Air in the abs system...how to bleed
 

Here is an interesting one...that I've not yet experienced...

A little background- i fully flush all the fluid in my C4 twice per year. Once in the spring and once in the fall. And as needed, i bleed the bomb and clutch and brakes through the season. Yes- I'm a little nutty about the car being "right"...and as was told to me a long long time ago "no car will ever hate you over fresh fluids"

So given that- there is virtually never air and never water in my system and as it gets two fluid flushes through all 13 points twice per year...all fluid everywhere is always fresh.

Now- it's snowing here in New England...

Attachment 1295719

And this is why we buy C4!!!!

I love to get out and drive before the plows come through. And while I'm out there, i like to use my abs - often...to get things moving, pistons pounding, and keep things alive. When we don't use things on the car, they will break themselves, and if something is on the verge of breaking, i make sure it does, so i know what needs to be fixed. It's just how i keep my 30 year old car "new"...

So out i went, playing around, slip sliding away, and then in safer areas, accelerate to a fair speed and hard on the brakes letting the little abs pistons pump away doing their antiquated magic, stopping the car hundreds of feet farther than an experienced human can, while convincing us that they do it better...anyway, they work.

But- here is the kicker. I started my evening with a hard pedal, and after some braking, i now have a very soft pedal. Leaving me to think there is air getting into the system or that there was air maybe somewhere in the abs pump or that part of the system that doesn't get bled...

Question is- is that possible? And if it is...how, beyond doing the full bleed, can we cycle out the fluid in that part of the system during a bleed?

yeah. When I swapped my pump I had a spongy pedal cause I couldn't get the brakes to bleed right. I used the c2 ABS test set I purchased here on RL to run the pump and cycle the locks while bleeding the 3 circuits. basically: pressurize w/ motive, get bottle ready on rear, open the bleeder, then run the pump, cycle the locks, then stop the pump, then close the valve. Do that all around. Then, do a regular bleed at each corner. That gets the air out of the pump, but you'll need to finish the bleed.

Make sense? I know the guy who made these made a C4 version.

I have no idea how the dealers would have done it back in the day. I assume their tools let them run the abs system? The newer cars have a function for this. On the durametric I can run ABS bleed on my 997.

^^^ wow awesome. Now, I need to look in my durametric again and see if it has brake bleeding function, I didn't think the durametric could activate the c2 ??

But the way you did above is same as I did but using the ABS test harness adapter. But same idea.

We have the C4 tester out here...so I'll have to get that and give it a shot.

As to shops properly bleeding a C4...i have lost all hope. So i do it myself. Now i just have to add at least an annual bleed of the abs as well...

We will be rebuilding an abs pump soon with 190 parts. I'm interested in getting to that. I also have a solendoid that I'll be doing a teardown on to have a peek inside. The abs is not mine, so that we will get to soonish, but the solendoid will have to wait until after intake madness is done. I'm half way!

+1 for an ABS madness thread!

I've worked through many of these thoughts as I developed the ABS test sets, and you guys are all right on point. The only thing that wasn't explicitly mentioned was that each of the three ABS servos are three state valves controlled by the supplied current and any cycling and bleeding should work through all states to bleed all the passages. The values I used came from Adrian's book (0 amps, 2 amps, and 5 amps) to put the valves in the "Normal", "Maintain", and "Reduce" states. For reference, a direct battery connection can provide the 5 amps, a series 5.6 Ohm (25 watt) resistor provides the 2 amps.

I've heard from a number of users (like Spyrex) that have had to just hammer the servovalves with the test set to successfully bleed the valves.

I've also disassembled the pump and servo-valves because I was curious. I expect that sourcing replacement seals will be the hardest part of us "enthusiasts" being able to do a full rebuild.

Good luck with yours Goughary!

Michael

Thanks Michael. Looks like I'll have to jump into abs madness sooner...although it's Dave's abs pump...soooooo dave? Let's get moving!

IIRC after looking at ABS circuits, the trick is to first push good fluid past the abs into brake lines so that when you open the ABS valves that let fluid flow back through the ABS you end up with good fluid in the ABS, but then old fluid upstream of ABS which needs to be flushed down past ABS again. Take a look at this thread. At Post #11 I come up with a procedure.

Seems like a lot of work, but I guess your experience with soft pedal is evidence fluid can go stale in ABS and have detrimental effect.

Dan

Spyrex- yes of course! Fun project...

Pm me and we can arrange..thanks!

On a C2 aren't the valves wide open (by default) when there is no power applied?

Hello Guys,
I have the same issue : air in the ABS unit that make a traditionnal bleeding fail : when contact is off, I can blead each caliper, but when I switch contact ON, the pedal goes soft again.
Note that I have bleed the pressure accumulator severail times successfully (no more air from there)

So I guess I have to use this jumper "1-3-5 VS 4 + 9" trick to open the valves.

I have questions though :
- 4 is common : I guess it means "ground" ?
- Is it necessary to open all 3 valves at the same time, considering you can only bleed on way at a time ? I guess I would need to know which pin drive which solenoid though...