norcal928 - Saturday Brake Master Cylinder Replacement and Bleeding
03-12-2009, 02:23 PM
#106
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Just in case someone isn't tuned in on bleeding the MC, the later cars 86.5+ have a single bleed port at the front of the master cylinder, at a high point where air in the front cavity can collect. Easiest way I've found to bleed that is to attach a small piece of clear tubing, and rout the other end right back into the reservoir, with that end down in the fluid some. Open the bleeder and pump the pedal to push any air out.
Note that the MC bleeder port only allows you to purge the front cavity, which I think is for the rear brakes only. There's no way to purge the top of the rear cavity; that one is really small, effectively filled with the piston.
I have a cute little air-operated vacuum pump that I need to plumb to my power bleeder cap. I'll try that on its own. If that doesn't get the pedal back up high, I'll put sealer on the threads of one bleeder screw on the rear, and one on each front caliper, attach a small fluid-filled bottle with a hose, and let the vacuum draw fluid from the wheels. Effectively the reverse bleed that Wally suggests. Vacuum will expand any bubbles and cause them to move to the highest point. Hopefully that's the MC reservoir in front. Letting the fluid flow backwards might 'sweep' the bubbles back towards the reservoir a bit.
Note that the MC bleeder port only allows you to purge the front cavity, which I think is for the rear brakes only. There's no way to purge the top of the rear cavity; that one is really small, effectively filled with the piston.
I have a cute little air-operated vacuum pump that I need to plumb to my power bleeder cap. I'll try that on its own. If that doesn't get the pedal back up high, I'll put sealer on the threads of one bleeder screw on the rear, and one on each front caliper, attach a small fluid-filled bottle with a hose, and let the vacuum draw fluid from the wheels. Effectively the reverse bleed that Wally suggests. Vacuum will expand any bubbles and cause them to move to the highest point. Hopefully that's the MC reservoir in front. Letting the fluid flow backwards might 'sweep' the bubbles back towards the reservoir a bit.
I only recently appreciated that the MC bleed is for the rear brake cavity only. Therefore, the procedure for this system is to bleed the front brakes first.
I've read other suggestions to tape the bleeder threads and try to suck fluid back up. I decided to break down and buy a tool that injects fluid through the bleeders rather than try to rig something impromptu.
http://www.brakebleeder.com/product.php?pid=2
although I got it for $100.
I've swapped brakes line before, but as you mentioned, did it very quickly and had no problem bleeding successfully after that. In Nicole's case, the calipers appear to have been off the car for weeks w/o plugging the lines, so, this is an uncommon issue, except for the first time the factory fills the system.
03-12-2009, 04:33 PM
#107
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Bill, that's worth a try, of course. Thing is, there is a fluid path to the pump motor that cannot be bled without actuating the solenoids and running the pump. Without allowing fluid to flow through that path, it's possible that no amount of pumping(pressure or vac) will get all of the bubbles out. I just noodged Tom again about the connector I'm waiting on, and an additional ABS connector. Hopefully I'll be able to get the thing assembled this weekend.
03-12-2009, 05:30 PM
#108
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I wish someone would video tape these tech sessions you guys have for the rest of us that can't make them....That would be so cool. Download to you-tube and post them!!! Would someone consider that next time?
03-12-2009, 05:58 PM
#109
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I now have a video camera that I can use for this. I started up a YouTube channel(link on my homepage) but it's pretty sparse at the moment. Only DIY there is the camera mount that I'll be using on fundrives.
03-12-2009, 07:33 PM
#110
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Bill, that's worth a try, of course. Thing is, there is a fluid path to the pump motor that cannot be bled without actuating the solenoids and running the pump. Without allowing fluid to flow through that path, it's possible that no amount of pumping(pressure or vac) will get all of the bubbles out. I just noodged Tom again about the connector I'm waiting on, and an additional ABS connector. Hopefully I'll be able to get the thing assembled this weekend.
03-12-2009, 07:37 PM
#111
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We did try to set up an internet feed during a timing belt job one time, but we had technical difficulties.
03-12-2009, 07:51 PM
#112
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Just curious if this would be a possiblility or a waste of time... Does anyone have a set of known calipers? Perhaps swapping the calipers with a known quantity could shed some light... or at least rule the rebuilt calipers out.
03-12-2009, 11:09 PM
#113
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Looking at the diagram, there is a channel that leads from ABS pump, to a check valve, to the line between the MC and ABS unit. According to the schematic it is always in-circuit. Also, the check valve may not be sealing. There is an additional check valve between an internal accumulator and the pump. Also, in the main fluid path, it appears that the spring seats on the solenoids may trap air -- it's possible that fully actuating them may help jostle some bubbles loose, but no amount of flow in either direction would get the bubbles out. Ref. "928S w ABS Infotechnik pub 1985.pdf" pp. 12-14.
03-12-2009, 11:31 PM
#114
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The old calipers are still in the garage, untouched. The trouble is that one has a broken bleeder nipple, and therefore could not be bled. At least not until the nipple has been fixed. I also have the complete Brembo Rebuild Kits for those two calipers... and the epoxy paint.
03-18-2009, 02:45 AM
#115
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Someone had posted a few weeks back in another thread, advising to drive the car or move all wheels at 4-6mph to engage an ABS test, and then bleeding before turning off the engine.
I've been trying to find that thread again, so I can be sure I remember this correctly. However, I cannot locate the thread anymore. Does anybody remember what topic it was under?
I came across two other old threads that have interesting info, but not what I was looking for:
https://rennlist.com/forums/928-foru...edal-weak.html
https://rennlist.com/forums/928-foru...k-failure.html
I've been trying to find that thread again, so I can be sure I remember this correctly. However, I cannot locate the thread anymore. Does anybody remember what topic it was under?
I came across two other old threads that have interesting info, but not what I was looking for:
https://rennlist.com/forums/928-foru...edal-weak.html
https://rennlist.com/forums/928-foru...k-failure.html
03-18-2009, 05:29 AM
#116
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Nicole, I have the hardware assembly of my tool finished. For some strange reason I'm having trouble getting the connector I need from 928I... Once I have that it should go together quick and be ready for action.
My brother is getting married this weekend, so I'll be wrapped up with that, but we should be able to try this contraption very soon after that.
My brother is getting married this weekend, so I'll be wrapped up with that, but we should be able to try this contraption very soon after that.
03-18-2009, 06:52 AM
#117
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Awesome!
Still wondering why I can't seem to find that other thread where the poster talked about activating the pump by moving the car or wheels...
Still wondering why I can't seem to find that other thread where the poster talked about activating the pump by moving the car or wheels...
03-18-2009, 01:04 PM
#118
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I have the same problem with my Dodge SRT-4. I replace the MC 3 times and still get soft brakes. The repair manual states to bleed with power bleeder and use DRB III tool to activate ABS pump than bleed it again. The special dealer DRB III cost $3000 and they won't rent it. I took the car to a local park with a gravel driveway. Drove the car back and forth braking making sure ABS goes on. Bleed it again this time tons of bubbles came out. The pedal is now firm.
03-18-2009, 02:55 PM
#120
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