Braking Problems?

[Op-Ivy]

Lately, I've been having some problems with my brakes. It's a bit hard to explain but I'll give it a go...

*When at a stop, I will push the brake pedal in as if to apply the brakes, and it will go nearly to the floor. When the pedal comes back up I will be able to push it to the same point, but there will be no regular braking resistance in the pedal. (It will be as if the pedal is a dead pedal with nothing attached to it, up untill the point that it was at previously on the last push)

*If I am not at a stop (driving forward) braking is fine, a little spongy, but fine. If I continue driving for a few meters, after the pedal acts this way, then braking is again fine...


-It will only do this when the engine is warmer then the regular operating temp.

-Just replaced the Master Brake Cylinder a few months ago. (Braking was fine after this, up untill a few weeks ago)

-Vacuum pump is sucking ok.

-Brake booster seems to be holding pressure.

-Check valve on line from Vacuum Pump to Booster is letting air in the correct direction of the arrow indicated on valve.


I am pretty stumped here... Is there a certain vacuum amount that the pump should be creating? Maybe it isn't making it when it gets hot. Would it make sense with the hot engine part? Could it be a bad Master Cylinder  that I put in?

Any help would be much appreciated!  :?

Thanks, Matt

[hoyt]

*When at a stop, I will push the brake pedal in as if to apply the brakes, and it will go nearly to the floor. When the pedal comes back up I will be able to push it to the same point, but there will be no regular braking resistance in the pedal. (It will be as if the pedal is a dead pedal with nothing attached to it, up untill the point that it was at previously on the last push)

After the second push are your brakes holding the car?  Or is there
no braking affect?  It is not clear to me....

*If I am not at a stop (driving forward) braking is fine, a little spongy, but fine. If I continue driving for a few meters, after the pedal acts this way, then braking is again fine...

Spongy feeling brake pedel usally means air in the brake lines.
Have you tried bleeding the system?  Did you bench bleed the
new MC before you installed it?

[Master ACiD]

just a wild guess but spongy brakes could be air in the lines or just old lines. you didnt say what year youre car is....

[Darkness_is_spreading]

I had a simlar problem to you, but mine was a very visual leak

[BellCityDubber]

I had similar symptoms too....
turned out it was the drivers side front brake line was rusted at the clip that clips it onto the frame rail by the battery tray...

new line... 5 bucks...

[Op-Ivy]

I am not leaking fluid. I do have a problem with air in my rear left brake line. I think I have a leak there somewhere.. I get air bubbles every pump when bleeding...


Hoyt, after the 2nd push I still have breaking power but only at my last push and farther.

I figured out today that after I rev the engine, the brakes go back to normal. Seems to me this would indicate something to do with the Vacuum Pump?


EDIT: 1990 TD Jetta

[hoyt]

I would suggest you get yourself a pressure bleeder.  Way easier
to bleed the system.   And if you bleed and old MC with the pedel meathod
you will ruin the seals in the MC.  You want something like this:

http://cgi.ebay.com/ebaymotors/ws/eBayISAPI.dll?ViewItem&category=111105&item=4642226499


I think your biggest problem is the air in the lines, fix that first.

How hard is it to push on the pedel?  Is it different with the
motor on or off?  With the motor off push 5 or 6 times, does
it get harder? or stay the same?

If the pedel does to get harder to push with the motor off, then
there is something wrong with the power brake buster or the vacuum
supply to it.

[LeeG]

Sounds like air in the system is letting you bottom out one of the pistons in the MC.  Why it would change when you rev the engine I dont know.  A failed booster should not make much difference to pedal travel.  
I would go looking for leaks, or a hose that has a big bubble in it when someone stands on the pedal.   If you dont find it, then push harder on the pedal....if something is going to break, its better it doesnt happen while you are driving.

You should be able to notice a big differance in braking effort if you are loosing vacum at times.  If you are not sure what nobooster braking feels like, shut the engine off, pump the brakes a few times to use up the vacum in booster, then roll down a hill and try brakes.

IMHO unless you are doing an awful lot of brake jobs the pressure bleeder is a waste of money.  I used to do nothing but brakes for a living and we bled 99% single handed with nothing but gravity....let one wheel drip for 5 minutes while you assemble other side.  once all have done that, pump pedal a bit to raise any bubbles to the bleaders, then let em each drip a bit more.  if you have trouble or want to flush system, open one bleader, put finger lightly over it as a check valve  and have someone slowly push the pedal up and down, repeat for each wheel...but never bottom pedal out on an older MC or you risk forcing muck into the seals.

[fspGTD]

Moved to troubleshooting.  IDI engine is for power enhancement related discussion only.

[shwartzbewithyou]

Did you ever resolve this?  I have the same issue that you're having.  I have the proper braking pressure while driving but when sitting at a light in the summer heat after I've already stopped, I can feel the brake pressure suddenly start to drop like I've sprung a leak.  I looked at all lines and their all good.  I have a proportioning valve and rear lines that are no more than 8 month old and have worked fine ever since.

Anyone?

[saurkraut]

$44.00 is nutz for a pressure bleader.  Make your own from stuff at a typical hardware store.  its really slick, one man opperation.

Not my invention, some guy on a BMW web page posted the directions:

Garden pump sprayer, toss the hose and nozzle. (I prefere the "ACE" hardware brand)

1/4 inch nylon tubing, I use 5' so i don't have to put the pump bottle on the car.

Need a break resevoir cap with a level sensor hole in it.

1/4" hose barb, a couple of big washers and something to thread it into on the back of the break reservoire cap.  I use a brass pipe union so when i unscrew the top of the pump bottle, it siphones the reservoire to almost the right level.

I also bought a pressure gauge, cause the BMW guy's directions said to not exceed 15 PSI or you'll blow the reservoire of the MC.  I never got around to installing it, I just give it eight to 10 pumps, then go bleed all the wheels.

When i made mine a few years ago, i think the whole bill at the hardware store was $17.

When i bleed my brakes, i loop a clear hose from the bleeder nipple through the suspension spring, so I can clearly see when the bubbles stop.  If you don't do this, you may be so fascinated by the flow of the break fluid that you will go through a whole quart of break fluide befor you know it.

I jumped on this technique because my Frau would get pritty ornery when I would ask her to help me bleed the brakes at night in the winter.

[LeeG]

I can feel the brake pressure suddenly start to drop like I've sprung a leak.

Could this also be described as the pedal slowly sinking to the floor?
A common failure mode for a master cylinder is for it to leak when there is only light pressure on the pedal -like when you are waiting at a light with your foot lightly on brake.

With the engine off, pump brakes until vacum is used up, then very lightly press the brake pedal down with just a bit more force than you need to overcome the return springs.  If you can get it to slowly sink to the floor then the master is leaking.  Sometimes using you hand on the pedal is more senstive to the pressure needed.

[saurkraut]

Found the link:

http://www.bmw-m.net/TechProc/bleeder.htm

[veeman]

I had a similar problem in my rabbit truck.  No matter how much I bled the brakes, it always felt somewhat spongey and the pedal travel seemed excessive.

Long story short, I checked the brake proportioning valve (aluminum block on driver's side inner fender well) and it was leaking out of one of the ports.  It's a slow leak...takes 2 months to have the level of the brake fluid get below the minimum level, but I'm sure that this is the problem.  I'd imagine that air is getting in there and the brake fluid is not being distributed as it should front to back.

Not sure what car you have, but it's worth a look.  

Also... FWIW, I use the Motive power bleeder.  It's great, but as others have said, you can make one out of a bug sprayer or a spare MC cap with a fitting on it (just watch your input pressure)...