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What causes this brake issue?


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Hi All,

I have the typical Toyota 4x4 S12W front brakes and 240sx rear disk brakes on my 77 280Z. All parts are new and bedded in properly. I have bled and vacuum bled the system till I blue in the face using ATE fluid. I use Porterfield R4S pads and have excellent fade resistance. The master cylinder is a 280ZX 15/16 type. I have even participated in a HPDE track day where I punished the brakes with no issues or severe fade. I have an adj proportioning valve that is biased safely towards the front. The suspension is all new and urethane most everywhere possible. The car's alignment is spot on and tracks and handles well.

 

For some reason the front right brake will lock up if I mash the brakes in an emergency stop. At the drag strip the other night I hit the brakes a bit hard and again the front right will lock. Below is a picture of an emergency brake at probably 80mph.

 

Any suggestions what is causing this? Should all four lock if the pedal is pressed hard enough? BTW, this skid was not intended and was a bit unnerving. Thanks for your comments.

 

 

post-388-12738110823675_thumb.jpg

Edited by jgkurz
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Possibly weight jacking due to spring positions?

 

Cut springs not cut/seated perfectly even frt-bck-lft-rght, or coil-overs installed and when adjusted was not done with corner scales with the drivers weight in the seat...

 

My springs are cut but they were all cut so they seated perfectly in the original location. The spring rate at each corner should be close to identical. Obviously this isn't ideal but I needed to lower the car on a limited budget so I went the "cut spring route". My future goal is to install coilovers with new springs that have rates for my specific application. I also need to swap to shortened struts vs my standard Tokico Illuminas. Rtz has told me this on more than one occasion.. ;)

 

With that said, is it still plausible that the front left caliper is contributing to the issue? It's under warranty so the part is free.

 

 

BTW, How do coilovers change weight reading on a corner scale?

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Imagine a bar stool or four legged table with one leg that is a little long or little short. Cross weights, (X-weight), not being equal causing weight jacking.

Cross weights could easily be way off, especially with coil-overs being as they allow so much adjustment. Left to right vehicle weight or front to rear can't really be adjusted, even thought he car can be titled left or right or front to rear with coil-overs. But what if one corner is adjusted tall or it and the opposite corner are adjust tall, (or short, same thing applies). Back to the bar stool weight one or two legs at opposite corners that longer or shorter than the rest, the longer ones are bearing more of the weight.

Back to your car, lets say the left front and right rear were adjusted tall, (or just the left front). This would cause the Left front wheel to be loaded considerably more than the right front and with good brakes that apply equal pressures, the front wheels would end up with the skid pattern you pictured.

 

Cut springs, same thing could happen, one spring sagging more than the others, a car body that is the twisted from a previous accident, one tire taller/shorter than the others, one corner of the cars suspension is binding during movement (squat up front, lift in the rear under braking), same thing.

Using corner scales with coil-overs, you can fine tune the "X" weights to counter any body twist issues, long-short struts, etc.

 

Grass Roots article on this subject;

http://grassrootsmotorsports.com/articles/understanding-corner-weights/

 

 

 

grassroots-CW1.gif

Edited by BRAAP
Clarification...
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Where did you install the adj. proportioning valve, and what did you do with the stock valve?

 

Nigel

'73 240ZT

 

Here a pic of the mounting location. I purchased the valve from Modern Motorsports and followed the Z specific installation instructions.

http://www.modern-motorsports.com/catalog/product_info.php?cPath=60_61&products_id=53

 

 

post-388-12738547859519_thumb.jpg

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Imagine a bar stool or four legged table with one leg that is a little long or little short. Cross weights, (X-weight), not being equal causing weight jacking.

Cross weights could easily be way off, especially with coil-overs being as they allow so much adjustment. Left to right vehicle weight or front to rear can't really be adjusted, even thought he car can be titled left or right or front to rear with coil-overs. But what if one corner is adjusted tall or it and the opposite corner are adjust tall, (or short, same thing applies). Back to the bar stool weight one or two legs at opposite corners that longer or shorter than the rest, the longer ones are bearing more of the weight.

Back to your car, lets say the left front and right rear were adjusted tall, (or just the left front). This would cause the Left front wheel to be loaded considerably more than the right front and with good brakes that apply equal pressures, the front wheels would end up with the skid pattern you pictured.

 

Cut springs, same thing could happen.

A car body that is the twisted from a previous accident, same thing.

Using corner scales with coil-overs, you can fine tune the "X" weights to counter any body twist issues, long-short struts, etc.

 

Grass Roots article on this subject;

http://grassrootsmot...corner-weights/

 

 

 

 

 

Excellent post. Thanks for taking the time Paul. Looks like I will get the car on corner weights... : )

 

I might also replace the front left caliper since it's under warranty.

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Here a pic of the mounting location. I purchased the valve from Modern Motorsports and followed the Z specific installation instructions.

http://www.modern-mo...&products_id=53

 

 

 

Where did you find the Z specific instructions, and what did they tell you to do with the original valve?

 

Nigel

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Where did you find the Z specific instructions, and what did they tell you to do with the original valve?

 

Nigel

 

The instructions came with the valve from Ross @ Modern Motorsports. http://www.modern-mo...&products_id=53

 

I don't have access to the car right now but I know the original valve is retained. I believe the instructions had me put adj valve is installed inline with the back brakes.

Edited by jgkurz
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The instructions came with the valve from Ross @ Modern Motorsports. http://www.modern-mo...&products_id=53

 

I don't have access to the car right now but I know the original valve is retained. I believe the instructions had me put adj valve is installed inline with the back brakes.

 

I can't find the instructions on the MM site.

 

If the original valve is retained (it would be mounted on the firewall in your year of Z, to the left of where you have your adj. valve from the picture), do you recall if the instructions advised you to gut the original proportioning valve? If so, you've connected the rear brake line to the front right caliper!

 

Nigel

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I can't find the instructions on the MM site. If the original valve is retained (it would be mounted on the firewall in your year of Z, to the left of where you have your adj. valve from the picture), do you recall if the instructions advised you to gut the original proportioning valve? If so, you've connected the rear brake line to the front right caliper! Nigel

 

That would suck. The 4x4 brake upgrade has been an improvement over stock but not the night and day that I hoped for. I have put the brakes through some serious abuse so it seems to me that if I connected the adj. valve incorrectly I would have symptoms noticeable under all braking conditions. I will check when I get home. If you are right I'll be happy because it's an easy fix. Thanks for the suggestion.

Edited by jgkurz
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I can't find the instructions on the MM site.

 

If the original valve is retained (it would be mounted on the firewall in your year of Z, to the left of where you have your adj. valve from the picture), do you recall if the instructions advised you to gut the original proportioning valve? If so, you've connected the rear brake line to the front right caliper!

 

Nigel

 

 

Here's how my adj valve is installed. The stock proportioning valve is removed. Both front brake lines are routed to the brake indicator switch in the stock locations with no connection to a valve. The one rear line coming out of the brake indicator switch has the adj valve inline before it goes to the rear of the car. It looks like I connected all this up properly back in the day. My next step is to replace the left front caliper. Stay tuned...

Edited by jgkurz
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I am curious if your brakes are warm/hot when this is happening? The R4s will pull your front calpers as described if they are cold as they would not be in the correct operating temperature range. These pads are definitely not for street because they don't get hot enought to be in the correct operating temperature. Differing loads on cut springs could exaggerate this as others have stated. You might want to test this before you swap out your caliper.

 

Good Luck.

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I have run R4S pads (S-Street...) for years without issue. You run the R4 compound and you will have exaggerated issues. But R4S's on the street dust a bit and squeal under light braking, but under panic situations, they bite just fine.

 

My immediate thought was either sticking caliper and weight differential.

 

The TUV inspection in Europe does a test and the braking force applied has to be within 3% side-to-side or they won't let you register the vehicle. If the thing is still locking when there's a 200# passenger, and a 160# driver, I'd lean more towards a caliper sticking. It is common enough. If the pads have been on there any length of time, you should see a difference in the wear side-to-side. My last set of R4S's on the Blue Turd went out due to a stuck caliper, so I can see that happening. One side was almost metal-to-metal while the other side still had a good 3mm left on the pads.

 

Make sure the pivot pin is lubed...

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I have run R4S pads (S-Street...) for years without issue. You run the R4 compound and you will have exaggerated issues. But R4S's on the street dust a bit and squeal under light braking, but under panic situations, they bite just fine.

 

My immediate thought was either sticking caliper and weight differential.

 

The TUV inspection in Europe does a test and the braking force applied has to be within 3% side-to-side or they won't let you register the vehicle. If the thing is still locking when there's a 200# passenger, and a 160# driver, I'd lean more towards a caliper sticking. It is common enough. If the pads have been on there any length of time, you should see a difference in the wear side-to-side. My last set of R4S's on the Blue Turd went out due to a stuck caliper, so I can see that happening. One side was almost metal-to-metal while the other side still had a good 3mm left on the pads.

 

Make sure the pivot pin is lubed...

 

Thanks Mark and Tony. My next step is to pull the front wheels off and check the pad wear between sides. The right side should have more wear if our assumptions are correct. I'll likely replace the LF caliper regardless. I do use R4S pads not the R4. One the street I'd hardly ever get the R4 pads heated up safely. I also am looking for some corner scales to test weight differential and weight jacking.

 

BTW, shouldn't my rear brakes have locked also if the car was setup correctly? My assumption is that the rear brake bias should be slightly less than the front. Correct?

Edited by jgkurz
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