Is it possible that a brake booster can APPLY brakes on its own?

I know they can cause a stiff pedal...which I get after driving a while...but it also applies the brakes....which is bad bad news...

Not bad piston seals...replaced both calipers...not back drums...totally replaced...not swelled brake lines...checked those...

I can drive the car on the freeway without hitting the brakes and the brakes will grab and the pedal will get stiff. So it has to be vacuum related. When the brakes grab and I pop off the check valve...I get barely any "poof" of saved vacuum.

Check valve is also new...

And when I "poof" the vacuum line to the brake booster, I can drive another 15 miles before it starts grabbing again...

I want to TRIPLE CHECK and make sure its the actual brake booster itself...like an internal diaphram thingy issue and not something else related to the booster.

 

What engine/intake manifold?
Possible that you have the check vavle in backwards?

 

Stock boxed intake. DPFI manifold. on a b7 block. What I assume to be the stock booster...and what I assume to be a stock replacement master cylinder.

Check valve is NOT in backwards...

 

bump...only gotta a couple hours to work on this

 

Dang.. basically stock.
I know some swaps have vacuum issues as to why I was asking.

Did some thinking and a booster shouldn't have anything to do with brakes randomly grabbing.
It's just an assist - if if failed then your pedal woudl be really hard to press down (like when the engine is not running).

1) With the engine off, press the brake pedal a couple times to clear all vacuum from the booster, it should become progressively harder to push. Press down firmly on the pedal and start the engine. The pedal should drop slightly if the booster is working properly. Allow the engine to run for about 30 seconds and the pedal should have a normal feel to it.
2) Wiggle the plastic fitting that connects the vacuum line into the booster. Be gentle so you don't break it but pushing it to the side a bit breaks the seal and you should hear a definate hiss. This hiss tells you it is holding vacuum and is good.
3) Disconnect the vacuum line to the booster (and plug it off) start the engine and after the first one or two pushes of the pedal it will be hard to push. This simulates what a bad booster feels like, plugging the vacuum line back in it will return to normal pedal effort if the booster is good.


-Checking the check valve -
Run the engine for a few minutes and turn it off. Push the pedal with normal pressure a few times - waiting about 5 seconds between each push. The pedal should travel less distance each time to achieve the same amount of pressure you need apply.
If there is little or no change then the vavle is defective.
With the engine off, remove the hose from the booster and block it off with your finger. Start the car and a strong vacuum should be felt. Turn the car off with the hose still plugged - the vacuum pull on your finger should remain for approximately one minute.


If all that checks out, then the only guess I have is that one of the brake cylinders is sticking.

 

Thanks for all that information!

My only other question is...what numbers or markings should be on my master cylinder and brake booster. I want to make sure they are the right ones. Midas might have given me the wrong master cylinder. Or the previous owner might have swapped out my brake booster for something else...

I know for the fact none of my brake cylinders grab. If I remove the vacuum line from my brake booster my brakes stop grabbing. I can then re-attach the vacuum line and have another great 15 miles of driving.

 

Got me confused and still have me thinking it's that valve.

Kiss test?

Disconnect the booster valve. You should be able to blow through the valve
from the booster end. You should not be able to blow through from the engine end.

 

I just tested everything you have said.

With the vacuum line disconnected from the brake booster. I get oodles of vacuum pressure. If I hold my finger on the line and turn off the engine. I get about 45 seconds of pressure on my finger. But fingers don't have the most leak proof seals So that's probably fine.

The vacuum line going to the brake booster is fine. I removed it totally and did a kiss test on it. It works like it should. But it's kinda hard to suck air from the booster.

I did the pump the brake and see if the pedal falls on start up...that works fine.

And I did the check valve kiss test...and I CAN blow through the valve on the booster side, and not on the engine side...which is the way it's supposed to be. However, how easy should I be able to blow through it? It takes a little bit of effort to blow through, but once I open the valve with my blow it's easy to blow through...

I checked my friends DA check valve its similar in resistance when blowing through it...

Also, with the engine off...when I pump the brake to where it is super hard. I can still push the pedal down all the way to the floor if I push hard enough. And it makes a whistle/Hmm/Horn noise as if air is escaping somewhere...I think this is normal haha.

 

just to be sure, this is the full booster check. youve already mentioned a few of the checks already tho.



but i somehow dont see a booster adding power to the brakes without moving the pedal directly.

 

Seems the booster is fine.

I'm outa ideas.

 

We're getting somewhere!!!

Tyson: On test #1 in that picture.

I don't know exactly how hard I am supposed to hold the pressure. But if I press hard enough I can "man-handle" the pedal all the way to the floor. (Using about 60% my total leg strength) And the pedal feels somewhat mushy. quite firm but mushy.

Also, THANK YOU GUYS! This is the first time I've gotten sincere replies on any of my threads.

 

test 1 is just like if you were waiting at a stoplight and holding the pedal down. no gorilla push. but it really shouldnt move at all tho.

 

I guess I'll ask again.

If anyone knows what codes or markings are supposed to be on an EF master cylinder and brake booster, please let me know!