Brake Hydralics - HELP!!!

benspeed

benspeed

New member
OK - I've got this 1992 968 I bought for parts and the car was just too nice so I've restored it to be a street car - something to beat around in that's the same year and model as my race car.

It sat for 6-8 years. I got it running well and drove around some country roads. Checked the brakes - all four rotors were hot so brakes seemed to be operational. Rotors and pads were just cruddy so I replace with new rotors and pads. Go to bleed the brakes - cannot get hardly any out of the rear calipers. Here's my symptoms:

  • Can't get any fluid out of rear calipers - only a spit or two - pedal will not go down with bleeders open
  • Front calipers - I can pump fluid through but it takes a mule to push that pedal
  • When the front bleeds it feels erratic - pedal won't go, then will drop, next pump smooth, then pedal is rock hard won't move, next pump it'll go down but not smoothly - sorta weird
  • Front calipers will function
  • Rears calipers do not function with pedal depressed
What I've done:

  • Bought a motive power bleeder - this did nothing alone - but it did help when I also push the pedal
  • Started disconnecting the rear lines where there were fittings to see if there was fluid/blockage - no fluid at the rear fitting where the lines disappear under the chassis -
  • Disconnected the lines at the master cylinder and looped hose back into the resevoir - brake pedal works nice and smooth - fluid moved from both lines no problem
I figure there's a blockage somewhere - not sure if I can run compressed air into the system without damaging anything. I was informed not to run more than 20-30 PSI into the lines.

Can I use the power bleeder to push fluid from the rear back towards the resevoir to unseat a blockage?

Any suggestions on this odd problem?
 
Ben--Still have rubber lines in the system?

Suspect a collapse.

If not, does it have a prop valve?

I would remove the rubber lines before a power bleed.
 
Certainly right to suspect the rubber hoses; that said, it's quite common for the rear stub steel lines (go from the trailing arm pivot, where the hose is, to the wheel caliper) to get crushed during service. I've outright replaced mine - easy to bend up a new section of pipe. So check those hard lines for damage.

I don't quite understand the concern for running more than 20-30psi through the system; brake systems routinely operate at 100bar (not psi!!), and it's possible to generate over 200bar at the master cylinder.

That said, I would not try to use compressed air to push through any blockage until first disconnecting both ends of the line! Sucks to just push the block further down the line! ;)

I expect the car has ABS; though this shouldn't even impact anything, I assume you're doing this with the car turned off?
 
Ben this is the barn find?? Check the lines front to back, top to bottom. The yahoos might have jacked the car up on them, who knows. I second the rubber line thoughts too. Stainless versions are very available and cheap. A must do.
 
Thanks guys - really appreciate the help. This is the barn find and will just be a street car.

Looked over the rubber hose sections in the back and they don't seem collapsed - also opened up a fitting before the rubber hose - no fluid.

Tracked the lines and don't see anything crushed.

The mystery to me is this is a closed system so what could get in there and stop it up? Congealed brake fluind?

Figure I'll take the line off the master cylinder and off the back of the calipers and try some compressed air from the master cylinder side. Would that damage ABS? I want to leave that working.

I don't know where to hunt for a stock proportioning valve - that's been on my mind - some kind of valve being stuck. Where would I look for that? I haven't been up underneath the front yet - been working my way from the back to the front.
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Got a total of $2200 into this car now - magnetic numbers are just for fun.

Thanks again for any suggestions.
 
um, if you think about it a bit, the rubber line idea is not plausible. Does the car have a distribution block? some cars have an automatic prop valve/distribution block on the firewall, which also cross biases the brakes. Sounds like a stopped up plunger preventing both rears from working. Your rear pistons might be seized which could explain both symptoms in the rear of not bleeding and not working. If you power bleed it, and the rears will bleed out but still won't move, then likely your rear calipers are seized. Do you have a repair manual? It should cover all that stuff...
 
Most late model MB and Audi with stability control/ABS systems will not manually bleed. They require flushing with a (relatively) high pressure pressure bleeder (2-3 bar/30-45psi) with system enabled (ign on) The appropriate tool is $1200 and has an electric pump. One of the BMW tuners (Turner Motorsport?) sells a nice hand-pump pressure blleder tank w/resevoir adapter for around 80bucks that works well-just need to keep it pumped up enough. phil
 
Caplipers aren't seized - the pistons move no problem.

I'm leaning towards a clogged up ABS pump - one Porsche expert suggested that enough moisture may have seeped in while the car was sitting that the ABS pump got corroded. I believe that's up behind the passenger fender - going to pull that off and see if I can get any fluid before the pump. That would also explain why the fronts barely bleed.

I have the factory manual - not the greatest - I was suprised how little it had on the braking system.
 
Yikes... sucks/wierd that you'd have to have the ign on to bleed - all our Bosch systems (at least for sure 5.X-onward) are all flow-through - can bleed with ignition off.

If you pull the rear lines off after the ABS unit and see about blowing air through there, you should be safe and also isolate any issues that might occur with the ABS unit; it should be safe against sticking, but hey, who knows...
 
Must have said something wrong - don't need the ignition on to bleed although I tried both off and on. Didn't make a difference.

Thanks Vaughan - will let you know how this turns out - likely will post more info for help!
 
Ben-if it's the late German syndrome, you need to force the fluid thru with the hi pressure type bleeder, not pump it thru w/pedal. If it's this case, you will not be able to get a decent peadal no matter how long you pump the pedal! phil
 
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Ah, I forgot about ABS... Used to my old cars :) Yep, there's a special procedure for that, and IIRC, it does require a pressure bleeder (I don't have it handy, but have it somewhere on my Audi service DVD somewhere)
 
OK - here's and update. I was able to isolate the blockage down to the abs pump. Fluid will not come through to the rear line. I checked EVERY line and the calipers with compressed air. Made sure to disconnect the ABS first.

Sure learned more about brake hydraulics than I knew before.

Looks like a new ABS pump will solve this problem.
 
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