An interesting thing happened to my car last weekend; I did an autocross event with it. The way the course was designed, I'd come into the "stop box" at about 65 mph, so the stop box became a pretty massive brake event; every time I'd have ABS engaged from ~65 all the way down to a walking speed. We did 7 runs, so there were 7 pretty extensive ABS events -something that's not happened in the 7 years that I've owned the car.

Anyway, car performed fine (first place in my class ), but when I drove it the next day, I noticed two "symptoms";
1. When the ABS pump cycles after start up/during the first drive away, it's very quiet now. For the past 7 years of ownership, the pump cycling has been very loud/prominent -even with the bin lid closed. How loud? Loud enough to be heard over engine noise. Louder than the AIR pump. Loud enough that people have asked, "What what THAT!?" I've always figured that it was normal...but now I wonder if it wasn't normal and the quiet cycle now (I can barely hear it IF I'm listening for it), is what should be normal. How loud is your ABS when it cycles at first drive away?

2. My pedal is soft on first application. I'm pretty sure that my 180k mile master cylinder is bad. If I do a light pump, then apply, brakes behave normally. If is simply depress the pedal, it goes about 1/2 way down before getting firm and I sometimes get a brake light on. As I depress the pedal on that first application, pressure does start to build at the point it should, I can feel the pads hit the rotors...but then it's as if some seal gave way, pedal goes soft for another 2" of travel or so, before building pressure and braking in earnest begins. Anyway, I can diagnose the soft/first pedal part fine -I only posted in case someone felt that there was a correlation between the now quiet ABS cycling, and the pedal. I'm guessing that the ABS shouldn't be loud, and has always needed to be aggressively used...and I haven't done that until now.

 

Its possible the abs unit is having issues but what I bet happened is you boiled your fluid or at least stuff that was in the lines.

Try a powered brake bleeder and flush the lines with it and I bet it goes back to the way it was.

Let me see if I can find the link to the one I bought. Works miracles on abs issues like that.

 

This is what I used. I got mine from habor freight but this is the same one and this one comes with the air compressor fitting that I had to buy separate so this ones a better deal.

https://www.amazon.com/vidaXL-Brake-...437800&sr=1-72

 

Ive had that same issue happen on two different 4wd trucks after heavy offroading and bleeding with that gadget was the only thing that fixed them.

 

Yeah...thanks for the reply, but there is no way I boiled the fluid. C5 front brakes, Motul RBF brake fluid (fresh), Hawk HP+ pads....I can do an entire track day on that set up w/o any issue (I have, but no ABS intervention), so one 65-0 stop every 20 minutes or so isn't going to do it.

 

Don't get me wrong; I think you're right that step 1 is to bleed, and I'll do exactly that, to start with.

 

The bleeder gadget I listed is the only thing Ive found that really flushes the abs system is why I mentioned it. You don't have to have to boil the fluid traditionally to boil stuff that normally stays trapped in the abs unit. Its weird but I but I have had the junk stuck in the abs so stubborn that even after 4 back to back normal bleedings still had pedal issues and one bleed with that gadget and it was better than when I bought the vehicle.

If someone had of told me this before I bough the thing I wouldn't of believed them but it really does that good of a job. It probably even cleans out trapped air in the wheel cylinders too. Thing really works. That fill bottle that comes with it that keeps the reservoir topped off is handy too and the key to how it gets so much fluid thru fast enough to clean out the system.

 

Copy all of that. Thanks! Any input on the loudness of the ABS pump cycling after start up?

 

Are you able to confirm to see if there is actually a problem with the wheel speed sensor signals? The first place I would start is connect a scanner and see if there is an uneven signal between all the sensors during normal driving, normal braking, and hard braking.

 

Yep liquids carry sound differently than air does. So if it has air/foam trapped inside its going to sound very different then if its pure liquid.

As good a theory as any but it could be something else too. Its worth a shot power flushing. It has really solved several nagging problems I had of late with different vehicles here.

The worst was my 1999 f150 pickup. Its brakes sucked since buying it from a friend. Spongy till pumped at least once. No amount of normal bleeding helped. I even replaced the abs unit since it was acting a bit weird too. Still same spongy pedal.

I flushed it probably almost 20 times total between attempts before and after the abs unit swap.

One bleed with that gadget and it has a full brake pedal now. Horrible crap out of the lines too when I flushed with it.

Your mileage may vary so to speak but I wish Id of had that thing 20 flushes ago

 

I'm not sure where this is going....I have a soft pedal on first application. No codes, no ABS light, ABS functions fine and normally...other than the cycle-after-start-up sound...which I still don't know is normal now, or was normal before.

 

i dont want to get off topic but i was curious why this one is so good crowz? how would it suck other crud out yhat other hand pump vacuum systems would miss?

 

It uses your air compressor to provide a powerful CONSTANT vacuum pull. Hand pumps I have tried in the past have 3 problems for me at least.

1. You never get that much of a pull since your "pumping" up the vacuum applied to the line.

2. Its either problem #1 or its more a pulse effect in place of a steady constant pull. (pulls when you squeeze and lets off before you get another squeeze on the pump handle)

3. I give out long before the jobs done

100+psi of air shooting thru causes a nice constant and strong vacuum effect to blast the fluid thru stronger and more even than the hand pumps I had tried before. The lever on this one is not a pump. Squeezing the handle on this one causes the air compressors air to shoot thru the handle creating the vacuum. Squeeze till bled then let go of the handle.

It catches the old bad fluid in the tank that the handles on the other bottle you fill with fresh fluid and close the valve on it. Turn it upside down into the neck of the master cylinder and then open the valve. Whenever the fluid level drops from you draining it out it auto refills it as its draining.

 

I wonder if I could get a comment as to how noisy the ABS cycle after start up should typically be?

 

87 auto, mine is not as noisy as say the aeirial motor. its about as noisy as the motors that run the headlights.

 

Id go with the same noise as the headlight motors too. Very noisy to me, definitely louder than any other vehicle I have ever owned or driven.

Scared the crap out of me when I first bought my 86 when it cycled for the first time.

 

I figured it out.

Brake fluid was low in the res. Under hard braking it would slosh/move forward in the res, exposing the rear port (front brakes) to air. It ingested air.

Bled out this past weekend....back to normal.

 

Since you are doing these events have you considered the larger bore MC? Several here have done it this is one review I know of. I want to do it someday too

from this thread:

https://www.corvetteforum.com/forums...r-upgrade.html