Brake booster or master cylinder?

Posted by Ace_gst, Mar 15, 2012
Handling Tech - 4G63 suspension, steering, brakes, tires, lightweight wheels, bushings, etc.

  1. Ace_gst

    Ace_gst Proven Member

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    I am having a problem with my brakes, they don't work. I have blead them like four times but they don't stay stiff or really get stiff. Just falls to floor unless i pump them but they always goes back to soft. How do i know if its the booster or master cylinder. Iv checked for leaks every where too.
     
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  2. SmoochDarling

    SmoochDarling Proven Member

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    My bet would be on Master cylinder. Sometimes they will leak very slowly out the back, and not be able to effectively seal. Brake boosters rarely go bad.

    Go over every inch of your lines. If your fluid is going down, find where it is going.
     
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  3. Ace_gst

    Ace_gst Proven Member

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    That's what i was thinking was master cylinder. Plus im not lossing fluid. Just could never.get peddle to stiffen up.
    Just to add some info this is my first time messing with the brakes on the car i just got it running. :)
    Ill let every one know if it was the cylinder.
     
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  4. gusu

    gusu Proven Member

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    What method of bleeding are you doing? If you don't bleed each caliper the right way sometimes it wont get all the air out. I've gotten away with bleeding a single caliper but you can't rely too heavily on that.
     
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  5. TrevorS

    TrevorS Proven Member

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    The booster isn't required in order for the brakes to function properly, all it does is make them more sensitive to foot pressure -- hence the word "booster". Your brakes should function correctly with the engine off parked in the garage, you'll just have to push harder. If they don't, then the problem is somewhere in the hydraulic system.
     
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  6. Ace_gst

    Ace_gst Proven Member

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    I bled all four calipers like four times. I already have the master cylinder orderd well see if it fixes it.
     
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  7. TrevorS

    TrevorS Proven Member

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    Well, if the brakes don't work (with or without the booster), you can be assured you have an hydraulic leak. Hopefully it's the MC, but if not, then it's something else in the chain -- best wishes, dude :)!
     
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  8. Ace_gst

    Ace_gst Proven Member

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    Well i installed the new MC today and i got it stiff while the car was off but as soon as i started it up the peddle will fall to the floor. When i tried to pump the brake with the car running it sounds like it wants to die and it reads 16-18 afr. Once i let off brake the afr goes back to 14.3-14.8ish. I didn't notice any leaks and my fluid is not going down so im lost.
     
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  9. jtmcinder

    jtmcinder DSM Wiseman

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    Your check valve is probably either backwards or broken.
     
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  10. miguelmcv

    miguelmcv Proven Member

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    How did you bleed them?.
    Let us know how you bleed the line, so we can think on what could be the problem.
    It looks to me like, there still air in the lines.
     
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  11. Ace_gst

    Ace_gst Proven Member

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    I bled the passenger rear then driver rear, not much air there. Then i bled the front driver side and passenger which had alot of air. They felt stiff while and worked until i started the car.
    Where is this check valve at?
     
    Last edited: Mar 19, 2012
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  12. miguelmcv

    miguelmcv Proven Member

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    The valve is on top of the Fuel filter, all brake lines are connected to it.
    How did you bleed them?. What method did you use?.
    Did you drain the oil and put more oil in the reservoir?.
    Did you bleed it from the line where meet the caliper, or from the valve bleeder?.
    Did some one help you to push the brakes while you bleed it?.
    Let us know how exactly you bleed each line.
     
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  13. Ace_gst

    Ace_gst Proven Member

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    I drained the resivor completely. I bled them from the bleeder valve till i didn't see air. I did the old "pump.... OK hold it" to bleed from the bleeder valves while my buddy worked the peddle. The backs had like no air but the fronts where full of it. Sure it because i only primed the back out let of the MC OMGhdamn but i think i worked all the air out the fronts.
     
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  14. miguelmcv

    miguelmcv Proven Member

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    If you drain the reservoir, then there should be more air in the line.
    You shouldn't drain the reservoir, Draining the reservoir makes the system to get more air.
    Did you bleed the lines at the cylinder?.
    If you didn't, then you still have air in the system.
    Draining the oil in the reservoir, will allowed the air to get into the lines and more bleeding is required.
    Imagine, Air going into the cylinder and getting out of the caliper, that will take a lot of oil bleeding, Keep bleeding until you think that is enough oil bleed it.
    The air has to run from the cylinder to the lines (from the cylinder to the Valve), then passes to the valve, then goes to the lines again (from the valve to the caliper), and last, goes out to the caliper.. That requires some kind of oil,,, Keep draining But never let the reservoir to get low, other wise you will never bleed the air from the lines.
    The rear lines will take more time to bleed which they are more longer then the front, so be patient.
     
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  15. bryanwheat

    bryanwheat DSM Wiseman

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    IT is leaning out when you push the brakes which would indicate that there is a vacuum leak in the brake booster.
     
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  16. miguelmcv

    miguelmcv Proven Member

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    It shouldn't be backwards?.
    If there is vaccum leak at the booster, then after you push the brake, it has to be harder, not lighter, which leaking air from the booster gives less assistance from the booster and makes the pedal harder, which is totally opposite from air in the system, which it is a light felling in the pedal.
    I don't know if I understood what you were trying to say.:confused:
     
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  17. bryanwheat

    bryanwheat DSM Wiseman

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    IT could be that it is only leaking when you push the pedal. There is no other reason for the engine to go lean other than a vacuum leak in the booster.
     
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  18. kchaazz

    kchaazz Proven Member

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    If you pump the brakes on any car it will lean out a bit and also youll notice a big fluctuation in vacuum on your boost gauge, this is normal. It leans out because the brake booster forces some air into the intake manifold when you push the pedal. The booster gets air through the pedal side and draws in air every time the pedal comes up. There is a check valve in the hose coming from the booster to the intake manifold. Check valve prevents turbo boost from entering the booster, and the valve also requires a little pressure on the pedal side combined with engine vacuum to crack open and only does so when you push the brake pedal and force air into the valve. Basically: push pedal, valve opens, engine vacuum pulls air from the booster, thereby making the brake pedal easier to push. An easy way to check for a bad booster/check valve is to turn off the engine and pump the pedal- the pedal should get rock hard, and immediately soften when you start the engine.

    OP: You have air in your lines for sure. Try bleeding the master cylinder by cracking one line at a time on the master while someone gently pushes down on the pedal- dont pump it, itll make a mess. Have a towel to hold and catch the fluid so it doesnt spew everywhere. After the master is free of air, bleed the Passenger rear, Driver rear, Passenger front and Driver front.
     
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  19. bryanwheat

    bryanwheat DSM Wiseman

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    A little lean is normal, but 16-18:1 is very lean. He also mentioned that the car almost dies when you push the brake pedal. Air in the lines won't cause either of these problems.
     
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  20. kchaazz

    kchaazz Proven Member

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    Well, of course air in the hydraulic system wont cause this at all, however, if you sit there and pump the pedal continuously, it will lean the car out, engine vacuum will drop and the engine will start to bog down.
     
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  21. jtmcinder

    jtmcinder DSM Wiseman

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    The valve that Miquel is talking about is the proportioning valve, not the check valve. Ignore him.

    The check valve is the lump in the hose between the intake and the booster. When it's broken (or backwards), you'll have the symptoms described by the OP. Verify that air can flow from the booster to the intake, but not the other direction. If the valve passes this test, move on to checking the booster, itself, for leaks, as suggested by Bryan.
     
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  22. Ace_gst

    Ace_gst Proven Member

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    Ok ill bleed them out today after work and check the valve. I don't remember a lump in the Jose to ill have to check that.
     
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  23. SmoochDarling

    SmoochDarling Proven Member

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    1+

    I had a no-brakes situation after i left my car sit for over a year. At the time I was also diagnosing a poor idle and running condition.

    Whenever I pumped the brakes my car would stall out and die. The only way I could build pressure in my brake system was to rev it up and pump the brakes repeatedly. It would immediately want to die, and pressure in the brake would decrease quickly.

    I immediately figured my Brake Booster was leaky cause of the idle dropping. So i popped another one in the car and had the exact same situation. I checked the one way valve in the booster line and it was good. *note a good check for the brake booster is to put a vacuum line up to the BB and bypass the check valve and then attach it to pressurized air. it should not leak! Just don't over pressurize it.

    So I ordered a master cylinder after bleeding all my lines and still having the same problem. Once I got master cylinder hooked up, it was all good. But make sure you bleed the lines correctly, just like kchaaz said.
     
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  24. Ace_gst

    Ace_gst Proven Member

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    Ok thanks every one im going to bleed a whole bottle of fluid and see where it gets me.
    Also iv been bleeding with car off should i try with it running.
     
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  25. kchaazz

    kchaazz Proven Member

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    ^^ Its easier with it running, in my opinion...
     
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