Brake Master Cylinder Issues...

[93Rubie]

I did a SCR Region auto-x drivers school today and I had a blast. Learned a ton about me and my Corvette. Counting this event that is 10 so far. I love it, and I'm hooked.

Now for the bad part. My instructor took the wheel after my first 6 runs and he put down a 7th. On the 8th run, the brake pedal went to the floor, suddenly. This was NOT brake fade, and we concluded it was NOT air or anything similar. When this happened the red brake light came and on I noticed the ABS Active light was on for a little bit. He got the car stopped by coasting in gear and using the hand brake. W

We popped the hood, no leaks, plenty of brake fluid. Another guy pushed on the pedal and it came right back. However, we elected to park it for a bit and let the car cool and asses the situation.

After lunch, I went back to the car. Pushed on the brake pedal, pumped up as it used up vacuum in the booster and got hard. Did not bleed down either. Good firm pedal. Started the car, still had decent pedal, did not sink. No leaks. I did notice that there was a "bump" feeling in the pedal. Not sure if this was there previously or not.

We decided to give it a try. I took the first lap kinda easy, but the brakes where find. I went balls out the rest of the afternoon with NO issues. Using the brakes hard.

The theory is this: Some junk in the master cylinder (I vacuum flushed the entire system this spring via Mity Vac with 1 qt. ran thru it) got into the seal area and caused it to leak internally. When this happened the pedal went south and the pressure differential switch did its job with the red brake light on. When we hit the pedal again, it moved the junk and worked fine. I did observe lots of extra junk in the resivoir that was NOT there before.

Comments on this?

Also, what brand do you recommend on master cylinders? AcDelco seems to be non-existanct. Seems the Corvette parts places our low on them as well. I tend to shy away from the Auto Zone, etc...remans, considering the driving I do. Corvette Central does sell a rebuild kit. Assuming I can get the resivior clean the and cylinder bore is good, this might be the best option.

Comments on this?

Also, would you do a complete system flush again or just bleed at the fittings on the master when installing? Its obvious which is less time consuming.

The ONLY other thing I could think it COULD be is a ABS Malfunction some black magic one in a trillion chance thing. No codes when I got home so....I doubt it.





FYI, I asked the instructor what he thought of my C4. He said it was very well balanced. Easy to drive, no surprises, very controllable when out of shape. Turns well. Lots of torque.

He did ask me what kind of alignment I had on it. I told him -.5 a degree at all four, zero toe in front, and as much caster as I can get.
He made the comment to the effect, its amazing what a alignment will do for these cars.

I asked him what mods he might do to it? He said, tires and maybe shocks. Leave the rest alone.

He typically drives a fast Honda, this year a Porsche GT3. Nice guy, very good driver.

I continue to be amazed by the C4 how good it handles, (when driven right), brakes, accelerates. Its easy to drive nature, communicative (to me) chassis feel/steering feel, (I'm never surprised by it).

I was nailing the 1st half of the course, 5 cone slalom medium spacing, skidpad, and Chicago box. I came out of the box so fast I didn't brake soon enough for the next turn and kept screwing up the last part of the course because of it.:eyerole

I gotta get this thing fixed, when is the next Auto-X?

 

[captboatwrench]

I bled about a qt of nasty black fluid from my brake system this weekend , its so dark that you cant see through it in the jar that I put it in.
my brakes are still a littlle mushy feeling to me and my light still comes on periodically.
Im also in the market for a good master cylinder

 

[93Rubie]

Anyone?

 

[Hib Halverson]

Your car has the late C4 base brake system (ie: small front brakes) so, based on the symptoms (pedal to the floor after 8 runs then near full pedal after a cooling period), I'm going to guess there was nothing wrong with the brakes other than you boiled the fluid in the front brakes. If that happened, the brake light might come on because the fronts have boiling fluid and the rears do not.

When you boil the fluid, you "kill" it so first thing to do is flush the brakes including the ABS modulator valve then bleed and road test.

The late C4 base front brakes are notorious for this problem.

That said, if you're going to start autocrossing a lot, doing a major overhaul of the brake system, including the master, can't hurt. Another thing is to do some stuff to get cooling air to the front brakes.

 

[93Rubie]

Your car has the late C4 base brake system (ie: small front brakes) so, based on the symptoms (pedal to the floor after 8 runs then near full pedal after a cooling period), I'm going to guess there was nothing wrong with the brakes other than you boiled the fluid in the front brakes. If that happened, the brake light might come on because the fronts have boiling fluid and the rears do not.

When you boil the fluid, you "kill" it so first thing to do is flush the brakes including the ABS modulator valve then bleed and road test.

The late C4 base front brakes are notorious for this problem.

That said, if you're going to start autocrossing a lot, doing a major overhaul of the brake system, including the master, can't hurt. Another thing is to do some stuff to get cooling air to the front brakes.

The weird thing was it just happened all of a sudden. No warning. I would think boiling fluid would cause a spungy pedal. It never presented any symthoms of fade or air in the system. We stopped the car and with it still running it was not even 1 or 2 minutes and the pedal was back good.

The I bleed a 1qt. of fresh DOT3 thru the system this spring in March. SO the fluid is realtivly fresh.


This happened at a drivers school so I was having back to back to back runs with very little cool down time. Usually, I only get 5 runs in the morning and have at least 5 mins or so between runs.

As I said in my 1st post, in the afternoon the brakes where fine. However, we did stop mid way thru my session and let the cars sit for maybe 5-10 mins and cool down. I had no further issues.


I am going to rebuild the master and add in the bias spring to get more rear brake. I'm also going to upgrade to DOT 4 fluid.
The brakes (before they went out) where working very well with lots of good bite.

You cannot be too careful with brakes. Maybe time to upgrade to a J55 setup?

 

[Hib Halverson]

All I can say is your symptoms sound like the front brakes boiled the fluid. If you were using DOT3 fluid that adds strength to my belief. DOT3 fluid is crap for any performance application. For motorsports use on a car which is "under-braked" in the front, you need at least DOT4 (I like that Valvoline Synthetic DOT4) and preferably a street/track fluid such as Motul DOT5.1.

I suggest you "cover all the bases" and look at other possibilities, ie: failed master cylinder, problematic ABS and so forth, but if it were me, there were no visible exterior leaks and after bleeding with fresh fluid you have a hard pedal, I'd have been looking at boiled fluid as what caused your problem.

The brakes will work fine until the fluid starts to boil then the pedal will go away pretty quick. As soon as the fluid stops boiling, pedal will return but it may not be as high or hard as before.

When the fluid boils then stops boiling it is "damaged" and needs to be changed. Also, after a boiling incident, a fluid will boil the next time at a lower temperature.

 

[93Rubie]

Hib, after doing a bit of research. I think your right. I never would have guessed that the fluid boiling would let the pedal go away so quickly. I guess a lesson learned. I know how brake fade feels and this was NOT it.

I'm going to go to some DOT 5.1. I do not believe I have any master cylinder issues. The brakes work fine once they cooled down. Like I said, I ran it hard all afternoon as well, however, we did stop about 6 runs in and let the cars cool down. So....

The pedal does not sink and I had no issues driving home. NO ABS codes and I test drove it and ABS is functional. So I doubt at this point a mastery cylinder or ABS was the issue. I thought that earlier as I could not believe relatively fresh DOT 3 could boil that easy. I did not know the late C4 base brakes where this bad, makes me wonder how bad the early 11.5" brake are? :ugh

I'm going to drain the system and refill with DOT 5.1 and see how it does. The brakes where biting hard until the pedal went away. So evidently the pads can take it. However, I will pull them and check for heat cracks.

Like I said, this was a drivers school not a normal auto-x, no cool down time, lots of back to back runs.

Thanks. I didn't know the limits of this braking system, now I do.