[Crumpler]

Hey guys, some input if you have a moment.

I have had ‘observations’ about the S3’s braking for awhile.
I don’t know if soft is the best word, the pedal travel seems long before they bite. The pedal does not slip towards the floor when braking — like MC failure.
There is no squirrelly behavior, stock brake bias and booster.
Brake upgrade to GTS on front, but issue predates that.
I would say generally, the first 1/3 of pedal travel has no real resistance. You can feel braking start around 1/2 the travel, and biting hard at final 1/4.

I gotta power bleed and change fluid again for the track inspection, I will let you know if any additional air found.
I have not driven any other 928’s, only mine, is this what brakes did in 1986?
I have a rebuilt MC here in a box assuming it would have to be replaced sooner or later but where would you go with this first?

[Hey_Allen]

Time to put beads on the end of the pipes!

At least the failure was easily found and easily mended...

[maddog2020]

possible booster leak? Lack of vacuum? is this the 928 with boost? how is vacuum set up?

[worf]

possible booster leak? Lack of vacuum? is this the 928 with boost? how is vacuum set up?

This-^

and check operation of vacuum check valve at booster.

[Crumpler]

Thanks for following.

S3 has the 1/2 inch(?) line off the side plenum. I did delete the Venturi ancillary line. Factor?
Yes, low boost. Check valve would nullify if working properly?
HVAC line capped off on check valve.
Soft lines replaced sevenish years ago with SS.
Vacuum on manifold 20 mmHg at idle.

Edit:
Ok I will start there when I get home and post.
Thanks.

[maddog2020]

what happens to vacuum at idle when you press the brake pedal?

[Crumpler]

No change in vacuum when stepping on brake at idle.
Verified good vacuum to check valve.

Pulled check valve. Grommet looks good, valve good.

With engine off, sucking noise when valve removed from booster.

After shutting off, assist there, each push shorter.

Objectively, at idle, the pedal travel is about three inches and is firm about one 1 inch past the throttle pedal arm.
Engine off and assist gone, travel stops one inch in front of the throttle pedal arm. If that makes sense.

I don’t know maybe I’m creating a problem where I don’t have one.
If above cursory diagnostics done, have I ruled out booster and check valve?

I’m left with potentially MC weakness, pads not bedded, air still in lines?

[N_Jay]

If it is frim at a stop, I would doubt air in the lines.
Pads not bedded or something else adding to the take up between the pads and the rotor.
(Or just the way they are built)

[worf]

Tell us about your flexible brake caliper hoses.

[Crumpler]

Braided stainless steel. Some sort of clear plastic coating on outside. Roughly 7 years old.

I took it out and tried to bed the brakes. Seemed to help FWIW.

[worf]

Does your flush procedure include the MC bleeder?

Do you have any indication that the ABS works (regardless if no warning lights?)

Do you appear to have good brake retard even with the “long” pedal?

[worf]

No change in vacuum when stepping on brake at idle.

If I suspected vacuum (and because of the ‘modified’ brake vacuum), I’d run a line and gauge on a tmp basis into the cabin and observe brake vacuum under various load conditions. Ideally I’d get a measure of booster vacuum. That would require and adaptor between the booster and its check valve.

You might also see if you can locate another 86.5 928 and to back-to-back tests for comparison.

[Crumpler]

Mmmmmm.
Good point, I haven’t bled the MC before.

ABS: harder to gauge. Bigger brakes.
Ok, got ABS to kick in. Mechanically they work, I didn’t see an indicator light on the cluster. Does the lamp go on with use or just if there is a fault?

[Zirconocene]

When I first got my S4, I also felt like the brakes were not up to the standard that I was getting in my other cars. Like you, it was not clear to me if it was just how the 928 was, or if there was something a little off with the set up.

What really helped was taking Stan's advice and procedure to bleed the ABS module (it's on TOS). From memory, you jam the brake pedal down with a 2x4 and your seat over night, to compress any air in the system, then start cracking the B nuts on the ABS module. There might be some pretty important details I'm forgetting, so it's worth finding the original post. Some of the connections sort of suck to get at but it's not terrible.

It really helped the brake feel on my car and the brakes now bite where I want/expect them to, up high in the pedal travel.

Good luck

[worf]

… taking Stan's advice and procedure to bleed the ABS module (it's on TOS)…

Stan? @Mrmerlin Can you post that here?

[Zirconocene]

Here's the first procedure of Mrmerlin's that I could find, from this post: https://rennlist.com/forums/928-forum/9 ... 90-s4.html

"A new MC will usually precede air getting into the lines.
More than likely you have air at the ABS unit.
SO fill the res,
then open the two B nuts one at a time till you see no more bubbles,this is a gravity bleed.
NOTE work from the booster forward.

then open the tip bleeder. make sure no air.

Then move to the ABS unit.

NOTE if you have pressure bleeder that screws onto the reservoir then use it no more than 10 psi.

What your looking for are air bubbles to come from each B nut connection at the ABS unit,
open one at a time .

Then remove the pressure bleeder.

Go back to the MC have the helper press the pedal slowly,
and crack the B nuts one at a time to verify the air removal, do the tip last,
then move to the ABS unit,
crack each line, with the helper on the pedal go slow during this process.

then bleed the wheels LF RF LR RR. This should remove the air in the lines."

Let me go find the other one that I followed, where I had pressure on the system for many hours.

ETA: Here's the post that I asked some follow up questions about the procedure: https://rennlist.com/forums/928-forum/1 ... ption.html

In that follow up post, this is the actual procedure that I followed:

"first I would crack each of the B nuts at the ABS unit to let out any air ,
you will see the air come out.

Once you have the system bled then put a piece of wood on the brake pedal and push the seat forward not super hard but so the pedal stays compressed,
leave it sit overnight, in the AM remove the wood and check the pedal it should be hi and hard.,
try the brakes a few times,
then go open the bleeders at the MC tip, then at the ABS unit crack the B nuts again,
if it has air you will see little bubbles with the leaking fluid
then open the bleeders at LF RF RR LR, any remaining air should come out.
What your doing by putting the system under pressure is compressing the air bubbles ,
once they are small then they will easily flow up past areas that might capture air ."

Cheers

[MFranke]

Could it be that one of your Master Cylinder stages has failed? This may explain a soft pedal before the remaining MC stage engages the brakes.

[Crumpler]

Two things have come to light.
One: there was an amount of air still in forward GTS calipers. It took about 15 min of power bleeding with the motive handheld bleeder at 8-10 psi. I also pumped pedal a few times (not sure if that is best practice).
Two: brake pads are Hawk ER-1 compound.
Sold as light to moderate track use, but heat range is 700-1500F.
I had someone hit my rotors immediately with an IR gun after 20 minute session. Average was 320F or so.
Granted they can cool fast but I doubt I’m in the spec range for optimum performance. I will likely go back something with lower heat range.

[worf]

I also pumped pedal a few times (not sure if that is best practice).

The WSM for current Porsche sports cars directs technicians to pump the pedal 5 (IIRC) times for each bleeder.

However, the WSM assumes that brakes get flushed every two years and thus, there’s no rust or rust-like particulates in the system that might directly or indirectly (rusted bores) chew seals.

For me, best practice on 928s(*) is to pump the pedal during pressure bleeding but *only* as far as the pedal would normally travel under medium braking, which isn’t much.

(*) Except when the BMC is new, or on my ‘91 since I *know* the brakes have been flushed every couple of years since new.

[Geza-aka-Zombo]

Two things have come to light.
One: there was an amount of air still in forward GTS calipers. It took about 15 min of power bleeding with the motive handheld bleeder at 8-10 psi. I also pumped pedal a few times (not sure if that is best practice).
Two: brake pads are Hawk ER-1 compound.
Sold as light to moderate track use, but heat range is 700-1500F.
I had someone hit my rotors immediately with an IR gun after 20 minute session. Average was 320F or so.
Granted they can cool fast but I doubt I’m in the spec range for optimum performance. I will likely go back something with lower heat range.

I would think that when braking in the hard brake zones, the rotor and pad temperatures will be well within that heat range. I don't think I'd go lower; I presume that 320F is after a cool down lap?

[Crumpler]

Two things have come to light.
One: there was an amount of air still in forward GTS calipers. It took about 15 min of power bleeding with the motive handheld bleeder at 8-10 psi. I also pumped pedal a few times (not sure if that is best practice).
Two: brake pads are Hawk ER-1 compound.
Sold as light to moderate track use, but heat range is 700-1500F.
I had someone hit my rotors immediately with an IR gun after 20 minute session. Average was 320F or so.
Granted they can cool fast but I doubt I’m in the spec range for optimum performance. I will likely go back something with lower heat range.

I would think that when braking in the hard brake zones, the rotor and pad temperatures will be well within that heat range. I don't think I'd go lower; I presume that 320F is after a cool down lap?

Half lap but essentially yes.
I actually have some afternoon sessions today.
ER-1 still on, I will see how much difference there is without air in the system;)

I also pumped pedal a few times (not sure if that is best practice).

The WSM for current Porsche sports cars directs technicians to pump the pedal 5 (IIRC) times for each bleeder.

However, the WSM assumes that brakes get flushed every two years and thus, there’s no rust or rust-like particulates in the system that might directly or indirectly (rusted bores) chew seals.

For me, best practice on 928s(*) is to pump the pedal during pressure bleeding but *only* as far as the pedal would normally travel under medium braking, which isn’t much.

(*) Except when the BMC is new, or on my ‘91 since I *know* the brakes have been flushed every couple of years since new.

Got it. That makes sense.

[Geza-aka-Zombo]

Thinking about brake temps, I threw the attached spreadsheet together to give an approximation of how the rotor temps change with braking. Items in green are adjustable. It's a simple matter of converting vehicle kinetic energy to thermal energy in the rotor friction discs.

[worf]

It's a simple matter of converting vehicle kinetic energy to thermal energy in the rotor friction discs.

Awesome. A long time ago, in a Century far, far away, I could throw that math together. But, managing software engineers for 25 years ruined my brain

[Crumpler]

Cool chart Geza!
Thank you for the help.

The ER-1’swere good today. Worked well, proper bite in the right range of pedal.

Because I’m an idiot, I can’t have a perfect track day.
I passed everything in the first session. Then I couldn’t figure out why the group as a whole was getting faster.
I was driving the piss out of the car and getting passed.
The last session I figured out, as I was standing on it and going 80 in fourth, that the boost gauge was flat.
Sure enough an elbow pipe had blown off.

[Hey_Allen]

Time to put beads on the end of the pipes!

At least the failure was easily found and easily mended...

[Crumpler]

Time to put beads on the end of the pipes!

At least the failure was easily found and easily mended...

I also could have just looked at the boost gauge right in front of me ;)

Yes the beader is already ordered!

[Hey_Allen]

I think anyone who's worked on a boosted engine has likely done this at some point along the line.

Finding the hose popped off is preferable to a split hose or a failed connector that can't retain it...

(My first turbocharged engine rebuild, I was scratching my head for a bit, wondering what was wrong that caused lots of noise and no power... One hose not tightened, that looked perfect when the engine was off.)

[worf]

I think anyone who's worked on a boosted engine has likely done this at some point along the line.

It is architecture dependent. It depends upon where the compression occurs. In a roots-type supercharger there are no pressurized induction hoses. In a centrifugal supercharger or turbocharger, there’s no choice.

[Crumpler]

It’s interesting that this was just past the BOV. You can’t see it because of the radiator hose in that pic.

[Crumpler]

Still questing for perfect brake feel.

Did combination of motive power bleeding at resi. and vacuum bleed at caliper.
This did yield a little air.
Slightly better feel.
I noticed it was pulling slightly to right on hard stops, so I replaced the MC with a rebuilt unit.

I power bled MC and all AB’s connections, calipers again.
I don’t know you guys maybe this is what it is.
Once the HPS pads are hot they bite at level of the gas pedal. Progressive into abrupt braking from there.
It still has this initial dead feel because the brake pedal sits about an inch forward of the gas pedal.
I have GTS calipers on the front with stock MC, does that affect dynamics?

[worf]

I have GTS calipers on the front with stock MC, does that affect dynamics?

Vacuum booster is the same from ‘95 down to at least ‘86.5 (as far as current PET is concerned.)

So, I think you can rule that out.

Have you checked adjustment of the rod from the pedal to the booster? (WSM 46-10)

Probably not the “problem” but it doesn’t hurt to check.

[worf]

A long, long time ago, in a century far away, I was working on an ‘86 that had long pedal and poor brake performance.

For no apparent reason.

Long story short, I replaced the two relays on the ABS pump and like magic it had brakes again.

Reason I asked if you could engage ABS.

[h2pmr]

in my day job as a mechanic and anybody complains of poor feel of the brake pedal, if they have just bought the car and the pedal feel is different etc i pump the brake pedal with the engine switched off, to get rid of all the vacuum then see how the pedal feels then.
which rules out any servo / vacuum possible issues and just leaves the mechanical side of things
then clamp the flexible brake lines one by one to see that makes any difference, it will make the pedal harder but need to see if any one caliper makes more difference than the others
although i am lucky to usually have another model the same to test it against and then show the customer its the same.
just my 2p
cheers
Phil

[Crumpler]

I have GTS calipers on the front with stock MC, does that affect dynamics?

Vacuum booster is the same from ‘95 down to at least ‘86.5 (as far as current PET is concerned.)

So, I think you can rule that out.

Have you checked adjustment of the rod from the pedal to the booster? (WSM 46-10)

Probably not the “problem” but it doesn’t hurt to check.

Thanks Worf.
I will look at ABS relays. Last go round I did get them to engage but it won’t hurt to go through system again, that was a while ago.

I pulled the WSM tonight, appreciate the tip.
46-9 says the pressure rod adjust went from internal adjustment at pedal to the adjustment at the booster end in MY 86. Which I never knew.
Looking at mine tonight, it does look like there is still a shorter thread and lock nut on the pedal end of the push rod.
I won’t mess with it, WSM says minim play is 10 mm, and that is what mine is now.

in my day job as a mechanic and anybody complains of poor feel of the brake pedal, if they have just bought the car and the pedal feel is different etc i pump the brake pedal with the engine switched off, to get rid of all the vacuum then see how the pedal feels then.
which rules out any servo / vacuum possible issues and just leaves the mechanical side of things
then clamp the flexible brake lines one by one to see that makes any difference, it will make the pedal harder but need to see if any one caliper makes more difference than the others
although i am lucky to usually have another model the same to test it against and then show the customer its the same.
just my 2p
cheers
Phil

Thanks Phil!
Good point.

[worf]

I have a plastic dead blow hammer. From time-to-time, depending upon ‘my gut’ I will use it to ‘thump’ the caliper during bleeding. Sometimes, I’ll get bubbles out that were dislodged by the thumping. Usually on calipers that haven’t been flushed in years.

Given what you wrote, about not getting all the air out of the GTS calipers the first time… <shrug>

[Crumpler]

Mucking through this slowly.
Replaced SS soft lines. The set on there was almost ten years old, could have been a little tired.

I have reached out to Roger to try and source the ABS pump relays that I didn’t know existed until Worf told me.

I replaced CE abs system relay on CE panel.
Which is weird. WSM says fuse no longer in relay (10A on top), starting MY’85. But this 86.5 has just that, stamped 1984 West Germany.
I should have a 10A in fuse slot 16 for abs system instead, per WSM. Per fuse diagram that came with car factory it says 15A in slot 16.
None of this probably matters but interesting how this stuff in print contradicts itself.

[worf]

None of this probably matters but interesting how this stuff in print contradicts itself.

The ‘86.5 is half ‘86 and half ‘87.

The trick is always figuring out which wiring diagram or WSM section applies: ‘86 or ‘87.

Pull out the ‘87 diagram and see if it makes more sense based upon what you see with your eyes. If there’s an ‘87+ WSM section, look at that.

[Crumpler]

Roger sourced the relays. Thank god. I doubt that’s a common request.

[Crumpler]

I found an old thread on RL.
Now I’m more confused.
The gist was the bore size on early MC was actually larger.
I would assume bigger calipers bigger bore. But is it dependent on piston size as well?
It also seems ATE cast numbers don’t mean much or the cast numbers have nothing to do with bore size.
Mark posted a medley of numbers he has seen come off 928’s over the years.

[Crumpler]

Updating for the record.
Going back through bleeding process again, decided to bleed clutch slave as well.
I pushed clutch pedal just to see. It went to the floor, I pulled it back out. The trickle at the slave was now a full gush.
Then I got some real air out of the calipers.
I cannot justify even if air in master, slave, or blue hose: why it would effect brakes. Clutch always felt good btw.
The brake system now feels better, pedal shorter, responsive.
I went out and tried to activate ABS.
I could brake my brain out of my nose, but I couldn’t lock tires or get abs to engage. Previously, I would get some chatter.
I’m hoping and assuming this is good.
Big, sticky tires on a warm day.

[worf]

… decided to bleed clutch slave as well.
I pushed clutch pedal just to see. It went to the floor, I pulled it back out. The trickle at the slave was now a full gush.
Then I got some real air out of the calipers.
I cannot justify even if air in master, slave, or blue hose: why it would effect brakes.

Yeah. That’s a puzzle since the only thing shared by the brake and clutch system is the top third of the fluid reservoir.

However, I have heard this same story about a different 928 last week.

I can think of one way to cause the behavior: plumb the CMC from a BMC output line. Maybe?

… decided to bleed clutch slave as well.
The brake system now feels better, pedal shorter, responsive.
I went out and tried to activate ABS.
I could brake my brain out of my nose, but I couldn’t lock tires or get abs to engage. Previously, I would get some chatter.
I’m hoping and assuming this is good.
Big, sticky tires on a warm day.

Ultimately it is the result that counts.