IainM

'95 C2-6sp 120k, runs great.
I've been flushing the brake fluid every 2 yrs as recommended and replaced pads, rotors etc. a year ago. Brake feel and action is good in normal use, but if I brake hard to activate ABS the pedal drops a little and the cadence of the ABS seems to have changed, not as regular/rhythmic as I remember.
Also, I recently flushed the clutch slave circuit and ejected 25cc or so of black fluid, not a good discovery. My guess is that this is old, degraded fluid from the ABS circuit that somehow made its way into the clutch line? Maybe I just started using ABS?
I've had problems bleeding the clutch so I know about the clutch circuit pick up point in the reservoir being above the brake circuit and so I use the higher clutch point as the refill level when flushing the brake lines to ensure that no air gets anywhere into the system.
I'm also not sure I'm completely flushing the system and maybe even worst case, pumping the fluid from bottom of reservoir (with highest water content) into the calipers. I can't get any colors to use, must be a CA thing. Can I put food dye in my brake fluid? How do you stain ATS?


My questions are:

1: is it necessary to actuate the ABS system using hammer, PWIS etc to correctly perform the brake/clutch flush/bleed process?

2: .what is the capacity of the brake/clutch hydraulics circuits? I suspect it's more than 1 litre.

samurai_k

@IainM

I recently flushed and overhauled my clutch hydraulics.

I agree it was about 25CL to flush the clutch. However on my car, I had separate problems where black debris kept on coming out of the clutch circuit after flushing a 1L through the clutch circuit and the clutch pedal was still sticking after a few bled attempts. Eventually replaced the master/slave cylinders to sort my car out.

Kein_Ersatz

Normal brake flush will not fully flush the ABS without the "PST Time" (need to activate the ABS during a flush to remove old fluid surrounding/inside the ABS pump mechanism - depending on model/year/ABS version/OBD Hammer many not run the proper ABS cycle test). The step is usually skipped by the backyard DIY crowd, but needs to be done on a regular basis (YMMV) otherwise over 20+ years the wax in brake fluid gums up the pump. So at least every few full blake flushes, find a independent or advance DIY mechanic (with a PST tool) to perform a full flush to clean out the ABS paths/pumps. Run the cycle test at the start of flushing out each wheel/line to get new fluid in the ABS paths.

Having said the above, not aware of that brake fluid circuit impacting the clutch fluid circuit.

There are rennlist threads on this ABS flush topic and some suggestions on cleaning out the wax build up. YMMV.

nine9six

I would say no, to the food coloring in your brake fluid, despite beingmade in a lab with chemicals derived from petroleum, a crude oil product, which also happens to be used in gasoline, diesel fuel, asphalt, and tar.

1. Not "necessary" per se, but it's a good practice to avoid replacing a wax clogged ABS pump.

2. From a Geolab previous post:
The system holds precisely 0.9litres with level in container at top. (workshop manual), but the bleeding proceedure takes so much more.
I make sure I use a brand new can of brake fluid, and not an open can.

Brake fluid highly absorbs humidity, so use a new can. Do not use DOT 5.0 brake fluid if you have ABS/ABD, since DOT 5.0 is not for cars with ABS.

SpeedyC2

I don't mean to hijack the thread, but I do have a related question...

Does anyone know the recommended pressure to use with a Motive pressure bleeder when changing brake fluid?

The common practice with my Cayman is not to exceed 15 psi, to limit the possibility of damage within the system from over-pressurizing. Is there a similar limit for the 993?

Thanks!

pp000830

Can one simply actuate the ABS pump as in make a low speed sliding stop to clear the pump?
And the flush the system again at one of the wheel ends?

Foxman

Fifteen's good, don’t go above 20.

SpeedyC2

Perfect. Thank you!

nine9six

I have recommended this alternative, although the ABS pump has to cycle/actuate, but I would suggest only on wet, snow, ice or sandy pavement. You don't want to flat spot good rubber, should the ABS pump fail to actuate properly.

If the ABS pump is currently plugged with waxy buildup, the secondary methodology will not purge the ABS pump. It will only function to cycle a free flowing ABS pump of the existing internal hydraulic fluid.

HTH

IainM

I had the hanging clutch problem when I got my car and replaced the kinematic lever on the pedal for the updated one, also replaced master cylinder as it was weeping. You've probably read this one:
https://rennlist.com/forums/993-foru...-bleeding.html


what could possibly go wrong!?!

Ate 200, DOT 4

That thought had gone through my head too.

nine9six, are you saying that when the ABS activates due to wheel skid, the fluid only circulates within the ABS unit? Whereas the PST tool cycles the ABS valves to allow a fluid flow through the ABS?

IainM

I know the T-ODB can't activate the ABS "bleed" function and checking with Durametric confirms (I think) that they can't do it either:


Looks like the only affordable tool is this one:

http://jenniskens.livedsl.nl/Webshop...&product_id=50

Which assuming the greyed out box ungreys when connected looks promising.


Has anyone tried this tool?
I'll send the vendor an email to ask.


<edit> sorry, those screen shots went horribly wrong. I'll redo them

nine9six

No. Im saying existing waxy buildup may be to much to clear by functionally street cycling the ABS pump.

I have a PIWIS and it actuates the pump for about 15 seconds per test cycle. You will not get that durationin in a functional street cycling of your ABS pump.

IainM

Thanks for the clarification.
a further complication is that street activation of ABS may only be on one circuit/wheel
(your comment to do it on ice would help mitigate but not solve)

whats the process then? Pressurize the reservoir, open one bleed nipple and activate PIWIS 15 secs, close nipple, refill reservoir.
lather, rinse, repeat?

e9stibi

I have the Jennikins tool for exact this function. It works and cycles the pump. Because I have 2 993 (one with custom ECU), they needed to do some custom programming that it connected to both 993 but it took two cycle via email to resolve. Looks like they know what they do.

IainM

That's the piece of info I was waiting for!
thanks!
the fact that they could build out what seems like all the functionality Porsche built into the early ODB speaks highly of their engineering skills