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TOPIC: Brake help! - odd problem (as usual for me)

Brake help! - odd problem (as usual for me) 23 Jul 2015 10:30 #18389


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Hey guys,,, I posted this some time ago I think, over a year,,, but here is the background as we have not been able to sort it out yet.

1983s, early VIN 1108
Symptoms -
1 - For over a year or more, my brakes have been releasing slowly. Approx 2-3 seconds before fully released.
2 - I have also driven in the 95+ degee heat for a little while and found the problem sometimes worsens, to the point that after a full stop and trying to hit the gas,,, the car would not move,,, brakes fully engaged without my foot on the pedal, had to wait 6-7 second it seems before I could move,,,, then took a few hundred feet to fully disengage ,,, could tell there was drag while driving for a little while. - This condition has only happened once howeve,r to this extreme
3 - The car does NOT pull or brake unevenly.... ever. When the brakes release, it seems like all are releasing the same.
4 - My mechanic has 1 928 and 3 944s and does brakes and suspension for a living and this one has him stumped right now, he has not seen it before.
5 - The harder you brake, especially to a full stop,,, the longer it takes to release. Light pedal = quicker release, but always a little lag. This started a little over a year ago and has acted the same way ever since, no better, no worse. I have driven around VIR 2.5 laps with the Porsche parade last August and ran for 2 straight hours on a fun run at SITM this year in the mountains, with it doing the same thing the whole time. Again no better, no worse. Braking is not the issue, only releasing.

Equipment -
1 - Current Brake Master Cylinder was new in 2011, still seems to be in good shape, but I do have a new one to use if needed, if I do install it and it does not fix it however, I am out a couple hundred bucks - so I am trying to understand before replacing more parts. I understand that this could end up being the culprit if there is seal damage etc. So I am not writing off changing it out.
2 - The 2 brake pressure regulators, under the Master, used to control front to back pressure, were swapped out for a re-conditioned/used pair. That one we did just throw money at.. :) But no change afterwards. It was relatively inexpensive.
3 - New SS braided brake lines were installed a year ago
4 - DOT 4 Super Blue ATE fluid is being used. Flushing and filling have been done and went fine,, fluid moves in and out with no issues. Right now we do not suspect clogs or line issues.

Testing -
1 - The issue only appears when the engine is running, brake system charged.
2 - We did try this test, which to me, seems to indicate it is not stuck cailpers, pad issues or something mechanical like that.
- Car in the air on lift, wheels spinning free - engine OFF - hit brake, then release and another person turns the wheel. Seems to work normally and release
as soon as the brake pedal is released! Every wheel releases normally with the engine off. There is no problem with the engine off it seems.
- Car in the air as above, but with engine running - same sequence for every wheel,,, every wheel releases slowly, 2-3 seconds.
3 - My Mechanic attests that the wheel, brake hardware all appear to be normal and in good shape, rotor wear, pads are new etc, bleeding went fine etc.

So after looking at a few other posts elsewhere, it is starting to sound like something is not turning of the booster quickly or the master cylinder is not opening all the way etc. (Wally I saw one of your posts referring to pedal settings and how the master is working, that helped get me to this point)

So there you go guys - have at it! :) HELP!
1983 928S - 16v 4.7L L-Jetronic
Dark Grey w/ Blue Leather
85/86 exhaust manifolds
Y-Pipe
SS Hi-perf Cat
S4 springs
Bilstein shocks
Air pump delete
VIN: WP0JB0926DS861180
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Brake help! - odd problem (as usual for me) 24 Jul 2015 12:30 #18391


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Brake booster check valve and/or vacuum line to check valve issue? Unless of course I'm interpreting the info at the following link backwards?

www.freeasestudyguides.com/brake-booster-operation.html

Sounds like engine idle vacuum is holding the master cylinder side of the booster too much as you suspected?
Last Edit: 24 Jul 2015 12:31 by y33trekker.
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Brake help! - odd problem (as usual for me) 24 Jul 2015 17:02 #18392


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Yep, this is kind of where I was going with this myself, almost isn't too much left it could be given the current symptoms.

Just wanted to get some feedback to see what others say this weekend before we made further plans.

Thanks for the link, I will check it out.
1983 928S - 16v 4.7L L-Jetronic
Dark Grey w/ Blue Leather
85/86 exhaust manifolds
Y-Pipe
SS Hi-perf Cat
S4 springs
Bilstein shocks
Air pump delete
VIN: WP0JB0926DS861180
- - Isn't Life great? - -
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Brake help! - odd problem (as usual for me) 24 Jul 2015 19:15 #18393


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Well I'm by no means a brake system expert, so please take my suggestion with a grain of salt. Based on the other symptoms/possible causes listed for brakes slow to release at that link, the only other possibilities were things like brake pedal lever return springs and brake pedal linkages binding, which I suppose could still be the cause, but I'd imagined you'd already looked for the simple stuff like that.

The return spring INSIDE the brake booster was also listed, but if it turns out to be the brake booster, wouldn't the entire piece have to be replaced anyway?
Last Edit: 24 Jul 2015 19:17 by y33trekker.
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Brake help! - odd problem (as usual for me) 24 Jul 2015 20:59 #18394


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Good thoughts, but I am leaning away form pedal settings and springs etc because it works just fine with engine off. That is why I am thinking it is boost release related. Keep 'em coming though thanks
1983 928S - 16v 4.7L L-Jetronic
Dark Grey w/ Blue Leather
85/86 exhaust manifolds
Y-Pipe
SS Hi-perf Cat
S4 springs
Bilstein shocks
Air pump delete
VIN: WP0JB0926DS861180
- - Isn't Life great? - -
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Brake help! - odd problem (as usual for me) 23 Sep 2015 19:16 #18504


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Ok - so here we are (Where are you Wally? :)

- problem still occuring
- MC replaced - to remove possible bad MC and seal issue there
- Calipers rebuilt - was about time anyway and remove seal issue as well
- Repeated an earlier test, which was done incorrectly the first time,,,,,- pulled vacuum line out of the booster, capped it off ,,, went for a drive, no drag whatsoever! Release is normal. Just no boost as expected.

This puts me back to to the idea that it is keeping boost up too high when you let off. So unless there is some other regulate or valve that could do that, I am thinking the booster must be the culprit (we have pretty much taken care of all the other parts! :)

Best suggestion now - new booster? Is that difficult to replace,,, it looks tight in there (83)

Thanks
1983 928S - 16v 4.7L L-Jetronic
Dark Grey w/ Blue Leather
85/86 exhaust manifolds
Y-Pipe
SS Hi-perf Cat
S4 springs
Bilstein shocks
Air pump delete
VIN: WP0JB0926DS861180
- - Isn't Life great? - -
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Brake help! - odd problem (as usual for me) 24 Sep 2015 11:00 #18505


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Did you ever end up replacing the vacuum line going from the booster to the intake vacuum source itself?

Maybe it's simply a matter of a rubber line deteriorating over time and not being able to hold it's shape and temporarily collapsing?
Last Edit: 24 Sep 2015 11:01 by y33trekker.
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Brake help! - odd problem (as usual for me) 24 Sep 2015 11:23 #18506


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Thanks for the suggestion. Most of the vacuum tubes were replaced with silicone but that one I believe is still original. I will look into that but pulling the vacuum is not an issue, it is more
like the release of vacuum is not happening when it is supposed too.
1983 928S - 16v 4.7L L-Jetronic
Dark Grey w/ Blue Leather
85/86 exhaust manifolds
Y-Pipe
SS Hi-perf Cat
S4 springs
Bilstein shocks
Air pump delete
VIN: WP0JB0926DS861180
- - Isn't Life great? - -
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Brake help! - odd problem (as usual for me) 24 Sep 2015 13:09 #18507


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Maybe I'm not considering the operation of the booster correctly. What I was thinking is that once the booster has it's vacuum established via it's internal diaphram, a vacuum tug of war then takes place between the booster and the engine/intake vacuum that temporarily keeps the connecting vacuum line collapsed.
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Brake help! - odd problem (as usual for me) 24 Sep 2015 16:46 #18508


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To be honest, I am not sure myself, indeed you may be right. I really had very little experience with brakes prior,,, it has been a very good learning experience !
1983 928S - 16v 4.7L L-Jetronic
Dark Grey w/ Blue Leather
85/86 exhaust manifolds
Y-Pipe
SS Hi-perf Cat
S4 springs
Bilstein shocks
Air pump delete
VIN: WP0JB0926DS861180
- - Isn't Life great? - -
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Brake help! - odd problem (as usual for me) 02 Oct 2015 12:47 #18526


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Read through this one recently and wondered if a misadjusted brake pedal might still be a possibility in your case. It happens apparently.

928registry.928oc.org/forum/928-oc-techn...-front-brake-seizure
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Brake help! - odd problem (as usual for me) 03 Mar 2017 14:28 #19570


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FIXED<<<<<< Finally
Indeed (thanks goodness) it was the Brake Booster - If you have followed any of this thread we tested and worked our way back to the point that the only thing left was the brake booster. After almost a year and a 1/2 I finaly got it replaced. Just taking it out took about 1.5 hours! It is a real pain on par with the dashboard and front nose I think ... HA

I am going to a HPDE at the realtively new Domion raceway in Fredericksburg April 1st with the Va Bch PCA so I cannot wait. I have been drving minimally just to keep it road worthy with no boost hooked up. Now to ensure it is in shape for a short track day.

Needless to say I have missed the board and my car quite a bit. Although during that time, getting a new 2016 Mustang GT with performance pack has lessened the pain LOL

But I am ready to get back to Sharks in the Mountains and the Frenzy this year
1983 928S - 16v 4.7L L-Jetronic
Dark Grey w/ Blue Leather
85/86 exhaust manifolds
Y-Pipe
SS Hi-perf Cat
S4 springs
Bilstein shocks
Air pump delete
VIN: WP0JB0926DS861180
- - Isn't Life great? - -
Last Edit: 10 May 2017 19:10 by mm928.
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Brake help! - odd problem (as usual for me) 04 Mar 2017 07:34 #19572


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Good to hear of your persistence-based success. The brake booster issue is just another example of dealing with vehicles that are approaching antique status. Parts are just getting to the point of simply wearing out.

I'm getting ready to rebuild my brake calipers so please stay tuned since I may have a few questions myself. On a couple of occasions since the purchase of my 90 S4, the front passenger caliper has not wanted to release after braking. It's been suggested that the master cylinder is the cause, but I'm not sure if that quite adds up when it's only one caliper the problem is happening to.

One of the previous owners had the calipers repainted, so I've been wondering if something didn't get into or around the piston bore in the process that's been causing the issue. Either that, or they were simply painted and not also rebuilt during the process and it's just that time for this part on this old a vehicle. I'll find out soon enough I guess.

Congrats again on getting the issue solved.
Last Edit: 04 Mar 2017 07:40 by y33trekker.
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Brake help! - odd problem (as usual for me) 10 May 2017 07:23 #19704


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I've had a new development regarding my braking issue. Since purchasing the car, the only brake problem I'd had was with the front passenger caliper sticking, but the other day the front driver side joined the party.

I rebuilt the two front calipers recently (new seals etc), but the scenario sounds all too familiar with something that was said in the first post in this thread. When this happened on my car the other day, the outside temperature was also warmer than usual (mid 80's), which was about the same condition as when the front passenger caliper sticking had happened the very first time.

I think I'll try the same and disconnect the vacuum line from the brake booster to see what happens during a warm day drive, but the symptoms are sounding all too familiar so it seems a new brake booster may be in my future too.
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Brake help! - odd problem (as usual for me) 10 May 2017 19:13 #19706


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You never know, sorry to hear that. We tried connecting and disconnecting the booster and turning the wheels by hand without the booster,,,with the booster, without the booster etc to all 4 wheels. With it disconnected all 4 worked like a charm (although no boost of course.) That started leading us back to the booster. Good Luck!
1983 928S - 16v 4.7L L-Jetronic
Dark Grey w/ Blue Leather
85/86 exhaust manifolds
Y-Pipe
SS Hi-perf Cat
S4 springs
Bilstein shocks
Air pump delete
VIN: WP0JB0926DS861180
- - Isn't Life great? - -
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Brake help! - odd problem (as usual for me) 11 May 2017 06:53 #19707


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mm928 wrote:
You never know, sorry to hear that. We tried connecting and disconnecting the booster and turning the wheels by hand without the booster,,,with the booster, without the booster etc to all 4 wheels. With it disconnected all 4 worked like a charm (although no boost of course.) That started leading us back to the booster. Good Luck!

Was hoping you would clarify what was meant when you said the following about the brake booster test being done incorrectly the first time. Did you just pull and cap both vacuum lines going to booster (main vacuum line plus smaller vacuum line with check valve)?
- Repeated an earlier test, which was done incorrectly the first time,,,,,- pulled vacuum line out of the booster, capped it off ,,, went for a drive, no drag whatsoever! Release is normal. Just no boost as expected.

Thanks for the well wishes. It'll just be nice to get to a point where I can plan a day trip with the car without having a high probability the issue is going to rear its ugly head again.
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Brake help! - odd problem (as usual for me) 25 May 2017 18:18 #19754


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Still no joy on this one. Took the car to work today with both vacuum lines to the brake booster unhooked and plugged and could tell that the drivers front caliper had still hung a little on the way home because of that hot brake smell when I pulled into the driveway.

Is it possible that the brake booster linkage to the master cylinder is not adjusted correctly? I've read in the manual that the measurement from the bearing surface to the eyelet center of the connecting rod should be a very specific measurement (within a few mm).

Maybe just a master cylinder replacement?

Just curious how the fact that it mainly happens during warmer temps applies.
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Brake help! - odd problem (as usual for me) 06 Jul 2017 06:42 #19860


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The current diagnosis is indicating the master cylinder. I have one ordered and on the way so I'll find out soon enough.
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Brake help! - odd problem (as usual for me) 06 Jul 2017 13:06 #19861


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Takes a while to figure these things out sometimes! good luck!
1983 928S - 16v 4.7L L-Jetronic
Dark Grey w/ Blue Leather
85/86 exhaust manifolds
Y-Pipe
SS Hi-perf Cat
S4 springs
Bilstein shocks
Air pump delete
VIN: WP0JB0926DS861180
- - Isn't Life great? - -
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Brake help! - odd problem (as usual for me) 06 Jul 2017 13:15 #19862


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Meh. As blasphemous as this will possibly be taken by some (although it would likely be worse on Rennlist than it will here ; ), I've simply had other priorities lately, so doing much with the 928 just hadn't been of much concern.

The + to the shark being minor on the to-do list is that the odometer just keeps reading low numbers.
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Brake help! - odd problem (as usual for me) 23 Jul 2017 12:51 #19900


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Well when you're right you're right. Just the mere mention of a 928 not being top priority seems to have garnered a ding on the ol' karma meter here.

Further proof that some don't actually have the sense of humor they pat themselves on the back as having.

Anyway, I've also gathered the TB/WP parts for replacing the stock TB tensioner with the PK setup, so that's only a matter of time too.
Last Edit: 23 Jul 2017 12:55 by y33trekker.
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