Brake System Problem.

[SupraOfDoom]

So, I had done my front brakes about 2-3 months ago, rebuilt front calipers, rotors, pads, gr ss lines ect. I bled the brakes after this and made sure to get all the air out ( or so I thought ). Pedal felt fine 75% of the time, but the other 15% it would feel mushy. So, I end up having my rear brakes start scraping literally a week after I do the front brakes. I do my rear brakes, bleed the system again ( Again, no bubbles coming out ) and this time it feels considerably better, maybe 95% it feels perfect and stiff. Sigh, I don't get it but it's working pretty good even when mushy. Now, as of yesterday the pedal just went from that to 95% of the time being mushy. My fluid hasn't dropped, and as far as I can tell I'm not leaking anything. How is this happening? Where could I be getting air in the system and how all of a sudden? It actually got to that point right after a slight brake test I did.

Where should I be looking now?

I've replaced all the DOT3 fluid with BMW DOT4 fluid ( They are not mixed, it's purely DOT4 ).

I've bleed the brakes properly multiple times, starting from the furthest away from the master cylinder.

Could it be the ABS system holding air bubbles that I couldn't just bleed out?

Think the problem could be solved with a pressurized system that attaches to the master cyl? I'm doing it the old fashion way.

Thanks.

 

[Zumtizzle]

must be the damn abs system holding air.

can you get a mityvac bleeder and just bleed the hell out of it?

 

[suprabad]

You said you had to "bleed the brakes several times". This leads me to believe that you have a cracked line or bleeder or some breach that is alowing air to be sucked into the lines. That's where I'd start.

 

[SupraOfDoom]

Well, 3 times to be exact. I will re-check them all again.

 

[IJ.]

Sit in the car engine off give the pedal 4 good squeezes then Hold pressure on it for 4>5 minutes and let me know what happens.

 

[SupraOfDoom]

**Goes out to car for IJ**

 

[IJ.]

** waits **

 

[SupraOfDoom]

Within about 30-40 seconds after I pump then hold pressure it goes to the floor gradually.

 

[IJ.]

You have just performed a "Pedal Height Retention" test

Congrats your master cylinder is stuffed!

 

[SupraOfDoom]

"Stuffed"? New term to me.

I don't understand how this would happen after doing the brakes. It worked 100% fine before that point.

 

[IJ.]

"stuffed" ummm fucked would be applicable

You'll probably find some seepage between the Master and the Booster, what the test indicates is there's either a damaged seal or pitting in the Master cylinder bore allowing the pressure to leak by (sort of like engine blowby)

Brake fluid is "hygroscopic" meaning it attracts water in normal use, in time this will pit the master and is why you really need to flush fluid on a regular basis (I did mine yesterday) the other downside to the water in the fluid thing is it lowers the boiling point of the fluid and in extreme cases it turns to steam which can compress and you lose all braking.

(I worked as a Hydraulics Tech for a few years when I was a kid)

 

[SupraOfDoom]

Dang. Alright. Thanks for the help .

Now... where to get a cheapish mc....

 

[IJ.]

I'm sorry maybe it's my background but "cheapish" and "brakes" makes me cringe

 

[Who]

Dang. Alright. Thanks for the help .

Now... where to get a cheapish mc....

Call Jeff @ Champion

Cheap + brakes = :kickinthenuts:

My two cents. Once you replace your master cylinder don't be overzealous when you bleed or pump your brake system with the foot pedal. In other words don't stand on the pedal like a 500 pound gorilla when bleeding your brakes. I learned that the hard way.

 

[IJ.]

^ why not?

If the system is going to fail somewhere else I'd much rather it be during a static test than later on the road.

 

[Poodles]

Bingo ^^

If you can't stand on it, what happens in an emergency brake situation?

Make sure you bench bleed it before install.

 

[IJ.]

Never bothered with a bench bleed Poo, I don't introduce fluid to a system until I'm ready for a full fill and flush as this reduces the risk of a spill and paint damage.

Old systems I first flush with a litre of Metho (denatured alcohol) to remove any residual moisture (the alcohol surrounds it and transports it out of the system)

I then flush through 500ML of fresh fluid top it up and bleed using a 1 man bleed kit and a mityvac pump.

I get a perfect solid pedal this way.

 

[Poodles]

I'd be worried with introducing a lot of air to the ABS block. With the mityvac pump you could get away with it pretty easy, but for someone bleeding it the shadytree mechanic way I think it would be tough...

 

[IJ.]

What is this ABS witchcraft you speak of heretic?

 

[Poodles]

MKIII ABS rocks, you should configure a custom 4 channel system with your MoTeC (stock is 3 channel)

It's nothing like modern cars where the ABS kicks in way too early and pisses you off...