Brake booster testing ideas?

[Fairview]

My 84 Carrera did beautifully on its inaugural run early this morning except for the brakes. If I give the symptoms can you help me diagnose?

1) The brake pedal is quite firm, unusually firm,
2) With the pedal depressed, there is no change in the feel of the pedal upon start up,
3) The car stops, but not anywhere near as effectively as it should,
4) The brakes do not pull left or right, and no wheel seems to be dragging or getting hot,
5) Using a vacuum gauge and hand pump, the booster is holding vacuum, photo below,
6) When there is vacuum in the booster, and I depress the pedal, the vacuum is released, and
7) using the vacuum gauge at the connection where the line comes in from the engine to the vacuum booster area, the gauge shows an even vacuum of 15 pounds or more at cold idle, photo below, and
8)this is a turbo brake car, factory widebody that has been stored for 20 years, photo below.

I guess my question might boil down to: can a vacuum booster be bad but still hold vacuum? As always, I appreciate any guidance for you.

[proporsche]

i would start with replacing the brake fluid...then check your supply vac.hose from engine to the front booster....
Ivan

[Fairview]

Both done. Vacuum test results in my post above. Thanks

[Quasimoto]

Following as I’ve had similar thoughts about brake feel on my car.

[proporsche]

when you said both done..i do not see mention brake fluid change..also did you test the hose from the engine to the booster ??It exits near pedal cluster and goes up...
Ivan

[Fairview]

Originally Posted by proporsche

when you said both done..i do not see mention brake fluid change..also did you test the hose from the engine to the booster ??It exits near pedal cluster and goes up...
Ivan

Thanks, sorry I wasn’t clear. I have completely flushed the system with new brake fluid. Also, I tested the vacuum coming from the engine through the line to the booster and I am getting 15 pounds of vacuum to the booster with engine idling. That is photo 1 in the initial post.

The booster is holding vacuum and releases it when the brake pedal is depressed. But the brake action is weak feeling and wooden. I am familiar with 911 braking feel from ones I’ve owned before which were all outstanding. This is very different. My Model A with mechanical brakes stops better!

It feels like the booster, although getting and holding vacuum isn’t boosting and is actually hindering braking action and feel. Do these boosters lock up or get hard even though holding vacuum?

[proporsche]

usually they leak if they have a problem..How are your brake caliper pistons?
Just to come to think of it, if some water got in to the booster-do not know how ....

[T77911S]

pull the vacuum at the engine on the line and booster. make sure it holds.
make sure none of the pistons in the calipers are stuck.
how old are the brake lines.
the rubber lines can deteriorate inside

[Flojo]

Originally Posted by Fairview

8)this is a turbo brake car, factory widebody that has been stored for 20 years, photo below.

change the brake hoses near caliper, all 4.
then flush again.

drive safely.

[Fairview]

So after a few careful miles on deserted roads, I have felt a change in the brake pedal, like something stuck has freed up a bit. It is still not 100% though.

Taking the advice from you all, I have new pads and brake hoses ordered and will report back. Also, yes T77, that is a good idea to pull vacuum at the engine compartment and see if it holds. I will do that soon.

I will update with the results of all this.

[dannobee]

Did it have race pads on it before? Because this sounds more like a brake pad issue than a booster issue. Especially if driving it around helped. Race pads won't work until hot.

When boosters fail the brake pedal becomes incredibly hard when trying to stop the car. There's a LOT of surface area on the diaphragm of the booster to help.

One of the old standard tests was, with the key off, push on the brake pedal repeatedly until the pedal becomes very hard. Then start the engine and feel if the pedal sinks a little. If you still suspect a booster problem, clamp off the hose going to the booster, deplete the vacuum in the booster as outlined above, and drive it around. You'll know in the first block if it's working or not. Be VERY careful though, the pedal will be hard as a rock and the car will be very difficult to stop, even at very slow speeds.

[911obgyn]

Be prepared to change master cylinder I had similar issues with 85 Carrera and it ended up being MC. All the above checks lead me to mine. When bleeding on an old MC use a block under the pedal so cups in MC don’t travel over previously unused portions of the cyl and become damaged!

[proporsche]

good news ...but i am puzzled when i have asked you if you have checked the hose from the engine to the booster you said yes, now u r saying not? post #5
Ivan

[Fairview]

Originally Posted by proporsche

good news ...but i am puzzled when i have asked you if you have checked the hose from the engine to the booster you said yes, now u r saying not? post #5
Ivan

To check that line, I measured the vacuum at the booster end in the trunk with the engine running. It held 15 pounds at idle.

I assume your suggestion is something different: that I now check at the engine compartment without the engine running to check the integrity of the line forward to the booster.

Is that what you are describing? If so, that sounds like a good idea as my first test, with the engine running could mask a small leak in the line as the engine is constantly creating vacuum. A static test would be more informative.

[proporsche]

yup exactly.... i should have mentioned that;-)

Ivasn

[T77911S]

you have vacuum at the booster but its still a good idea to pull the vacuum from the engine bay to the booster.

[Fairview]

Originally Posted by T77911S

you have vacuum at the booster but its still a good idea to pull the vacuum from the engine bay to the booster.

Okay, so I unplugged the vacuum hose in the engine compartment that is at the white connector near the cruise control unit. I pumped the vacuum tester by hand to 5 pounds of vacuum and it holds the vacuum. Depressing the brake pedal will release the vacuum.

More to come...

[rbarton]

Fairview, just as a data point I had what sounds like the same symptoms you're experiencing on my '86 and a new (used) booster fixed them for me. My pedal seemed to hit a wall right at the point where the brakes were starting to bite. The car would stop, but to get it to stop quick I had to push so hard that it seemed the pedal would bend. I installed a new master cylinder with no effect. I tested vacuum to the booster and it was good. Replaced the booster and all was well again. I haven't yet looked closely at the old one to see what the failure mode was.

[Fairview]

Originally Posted by rbarton

Fairview, just as a data point I had what sounds like the same symptoms you're experiencing on my '86 and a new (used) booster fixed them for me. My pedal seemed to hit a wall right at the point where the brakes were starting to bite. The car would stop, but to get it to stop quick I had to push so hard that it seemed the pedal would bend. I installed a new master cylinder with no effect. I tested vacuum to the booster and it was good. Replaced the booster and all was well again. I haven't yet looked closely at the old one to see what the failure mode was.

That describes my symptoms perfectly. I was hoping the booster is good, might be out of luck...

[RedCoupe]

We had a TR6 with the same symptoms. Even did an upgrade to bigger calipers. Still wasn't satisfactory. Changed the booster, and, wow what a difference!