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Old 10-05-2020, 04:32 PM   #1
Kuutio
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Default Brake pedal goes to floor when car is running

Simple as that in the title, i hope So i have 940 that has soft brakes when car is running but when its not running i can stamp on the pedal with 1000 lamapowers and it wont budge. Also i think this is connected to this, my engine will change rpm more than is normal when pressing pedal and also has leak on intake which obviously makes the car run weird. I tried blowing air thru the valve on side of booster but couldnt blow through. So could my booster be done ? i dont think my master cylinder would do like this, will try to go thru calipers one or two times more but im thinking of busted booster since braking affects my rpm so much.
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Old 10-05-2020, 05:34 PM   #2
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If the booster is bad it will be harder to press the pedal not easier. You either have a bad MC or air in the system
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Old 10-05-2020, 06:18 PM   #3
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Does it go to the floor as the car is moving, or if the car is on and parked. If while driving it may be the sign of a bad wheel bearing causing the rotor to move back and forth, pushing the brake piston in.
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Old 10-05-2020, 06:31 PM   #4
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forgot to mention that my brakes do still work. Could be air but last time couldnt get any out. Doesnt matter if im driving or not.


edit: also sorry, should have written better, just catching if it could be fault somewhere. Will be trying to get air out maybe tomorrow if i get a friend to do it with. So im pretty much just asking if it could be fault somewhere....... also thanks for the replies to this point and sorry that i didnt clarify before!

Last edited by Kuutio; 10-05-2020 at 07:09 PM..
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Old 10-05-2020, 08:21 PM   #5
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Quote:
Originally Posted by Kuutio View Post
I tried blowing air thru the valve on side of booster but couldnt blow through.
Which direction?
Air gets sucked out of the booster by the engine, so blowing through the check valve toward the engine should work, but toward the booster should be blocked.

Quote:
Also i think this is connected to this, my engine will change rpm more than is normal when pressing pedal
Booster may be leaking internally.
With the engine running, stick your head under the dash and push the brake pedal down. Is there a constant hiss of moving air? Or just the initial whoosh as you push?

Of course...
Quote:
also has leak on intake
will make the engine change RPM more than normal, since it can't compensate quick enough for all that unmetered air.
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Old 10-06-2020, 07:54 AM   #6
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Quote:
Originally Posted by MasterBlaster View Post
Which direction?
Air gets sucked out of the booster by the engine, so blowing through the check valve toward the engine should work, but toward the booster should be blocked.


Booster may be leaking internally.
With the engine running, stick your head under the dash and push the brake pedal down. Is there a constant hiss of moving air? Or just the initial whoosh as you push?

Of course...

will make the engine change RPM more than normal, since it can't compensate quick enough for all that unmetered air.
So what i was thinking that maybe the leak could be in conjunction with the brake stuff but i will see that today since i will be changing gaskets and checking every hose on intake since it could be simple hole in a tube etc. but i will double check the check valve. Also forgot to mention that there was suction on the booster when i unplugged the valve from it and it felt like some small amount of fluid came out so it sure could be mc. Actually im starting to think that maybe the seals got smashed when we went to take air out of system if the rod had some corrosion etc on the shaft..... but i didnt see any brake fluid on the pedal side when i removed the rubber boot. Only some brown grease. I`ll snap the mc off the booster and will see if it has small leak.....

Edit: just a quick question of, if the mc is bad, should i rebuild or get new unit? Rebuild kit was 18€ and i think some sort of bosch unit was 90€. I havent rebuilt mc yet so i dont know how long that will really last. Ofc i would need to see what the condition inside it is first to really tell but what is general lifetime of rebuilt one ?

Last edited by Kuutio; 10-06-2020 at 08:01 AM..
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Old 10-06-2020, 08:57 AM   #7
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Quote:
Originally Posted by Kuutio View Post
So what i was thinking that maybe the leak could be in conjunction with the brake stuff but i will see that today since i will be changing gaskets and checking every hose on intake since it could be simple hole in a tube etc. but i will double check the check valve. Also forgot to mention that there was suction on the booster when i unplugged the valve from it and it felt like some small amount of fluid came out so it sure could be mc. Actually im starting to think that maybe the seals got smashed when we went to take air out of system if the rod had some corrosion etc on the shaft..... but i didnt see any brake fluid on the pedal side when i removed the rubber boot. Only some brown grease. I`ll snap the mc off the booster and will see if it has small leak.....

Edit: just a quick question of, if the mc is bad, should i rebuild or get new unit? Rebuild kit was 18€ and i think some sort of bosch unit was 90€. I havent rebuilt mc yet so i dont know how long that will really last. Ofc i would need to see what the condition inside it is first to really tell but what is general lifetime of rebuilt one ?
I wouldn't mess around with brakes, I just bought a new MC from Rock Auto for $43. Not sure what shipping across the Atlantic will cost though.
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Old 10-06-2020, 12:32 PM   #8
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Not a single drop of air out of brakes. Feels even worse now . Valve does work as intended. Didnt have time to pull mc. Will have a look tomorrow. Pressing brake i can hear whoosh when pressing and letting of, air not coming thru pedal side so i doubt that is the problem. Starts to look that the intake leak just happened exactly when i do something else.

Last edited by Kuutio; 10-06-2020 at 01:56 PM..
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Old 10-06-2020, 02:42 PM   #9
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Quote:
Originally Posted by Kuutio View Post
Not a single drop of air out of brakes. Feels even worse now . Valve does work as intended. Didnt have time to pull mc. Will have a look tomorrow. Pressing brake i can hear whoosh when pressing and letting of, air not coming thru pedal side so i doubt that is the problem. Starts to look that the intake leak just happened exactly when i do something else.
How much brake fluid have you put through your system? I just put 5L through mine and have a good pedal, are you bleeding all 4 corners?
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Old 10-06-2020, 03:10 PM   #10
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How much brake fluid have you put through your system? I just put 5L through mine and have a good pedal, are you bleeding all 4 corners?
All 4 corners, i guess there could be air pocket somewhere but dunno. Currently i think i have put around 1.5-2L. I have never heard anyone putting 5 liters thru brakes :D
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Old 10-06-2020, 03:32 PM   #11
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Quote:
Originally Posted by Kuutio View Post
All 4 corners, i guess there could be air pocket somewhere but dunno. Currently i think i have put around 1.5-2L. I have never heard anyone putting 5 liters thru brakes :D
I bought a 5L container of Pentosin LV from Rock Auto, it will just go bad sitting in the container. The pressure reducing valves that go to the rear brakes can hold some air, need to jack up the rear end as high as you can get it and let it sit overnight, then bleed while still in the air.
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Old 10-06-2020, 05:14 PM   #12
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Originally Posted by VB242 View Post
I bought a 5L container of Pentosin LV from Rock Auto, it will just go bad sitting in the container. The pressure reducing valves that go to the rear brakes can hold some air, need to jack up the rear end as high as you can get it and let it sit overnight, then bleed while still in the air.
That is something i could try because the reducing valve was leaking before for me if its the one sitting under the mc.
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Old 10-06-2020, 07:19 PM   #13
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Quote:
Originally Posted by Kuutio View Post
That is something i could try because the reducing valve was leaking before for me if its the one sitting under the mc.
That's the brake octopus, the reducing valves are in the back near the fuel pump with hoses coming off therm.
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Old 10-06-2020, 11:03 PM   #14
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Had the same problem, jacked up the rear and manipulated the hoses to the rear callipers up and down, bubbles can get trapped in the high point.
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Old 10-07-2020, 04:19 AM   #15
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That's the brake octopus, the reducing valves are in the back near the fuel pump with hoses coming off therm.
hmm, i almost had right part in mind :P atleast when i googled that part it said it was also reducer valve but dunno what it really is. Some kind of valve inside but i think it was meant to shut the other circuit if it goes boom ?
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Old 10-07-2020, 08:26 AM   #16
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Quote:
Originally Posted by Kuutio View Post
hmm, i almost had right part in mind :P atleast when i googled that part it said it was also reducer valve but dunno what it really is. Some kind of valve inside but i think it was meant to shut the other circuit if it goes boom ?
The octopus distributes braking pressure to the calipers, the switch inside indicates pressure loss in one of the circuits.
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Old 10-07-2020, 12:01 PM   #17
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900's don't use an octopus. They're only used on 200 and early 700 series cars.

They do use a reducing valve whose function is described here.

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Old 10-07-2020, 01:24 PM   #18
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Thanks for the answers up to this point i will try tomorrow to ram multiple liters of brake juice through system and before that ill see if there is air in mc as i think you can do that thru the 2 bleed nipple thingys on it just by pushing fluid thru those ?
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Old 10-08-2020, 10:56 AM   #19
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Thought to update since im on the car atm. No leak behind mc. Will try to ram fluid next when my buddy comes. Pretty sure its just air trapped in pipes. But could my lil fix make bad brake pressure? I dont think it would affect this way but how i fixed the leak on the valve under mc was to put hydraulic washer under it but could it affect pedal feel since it is very low cc difference and since if it is full of juice i dont think it would affect pedal that much ? Sorry if my text is hard to read, just throw questions if you feel like it :p
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Old 10-08-2020, 03:12 PM   #20
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New mc it is. No air, nothing but pure fluid and once again even worse pedal feeling. Still brakes work but ehh
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Old 10-11-2020, 05:55 PM   #21
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You can also gently tap on the metal lines and the pressure regulators with a small hammer to break loose any air that is stuck.

What kind of brake fluid did you use? Your car requires DOT4. The trouble you are having with bleeding the brakes is what made me buy a Motive pressure bleeder. It saves a ton of time and swearing. Makes bleeding brakes easy.
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Old 10-11-2020, 06:22 PM   #22
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I made my own pressure bleeder with a boneyard reservoir cap and a quick connect air fitting. I just drilled and tapped the cap for 1/4" npt and screwed the air fitting in. Just turn the pressure regulator down to 15psi on your compressor and make sure the reservoir doesn't run out of brake fluid.
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Old 10-11-2020, 06:38 PM   #23
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Quote:
Originally Posted by Kuutio View Post
New mc it is. No air, nothing but pure fluid and once again even worse pedal feeling. Still brakes work but ehh
You probably have air trapped in the lines just past the master cylinder. 940 master cylinders have bleeder ports on the side of the master cylinder in order to remove the trapped air. Bleed each of those ports before bleeding any of the other ports at the calipers.
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Old 10-12-2020, 01:19 PM   #24
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Quote:
Originally Posted by VB242 View Post
If the booster is bad it will be harder to press the pedal not easier. You either have a bad MC or air in the system

This is not always true. My wagon developed a leak in the booster, pedals were solid with no vacuum, with vacuum they were spongy as all get-out. Stuck my head under the dash, pushed the pedal in 1/2" and heard a constant hiss, which shouldn't happen. Given the booster was deemed failed at that point, I swapped in a Cardone which leaked even worse. Took a Mityvac to the yard, found one that held vacuum, pulled, installed in mine, pedal came right back, been flawless since. No other changes to the system.
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Old 10-12-2020, 03:20 PM   #25
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A vacuum booster will never be the source of spongy brakes. Take a look at a cut away of a braking system. The booster either applies assist to the braking force, or, it doesn't. There is a solid connection all the way to to back of the hydraulic piston in the master cylinder once the pedal is pushed far enough to close the valve on the booster.

https://www.google.com/search?source...8OQCnnEAOg24dM

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