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C4 Brake Hydraulic Pump Problems

K

K TUBBY

PCGB Member
Member
Hi, here is the problem, thoughts greatly received
Early this morning five miles down the road, brake boost failure, no warning lights nothing, pulled over (eventually) turned off ignition, pumped brake pedal several times to ensure that the pump if it wanted too would engage upon turning on the ignition, you guessed it, nothing.
Nursed the car slowly home, fluid level has risen in the reservoir by 6-8mm,
My initial thoughts were possible relay but, I don't think this would influence the fluid rising, not yet knowing the internal workings of the pump could a return valve failed and the fluid stored in the circuit returned to the reservoir that way.

Just a little history, the hydraulic fluid system was changed 2 years ago including the bleeding of the PDAS for the 4 wheel drive , the brake fluid was changed again 1 month ago after a full suspension refurb, 250 miles on and the problem has occurred.

Kevin

 
I would be suspicious of the fluid change you had last month. First port of call. Ask them the questions and tell them the problem only started since they did the work.
 
Maybe an airlock, did you notice if the pump was over hot? Odd that there were no warning lights, the C4 lights up like a Christmas tree normallY, but even a partial electrical fail should have got one of the alarms going. One for someone with a good knowedge of the PDAS kit and procedure for the C4.
 
Hi

I'm confident about the fluid change, I did it and have done previously on a regular basis over the 11 years of ownership, the oddity is the rise in fluid level, do you now what relay operates the pump or how best to check the pump operation by powering it up remotely?
Kevin
 
From what I remember of the C4 brake circuit, it has to be pressurised and there is a reservoir that holds the pressure. When the engine is off the pressure releases I guess. There may be none return valve that stops the pressure releasing and this may have failed. There are several bleed points around the pump, two I think, which may need to be bled.
 
Hi
Just had another look and fuse has blown, replaced fuse and yep blown, pulled pump out, split motor from pump and the motor does not rotate me thinks I need to raid piggy bank this is looking expensive, don't know if I can get the motor as an exchange unit or buy new (ouch) time for google to do some work.

Not unless anyone can offer another cunning plan?

Kevin

 
I copied this procedure from the web some time ago, can't remember where.
The last instruction talks about the pressure bulb near the brake pump.
Relay R51 activates the brake booster pump.

How to bleed PDAS without a hammer:

  1. Jack up rear of car, ensure to use stands so the car is stable, not only resting on the jack!
  2. Turn ignition on and wait for the pressure pump to finish.
  3. Make sure brake fluid is above max.
  4. Get someone to sit in your car, for turning the 4wd lock switch
  5. Slide underneath the rear of the car, and a bit in front of the rear wheels there is a hydraulic valve with a brake-pipe entering and a bleed-bolt above
  6. Fasten a hose to the tip of the bleed-bolt and the other end into a bottle to collect the fluid.
  7. Tell the person in the car to turn the switch clockwise (turn on the 4wd locking).
  8. Loosen the 11mm bleed-bolt to bleed the system underneath the car.
  9. Close the bleed bolt once the feed stops
  10. Tell the person in the car to turn the switch anti-clockwise (disable 4wd locking)
  11. Repeat 7-10 19 times or so checking the brake fluid is above max so that a total of 20 times bleeding.
  12. Bleed the pressure-bulb at the bottom near the spare wheel.
 
You posted while I was checking something so disregard previous post.

Well done in finding the fault. Check as much as you can whether there is any obstruction jamming the motor from turning. You'll have to take care as you can get rebuilt units and they will want yours as an exchange.
Check www.ebay.de the German Ebay, parts cost less and no import duty.

 
Any clues on refurbishment companies, I keep drawing a blank apart from Chinese web sites & module repairs?
 
BBA Reman in Kent does these types, don't know if they do it for a 964. Your local specialist may also know. I recall an article on Rennlist about repairing it by using Mercedes piston parts, apparently the same Bosch parts. Lastly, check the Porsche reman parts list, they keep adding units but have not looked at it for a while, it may be there. Good luck.
 
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