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Bleeding the diff locks on a C4


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My '89 C4 is currently in the shop for an ABS problem (i.e. it doesn't work) to be investigated. As part of the diagnostic work a full hydraulic bleed was undertaken but there seemed to be a problem in bleeding the hydraulic diff locks. The shop have (I beleive) a PST2 which acknowledges the PDAS system but the "Bleed" button remains greyed out preventing the necessary cycling of the locks. The bleed to the primary hydraulic system has improved the "pump time" from more than a minute and a half to less than 30 seconds.

Is there a known issue with the PST2 and the '89 C4 or can anyone suggest why the PST2 would grey out the "Bleed" button. Could it be related to the non-operational ABS?? The PDAS system appears to work as expected - the light flashes when driving over some bumps and on enthusiastic cornering. I've installed the PDAS bulb in the oil gauge just to make sure I can notice it!!

All suggestions appreciated.



Edited by Computamedic
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  • 2 years later...


For bleeding the both diff locks you dont need the PST anymore :lightbulb:

each time after putting the ign switch on "dont start the engine!!", you hear clicking the slave actuators once (both at the same time).

By repeating on/off the ign switch you will be able to bleed them w/o any tester , opening the bleeding nipple and closing. Each time it comes a litte amount of brake fluid out ...... ;)

I hope it could help even a little late :)



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  • 6 months later...

Yes there is a known problem with a KTS500 or Porsche PST2 tester that was updated as Porsche recommended.

Porsche removed the bleed function from PST2 software version 17 so if the shop did the right thing and updated the PST2 software up to version 24 they can not bleed the brakes with the PST2 however if they have the Bosch/Porsche hammer that will do the job.

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