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    Thread: Can't Solve: Very Low and Soft Brake Pedal!

    1. Member '88Jetta16v's Avatar
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      10-01-2009 08:11 PM #1
      Update/Conclusion at the bottom

      Okay, I have a full Corrado G60 11" front brake swap on my GLI. Here's a list of the parts in the entire braking system:

      -Brand new aluminum ATE master cylinder 22mm
      -Brand new (rebuilt) Girling 54 front calipers
      -Brand new (rebuilt) Girling 36 rear calipers
      -Brand new PBR MetalMaster pads (front and rear)
      -Brand new GLI proportioning valve properly adjusted to compensate for lowered ride height
      -Brand new Autotech stainless braided lines (all 6 of 'em)
      -Brand new hardlines (all of 'em)
      -Brand new Castrol GTLMA DOT 4 fluid

      What I'm trying to say is the entire braking system is NEW.

      The master cylinder was properly bench bled before installation.

      The entire system has been bled through completely THREE times now! The fluid is clear and clean each time w/ no bubbles.

      Brake fluid is at the proper level in resevoir.

      I have checked the system through entirely multiple times for leaks. There are NO leaks.

      Symptoms:

      1.) When I push the brake pedal, I get almost no response until the pedal is more than halfway through its travel.

      2.) It feels soft when it finally does engage the brakes. However, if I sit at rest, I cannot push the pedal to the floor (as if it were leaking) but it does feel soft.

      3.) It is too high effort to stop the car and I really need to stand on the pedal. This thing should stop like it slammed into a brick wall.

      My brother has the exact same setup on his '92 GTI and it is wonderful! The pedal is high, hard, and the car stops on a dime with little effort.

      Possible Fix

      The only variable left in the system that I can see is the vacuum booster. I'm not entirely sure how it would give the results I'm experiencing if it was worn out or leaking, but maybe?

      The booster is original to the car (which btw, came with 9.4" front discs) and has 205,000 miles on it.

      Please help!!

      Thanks


      Modified by '88Jetta16v at 8:13 PM 10-1-2009


      Modified by '88Jetta16v at 12:59 AM 10-11-2009

      1988 Tornado Red Jetta GLI 16v / 1989 Cinnabar Red BMW 325is / 1989 Tornado Red Jetta GLI 16v / 1989 Bright Blue Metallic GTI 16v /

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    2. Member '88Jetta16v's Avatar
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      10-01-2009 09:45 PM #2
      Autocrossing this weekend, hope to have this fixed.
      1988 Tornado Red Jetta GLI 16v / 1989 Cinnabar Red BMW 325is / 1989 Tornado Red Jetta GLI 16v / 1989 Bright Blue Metallic GTI 16v /

      1992 Montana Green GTI 16v / 1992 Black GTI 16v / 1992 Alpine White GTI 8v / 1998 Black Jetta GLX VR6 / 2001 Silver Metallic Audi A4 Avant TQM

    3. 10-01-2009 09:46 PM #3
      booster

    4. Member '88Jetta16v's Avatar
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      10-01-2009 10:10 PM #4
      Would you mind explaining why you think the booster is the issue? Thanks.
      1988 Tornado Red Jetta GLI 16v / 1989 Cinnabar Red BMW 325is / 1989 Tornado Red Jetta GLI 16v / 1989 Bright Blue Metallic GTI 16v /

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    5. Member regendub's Avatar
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      10-01-2009 10:14 PM #5
      i second the booster idea. In class last semester we had a Toyota truck that went through multiple new master cylinders until we figured out that the booster had a bad seal and was leaking, causing the pedal to be soft, and no brakes.
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      10-01-2009 10:47 PM #6
      Only thing left is booster. If everything is new (or like new) and the master cylinder was properly bled and the brakes are as well it has to be booster. You've got pressure just not enough right?
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    7. Member spoon!'s Avatar
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      10-01-2009 10:55 PM #7
      It doesn't seem like you're getting any help here... probably because there are not many tech heads in this forum. Try thesamba.com

    8. Member '88Jetta16v's Avatar
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      10-01-2009 10:57 PM #8
      Quote, originally posted by ennui_delphian »
      You've got pressure just not enough right?

      Correct.

      1988 Tornado Red Jetta GLI 16v / 1989 Cinnabar Red BMW 325is / 1989 Tornado Red Jetta GLI 16v / 1989 Bright Blue Metallic GTI 16v /

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    9. 10-01-2009 11:28 PM #9
      booster is leaking or filled with fluid

    10. Member F3t1sh's Avatar
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      10-01-2009 11:43 PM #10
      I say the booster might be the issue and I'd upgrade the MC to a 24mm one...
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    11. Member mimisan's Avatar
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      10-02-2009 12:11 AM #11
      First you can check the booster vacuum source and check valve simply by starting the engine.As the engine runs,depress the brake pedal.If the idle drops,you have a leaking booster diaphragm.

      Next,actually pull the hose off the booster.You should have LOTS of vacuum at hose from motor.Try it also with engine off after idleing for a few minutes.You should also hear vacuum from booster being replaced by air as you pull it off.

      Backing up a couple of steps,pull the master back of and make sure there isn't any fluid in the bottom of the booster.Usually the engine would suck this in,but just check.

      The next thing I would check is the freeplay in the pedal.Pushing down on the pedal,it should be about a 1/4 inch before you "feel" the pedal hit the rod.Adjust if necessary.

      The last thing and maybe what the problem may be is the rod between the booster and the master cylinder.The depth of where these would touch may be different now that you've got a M/C that was not originaly for this car! Your bentley has the measurement for what it should be,no matter what size the M/C is now.

      Good luck with the project,and let us know the fix!!!


    12. 10-02-2009 07:52 AM #12
      Quote, originally posted by mimisan »

      The last thing and maybe what the problem may be is the rod between the booster and the master cylinder.The depth of where these would touch may be different now that you've got a M/C that was not originaly for this car! Your bentley has the measurement for what it should be,no matter what size the M/C is now.

      good point, this happened to my buddy. the rod that contacts the plunger in the MC actually broke off and wasnt making full contact


    13. 10-02-2009 09:36 AM #13
      I would also double check and make sure the rear calipers have the bleeder screw in the proper orientation..

    14. Member '88Jetta16v's Avatar
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      10-02-2009 04:29 PM #14
      Quote, originally posted by weefek »
      I would also double check and make sure the rear calipers have the bleeder screw in the proper orientation..

      What do you mean by this?

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    15. Member '88Jetta16v's Avatar
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      10-02-2009 04:30 PM #15
      Quote, originally posted by mimisan »

      Good luck with the project,and let us know the fix!!!

      Great advice, thanks! I'll look into each of these.


      Modified by '88Jetta16v at 7:38 PM 10-2-2009

      1988 Tornado Red Jetta GLI 16v / 1989 Cinnabar Red BMW 325is / 1989 Tornado Red Jetta GLI 16v / 1989 Bright Blue Metallic GTI 16v /

      1992 Montana Green GTI 16v / 1992 Black GTI 16v / 1992 Alpine White GTI 8v / 1998 Black Jetta GLX VR6 / 2001 Silver Metallic Audi A4 Avant TQM

    16. Member '88Jetta16v's Avatar
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      10-02-2009 07:38 PM #16
      Quote, originally posted by mimisan »
      First you can check the booster vacuum source and check valve simply by starting the engine.As the engine runs,depress the brake pedal.If the idle drops,you have a leaking booster diaphragm.

      No problem here, it seems. Idle does not drop when depressing the brake pedal.

      Quote, originally posted by mimisan »

      Next,actually pull the hose off the booster.You should have LOTS of vacuum at hose from motor.Try it also with engine off after idleing for a few minutes.You should also hear vacuum from booster being replaced by air as you pull it off.

      Haven't tried this yet. I'm a little worried about breaking the old brittle plastic elbow fitting at the booster.

      Quote, originally posted by mimisan »

      Backing up a couple of steps,pull the master back of and make sure there isn't any fluid in the bottom of the booster.Usually the engine would suck this in,but just check.

      No problem here.

      Quote, originally posted by mimisan »
      The next thing I would check is the freeplay in the pedal.Pushing down on the pedal,it should be about a 1/4 inch before you "feel" the pedal hit the rod.Adjust if necessary.

      This checks out correctly, too. Almost exactly a 1/4" travel before pedal hits the rod.

      Quote, originally posted by mimisan »

      The last thing and maybe what the problem may be is the rod between the booster and the master cylinder.The depth of where these would touch may be different now that you've got a M/C that was not originaly for this car! Your bentley has the measurement for what it should be,no matter what size the M/C is now.

      Haven't check the measurement or anything, but...

      I have read in more than one place that the booster for all non-ABS Mk2s and G60 Corrados is the same exact specification unit.

      Further supporting that this would not be the issue is the fact that my brother's '92 16v GTI has the same brake setup I do and is not having this problem. And he's using the original booster. And that car also came with 9.4" front discs and a 20mm MC.

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    17. Member '88Jetta16v's Avatar
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      10-02-2009 07:44 PM #17
      Observations I just made:

      1.) When the engine is off, I cannot pump up the brake pedal enough to make it sit right at the top and move only a half inch or so under pressure as it should. Now matter how much I pump it, it still sinks 3 inches or so before becoming immovable.

      2.) Looking at the rear of the booster from inside the car where the rod passes into the booster: There is a lot of rotten orange foam around the rod that is crumbling. Much of it is missing already.

      3.) I hear lots of air rushing out down in the driver's footwell when depressing the pedal and sticking my head in the footwell.

      Does that help at all?

      1988 Tornado Red Jetta GLI 16v / 1989 Cinnabar Red BMW 325is / 1989 Tornado Red Jetta GLI 16v / 1989 Bright Blue Metallic GTI 16v /

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    18. Member waterwagon's Avatar
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      10-02-2009 08:19 PM #18
      Sound like the booster is bad to me. If you can hear air rushing out of it, than it is shot.

    19. Member mimisan's Avatar
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      10-02-2009 09:35 PM #19
      You can also try to isolate each corner by pinching off brake hoses.If pedal gets firm,problem at calipers,then release one at a time to find which one is bad.

      If pedal still soft,is it possible master cylinder is bad?Is this NEW master or used
      from????

      I think the foam was mostly a sound dampner and filter for dust not to enter into back half of master.

      Is a power booster required with disc brakes? No. Although a power booster gives a good pedal feel, manual brakes work fine.

      What are the symptoms of a bad power booster? A bad power booster will give a very hard pedal, it will feel like you need two feet to stop the car.
      What does a booster do? A power booster helps assist pushing the master cylinder piston when you apply the brakes.

      How much pressure should I be getting to the wheels with a power booster?
      Typically you should expect about 1000 psi. to the wheels for a disc brake system. A disc brake system requires this amount of pressure so be careful when using a smaller 7" booster that puts out only 900 psi pressure.
      How much vacuum is needed to operate a booster properly? For a power booster to function properly you will need at least 18" of vacuum. Anything lower will give you a hard pedal.

      I have power drum brakes, can I use the same booster if I change to disc brakes? Yes. Just be sure to use a disc brake master cylinder.

      What happens if the vacuum is too low?
      If your vacuum level is too low you will experience a hard pedal and it will feel like the vehicle won't stop.

      How will I know if my booster is bad? A bad booster will give a very hard pedal and it will feel like the vehicle will not stop.

      How do I check to see if my booster is operating properly? Shut off the engine. Depress the brake pedal a few times to evacuate the booster Apply a steady pressure to the pedal and start the engine. The pedal should fall slightly.


      Will I need a special booster with 4 wheel disc brakes? The main consideration with 4 wheel disc brakes is that you have plenty of power assist. Don't undersize your booster with a 4 wheel disc set up.

      When I step on the brakes the pedal feels good but I get no braking.What could cause this? One of the causes of this is a mismatch between the booster pin length and the depth of the master cylinder piston hole. Be sure the hole is not too deep for the booster pin.

      I have installed a new booster on my car and now the brakes drag. Again this can be due to a mismatch between the booster pin and the master cylinder piston. Too long a pin will cause this.

      I have installed power brakes on my origional manual brake car and now the brakes are extremely sensitive. There are two attachment points on the brake pedal.For power brakes you need to use the lower attachment hole. Using the upper hole will make the brakes too sensitive.

      Power boosters keep going bad on my vehicles. You are probably getting corrosive vapors back into the booster. Install a vapor trap and that should eliminate the problem.
      CanI rebuild my own booster? This is not a good idea. There are many parts inside the booster that will require special tools to assemble and re assemble.

      How does a booster work?
      A power booster uses atmospheric pressure to help assist the pushing of the master cylinder piston. Basically, a power booster is divided into two chambers internally with a rubber diaphragm seperating each. With no force applied to the booster there will be vacuum on both sides of the diaphragm. This vacuum is supplied by the engine either from the intake manifold or the back of the carburetor.

      When you press on the pedal, the rod opens a poppit valve in the booster that allows atmospheric pressure on the firewall side of the diaphragm. This causes the diaphragm to push on the master cylinder piston thru a plastic hub, providing power assist to the master cylinder.


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      Modified by mimisan at 7:41 PM 10-2-2009


    20. 10-02-2009 09:45 PM #20
      mk2 forum at its best


    21. 10-02-2009 10:07 PM #21
      Quote, originally posted by ’88Jetta16v »

      What do you mean by this?

      The bleeder screws must be pointed up , and be at the highest point on the caliper possible. I had mine on backwards once and bled the sh*t out of them, no air came out, but there was still air in there because the bleeders were pointed down


    22. Member '88Jetta16v's Avatar
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      10-03-2009 10:06 AM #22
      Quote, originally posted by weefek »

      The bleeder screws must be pointed up , and be at the highest point on the caliper possible. I had mine on backwards once and bled the sh*t out of them, no air came out, but there was still air in there because the bleeders were pointed down

      I see what you mean, now. Both rear calipers are on correctly.

      1988 Tornado Red Jetta GLI 16v / 1989 Cinnabar Red BMW 325is / 1989 Tornado Red Jetta GLI 16v / 1989 Bright Blue Metallic GTI 16v /

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    23. 10-03-2009 11:15 AM #23
      i had to fully compress my rear calipers and use a block of wood to hold it at full in, then bleed. there were tiny air bubbles hiding

    24. Member '88Jetta16v's Avatar
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      10-03-2009 02:32 PM #24
      Any more opinions or thoughts? Consensus seems to be the brake booster. I'll see if I can get a good used one. GAP doesn't carry them and AutohausAZ has them for $210.
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      10-03-2009 07:27 PM #25
      i got my booster at 800-vw-parts for about $80.

      Also i seen a few newer VW that i worked on at my job is to pull the e-brake (parking brake as some would say) as far up as it can go then bleed the brakes.

      If it's worth building, it's worth over building.

    26. Member '88Jetta16v's Avatar
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      10-11-2009 12:46 AM #26
      UPDATE:

      Went to the junkyard and all they had were two 1990 LA1Z 4-door Jettas. Here's the catch: one was built in 9/89 and had small bumpers and one was built 2/90 and had big bumpers. That also means, apparently, that they came with different brake boosters. I had been told repeatedly that all Mk2 brake boosters (except diesel) were the same. NOT TRUE!

      The early '90 had a booster just like the one in my '88 GLI. Made by F A G and originally cadmium plated; now looking rusted, corroded, and silver/grey in color. The key factor: the pushrod inside is NOT threaded for length adjustment.

      The late '90 had a booster just like the ones in my '91 and '92 GTIs. Painted black from the factory and made by ???. The key factor: the pushrod inside IS threaded and adjustable in length.

      I yanked the one from the later car (had about 110k) and installed it in my GLI without checking or adjusting the pushrod length against my master cylinder.

      Observations with new booster:

      1.) FAR less effort required to apply brakes. With a good booster in the car now, I realize just how ridiculously hard I had to stand on the pedal before.

      2.) Pedal height is much higher at initial braking point, probably 50% higher than before. Still not ideal, but that's what the adjustable pushrod is for!

      Conclusion: Old booster was shot and not adjustable


      Modified by '88Jetta16v at 10:21 PM 10-11-2009

      1988 Tornado Red Jetta GLI 16v / 1989 Cinnabar Red BMW 325is / 1989 Tornado Red Jetta GLI 16v / 1989 Bright Blue Metallic GTI 16v /

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    27. Member Greg_J's Avatar
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      10-11-2009 01:05 AM #27
      glad you got it solved

      i would have gone insane trying to trouble shoot that issue. i had similar symptoms and turned out i had a bad MC

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    28. Member mimisan's Avatar
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      10-11-2009 09:55 PM #28
      Thanks for posting the fix,now it's out there for everyone to benefit from!!!!!!!!!!!!!




      Modified by mimisan at 7:57 PM 10-11-2009


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