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The feel on the road is excellent braking. On jackstands the rear brakes can barely stop R or D from spinning at idle. Pedal firms up nicely (with foot pumps) when motor isn't running, doesn't go long if you sloooooowly ease on pressure, softens about 1-2 inches with engine running...
1. Bled brakes twice (DOT 4), all 8 bleeders, farthest to closest, etc. Great flow to rears (used a Motive)
2. No drips, no leaks
3. No air at MC bleed screw
4. ABS works (icy here, easy to test)
5. Rotors measured at 24mm
6. Pads thick
The rears hardly do anything with an 18 bar bias valve (which is what I believe the 86.5-95 came with from the factory)
If you want them to actually do something you will need to swap in a 33 bar bias valve. They were $100 for years, but I think they went up recently 92835530502.
Porsche did this back in the day to make the car easier to control for normal drivers since the fiasco of the "racecar" LSD in the 930 crash....
Long story short, it isn't you, it's just the way Porsche sent it from the factory.
The rears hardly do anything with an 18 bar bias valve (which is what I believe the 86.5-95 came with from the factory)
If you want them to actually do something you will need to swap in a 33 bar bias valve. They were $100 for years, but I think they went up recently 92835530502.
Porsche did this back in the day to make the car easier to control for normal drivers since the fiasco of the "racecar" LSD in the 930 crash....
Long story short, it isn't you, it's just the way Porsche sent it from the factory.
Wow! That is some oracle-level knowledge, and I really appreciate the reply. Thank you so much, I was ripping my hair out on this one. Have a good New Year.
A couple of questions need answered before you assume it’s functioning correctly.
Does the vehicle have the factory stock calipers and other brake parts?
Does it have a functional ABS system?
Brake biasing valves only change internal caliper pressure after they’re rated pressure. So if you have the 18bar valve then the internal caliper pressure will change or “knee over” above 18 bar of pressure.
The rear brakes are more effective at lower pedal pressure than at high pedal pressure so the fact it won’t stop the rear wheels at idle doesn’t make sense to me.
Im also not a brake engineer.
As a brake performance enthusiast I rely on a set of gauges like this. They screw into the bleeder and provide hard data.
BTW I did change the bias valve on my S4 brake setup and it does make a noticeable difference, car dives less but you have to be a skilled trail brake'r and can't rely on the ABS because the 928 has a rather simple, blunt and low resolution 3 channel abs system that will not save you in all situations like a modern ABS system can. The rear fluid channel is shared for the left and right side. ABS tech and processing power progressed exponentially in the early 90's. The 80's Bosche ABS system found on the 928 is a carry over from the Bosche system that Mercedes was using in the late 70's.
Last edited by icsamerica; Dec 28, 2022 at 08:22 PM.
The rears hardly do anything with an 18 bar bias valve (which is what I believe the 86.5-95 came with from the factory)
If you want them to actually do something you will need to swap in a 33 bar bias valve. They were $100 for years, but I think they went up recently 92835530502.
Porsche did this back in the day to make the car easier to control for normal drivers since the fiasco of the "racecar" LSD in the 930 crash....
Long story short, it isn't you, it's just the way Porsche sent it from the factory.
They will still stop the wheels from turning in free air in gear however...1000x over.
That's his actual complaint...something is broken, this is not normal.
Last edited by Speedtoys; Dec 28, 2022 at 08:08 PM.
Brake calipers (and likely akmost every other part of the braking system) are approaching 40 years old. Maybe it’s time to remove them, take them apart, inspect, replace, rebuild?
Careful there.
There's a fairly large percentage of the 928 community that will be upset that pieces of the 928 didn't last forever.
The feel on the road is excellent braking. On jackstands the rear brakes can barely stop R or D from spinning at idle. Pedal firms up nicely (with foot pumps) when motor isn't running, doesn't go long if you sloooooowly ease on pressure, softens about 1-2 inches with engine running...
1. Bled brakes twice (DOT 4), all 8 bleeders, farthest to closest, etc. Great flow to rears (used a Motive)
2. No drips, no leaks
3. No air at MC bleed screw
4. ABS works (icy here, easy to test)
5. Rotors measured at 24mm
6. Pads thick
What am I missing? ('Not a mechanic)
Thank you,
1986.5 928S AT
Maybe nothing is wrong, On jack stands the front wheels will be stationary so the rears, if turning, would trigger intervention from the ABS, lowering the pressure to the front and so perhaps you're not seeing the what you'd expect on the rear unless you were pressing with panic stop force. Did you hear the ABS kick in? Can you retest with the ABS fuse or relay disconnected?
Last edited by icsamerica; Dec 29, 2022 at 12:30 PM.
Maybe nothing is wrong, On jack stands the front wheels will be stationary so the rears, if turning, would trigger intervention from the ABS, lowering the pressure to the front and so perhaps you're not seeing the what you'd expect on the rear unless you were pressing with panic stop force. Did you hear the ABS kick in? Can you retest with the ABS fuse or relay disconnected?
Read this earlier today.
Been wondering if the ABS system is functional, at all, with the front wheels not turning?
Read this earlier today.
Been wondering if the ABS system is functional, at all, with the front wheels not turning?
ABS would, by requirement, never allow the rears to lock up under any foreseeable circumstances. The question in this context becomes “what are the requirements for rear bias if the fronts are locked.”
89+ ECU will shut off ABS and PSD on a dyno instantly. Obviously there’s some code/logic/requirements about what’s normal, what’s an anticipated error, and what’s “implausible.”
I don’t know what the earlier ones do.
Its basically a black box to me.
Fluid pressure measurement at the rear caliper(s) would be the data that would point a finger at the calipers or at an ABS system limiting rear pressure.
ABS would, by requirement, never allow the rears to lock up under any foreseeable circumstances. The question in this context becomes “what are the requirements for rear bias if the fronts are locked.”
89+ ECU will shut off ABS and PSD on a dyno instantly. Obviously there’s some code/logic/requirements about what’s normal, what’s an anticipated error, and what’s “implausible.”
I don’t know what the earlier ones do.
Its basically a black box to me.
Fluid pressure measurement at the rear caliper(s) would be the data that would point a finger at the calipers or at an ABS system limiting rear pressure.
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Weak braking, rears only?
