing behind the MC.

But first, check levels. May have too much moisture sitting for that long.

1. Checking MC, press the pedal hard with the engine running. The pedal should feel solid and remain solid. If the pedal slowly falls, you may have an internal leak or leaking somewhere else ( hopefully a line ).

2. Booster Check, Pump brakes 10 times with engine off, then hold and start the engine. The pedal should fall slightly, if not, you may have found the culprit. Also check the ONE-WAY check valve for air flow, that is in the MC unfortunately.

per Bentley. pg 470-4

rnln · 03-02-2011, 12:08 AM

on (1): If I don't touch the gas pedal, depressing the brake pedal only, won't change the pressure level.

on (2): will check.

On "Also check the ONE-WAY check valve for air flow, that is in the MC unfortunately". How to check?

911pcars · 03-02-2011, 09:49 AM

Symptoms sound like bad seal(s) in the master cylinder. The booster only helps apply pressure to the internal piston.

Sherwood

Steve@Rennsport · 03-02-2011, 09:54 AM

Quote:

Originally Posted by 911pcars

Symptoms sound like bad seal(s) in the master cylinder. The booster only helps apply pressure to the internal piston.

Sherwood

Totally agree with Sherwood,...this sounds like a bad MC.

rnln · 03-02-2011, 10:18 AM

Not to argue, just to identify the problem, if master cylinder is bad, shouldn't it build up and hold pressure when I pump the pedal mulitple times? And also lose all pressure when I depress the gas pedal?

rusnak · 03-02-2011, 10:28 AM

I wonder if all the fluid is getting sucked up into the booster? Either way, bad MC time to replace.

911pcars · 03-02-2011, 10:30 AM

Quote:

Originally Posted by rnln

Not to argue, just to identify the problem, if master cylinder is bad, shouldn't it build up and hold pressure when I pump the pedal mulitple times? And also lose all pressure when I depress the gas pedal?

It depends on the condition of the piston seal(s).

The vacuum canister should store the needed vacuum (neg. pressure) to help depress the brake pedal regardless of the status of the gas pedal. However, if the vacuum canister hose leaks, the gas pedal position (and thus engine vacuum) will have more of an effect on braking boost.

Sherwood

DRACO A5OG · DRACO A5OG's Garage

Oh boy, Rnln, we got our work cut out for us

kidrock · 03-02-2011, 02:25 PM

Quote:

Originally Posted by rnln

Not to argue, just to identify the problem, if master cylinder is bad, shouldn't it build up and hold pressure when I pump the pedal mulitple times? And also lose all pressure when I depress the gas pedal?

not necessarily. Most of the time, a bad MC will not hold pressure, regardless of how many times you pump and hold the pedal.

I'm going with a bad MC on this one.

rnln · 03-02-2011, 11:21 PM

Thanks everyone for input. I have not have the chance to test more on it.

Draco J, what do you mean?

Kidrock, umm... Maybe I am too dumb but let read what you were saying carefully : "most of the time when MC is bad, it won't hold pressure". Well, mine will hold pressure after you pump the brake pedal and don't touch the gas pedal. It will lose pressure only if you depress the gas pedal.

I will also test all cases when engine off and will report.

Thanks again.

DRACO A5OG · DRACO A5OG's Garage

May need to rebuild your MC

Did you get her to your new garage yet ,) ?

rnln · 03-02-2011, 11:53 PM

No, was doing lawn today with only an electric string trimmer, the whole yard. Grass was too tall, too much weeds too. Couldn't get my gass mower started arg. I never can get any gas tools started

Any expert around OC can give me a free lesson/trick?

DRACO A5OG · DRACO A5OG's Garage

Was your mower sitting for 6+ months too

rnln · 03-03-2011, 12:22 AM

yes

kidrock · 03-03-2011, 03:24 PM

use liberal amounts of ether (quick start) for your mower. You'll probably need to pull the carb and give her a good cleaning.

Maybe I wasn't reading your dilemma right....now I'm not quite sure if I understand. If your MC is holding pressure until you depress the throttle pedal, maybe your problem is your brake (vacuum) booster? Now I'm just guessing, and that isn't gonna help the problem.

DRACO A5OG · DRACO A5OG's Garage

Yeap, that is exactly what is going on with him, he pumps then holds the brake with left foot then in neutral he starts the car and press the accelerator then brake pedal sinks.

I am going to see him and test it myself per Bentley guide lines.

Quicksilver · 03-03-2011, 03:47 PM

Trying bleeding before you throw money at it. You want all that bad fluid out of there anyway.

The vacuum booster will pull the pedal down so that will be effecting things when there is vacuum in the system. I would pull the vacuum line and release any stored vacuum and then bleed it. If it works I would watch it carefully as corrosion and gunk knocked loose can cause even more problems.

DRACO A5OG · DRACO A5OG's Garage

+ 1, above

rnln · 03-03-2011, 10:31 PM

Tested it more careful tonight. There is a little change to the above I stated before.

1- When battery terminal is disconnected, depress brake pedal, no pressure. Pumpped it several times, pressure built up. Hold it there for around 2 minutes, I don't feel any change in pressure, but it's only my foot. I won't bet my head on it.
2- Lift up my foot, then push it right down again, pressure still there.
3- Tried everything again and Waited for around 15 to 20 seconds after I lifted my foot, stepped on it again, no pressure. So, the pressure released itself in around 1/2 minute or so.

4- connected the battery terminal, pumpped the brake pedal 10 times. Pressure built up at around 5-7 pumps.
5- held the brake pedal with pressure, started the car. After engine started, brake lost some pressure but not all. Pedal went down... maybe around 1". I think this is normal? Then I held it there and it stay there for several minutes. Lifted up my foot (same as above), waited for around 1/2 a minute, depressed brake pedal again, no pressure. So, pressure released itself after pedal lift for around 1/2 or so.

I also checked in the trunk near the MC, and in the pedal cluster area for leak but all are dried.

QuickSilver,
"pull the vacuum line and release any stored vacuum". You meant the brake lines?