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  #1 (permalink)  
Old 08-23-2018, 10:06 AM
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Default brake bias question

Took my 2012 BDR to the track for the first time last night. The rear brakes kept locking up on me with any type of heavy braking. The fronts were getting warm, but the rears were doing too much of the work.
I had changed out the fluid to high temp and I am pretty sure I got all of the old fluid out. The pads are the stock pads that came with the car. I don't remember seeing any kind of brake bias control under the car.

Has any one else experienced this on track? It was bad enough that it worries me that I may have a serious problem on the street in a panic stop situation.
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Old 08-23-2018, 11:57 AM
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tocheck,

Your worry is well founded. If your rear brakes lock, you can't steer.

Dunno what the problem is - don't know the BD brake system. Find someone who does, start with proper bleeding and go from there.

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Old 08-23-2018, 12:25 PM
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I don't have a BDR, however, if you have a separate master cylinders for your front and rear brakes there maybe a brake bias bar behind the brake pedal that can be adjusted.
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Old 08-23-2018, 06:06 PM
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Quote:
Originally Posted by pgermond View Post
I don't have a BDR, however, if you have a separate master cylinders for your front and rear brakes there maybe a brake bias bar behind the brake pedal that can be adjusted.
The car has a standard dual master cylinder.
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Old 08-23-2018, 09:16 PM
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Then you probably have a proportioning valve.

Since the proportioning valve decreases the pressure sent to the rear brakes, the main symptom the valve is going bad is the rear wheels locking up when the brakes are applied.

Just my opinion.
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Old 08-23-2018, 11:23 PM
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Or the fronts are inefficient.

I would expect the front brakes to be hot, not just warm, after a spirited drive.
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Old 08-24-2018, 08:06 PM
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The BDR uses a 1982 VW Vanagon master cylinder. with M10X1.0 ports front and rear circuits.https://www.rockauto.com/en/catalog/...+cylinder,1836
You could add a proportioning valve to the rear circuit. Tilton makes a few in that metric size. Valves - Tilton Engineering
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Old 08-25-2018, 06:04 AM
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A friend has a Backdraft (#250-350 range) and was unhappy with the braking. He changed pads, different on the back to partially reduce the rear braking forces and more friction on the front. Last I heard he was quite happy.

I'd start with trying to determine what pads are currently in use and then learning the braking force charts to get more front friction.

It's hard to test on the street because you really can't work the brakes as hard. Changing pads isn't usually that hard but does take time and new pads need to be "bedded" in.

Jim
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Old 08-25-2018, 06:38 AM
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Quote:
Originally Posted by tocheck View Post
Took my 2012 BDR to the track for the first time last night. The rear brakes kept locking up on me with any type of heavy braking. The fronts were getting warm, but the rears were doing too much of the work.
I had changed out the fluid to high temp and I am pretty sure I got all of the old fluid out. The pads are the stock pads that came with the car. I don't remember seeing any kind of brake bias control under the car.

Has any one else experienced this on track? It was bad enough that it worries me that I may have a serious problem on the street in a panic stop situation.
Your fronts should lock up prior to the rears. If the rears are locking up first and you have the same compound pads on both the front and the rear you have an issue with your braking system and should have it looked at.

Just adding a proportioning valve to the rears to limit their bite will only reduce the braking ability of the rears and this will resolve the rear locking up first part, but it will increase peddle pressure to stop the car and I am convinced you have an issue with the fronts.
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Old 08-27-2018, 05:17 AM
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Quote:
Originally Posted by spdbrake View Post
The BDR uses a 1982 VW Vanagon master cylinder. with M10X1.0 ports front and rear circuits.https://www.rockauto.com/en/catalog/...+cylinder,1836
You could add a proportioning valve to the rear circuit. Tilton makes a few in that metric size. Valves - Tilton Engineering
M10 pre-made lines are plentiful at any NAPA store
Thanks for the info Spdbrake. If I can't find anything mechanically wrong, I am going to try new pads all the way around before adding the valve. I had a thought: If that is a vanagon master cylinder, would the master cylinder be biased to the rear? I am really curious if this is a problem any other BDR owners have had, or if it is unique to mine.

Thanks to everyone for the comments and warnings.
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Old 09-10-2018, 10:03 AM
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No track, but had the same issue, rears locked up first. I have an older car and decided to replace the master cyl and add a proportioning valve at the same time. Interestingly, the replacement master cyl seemed to have different bore sizes front and rear, while the old one was nearly the same. Not sure if that's why, but the new cylinder works much better, and I never touched the proportioning valve, although its there if I need it.
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Old 09-12-2018, 12:46 PM
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Quote:
Originally Posted by Quentin Z View Post
No track, but had the same issue, rears locked up first. I have an older car and decided to replace the master cyl and add a proportioning valve at the same time. Interestingly, the replacement master cyl seemed to have different bore sizes front and rear, while the old one was nearly the same. Not sure if that's why, but the new cylinder works much better, and I never touched the proportioning valve, although its there if I need it.
Thanks for the info. Any chance you still have the part number of the master cylinder you bought? Wondering if it is the same as the one suggested by Spdbrake.
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Old 09-12-2018, 02:50 PM
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I ended up buying the expensive one from BDR because they are close to me, and I could get it quickly. But I also did some investigation and found that a BECK/ARNLEY 0726331 from a MERCEDES-BENZ 1978 300D 3.0L L5 DIESEL appears to be the cross-reference match to the new one. I bought one and shelved it just in case: Here's the rock auto link:
https://www.rockauto.com/en/catalog/...+cylinder,1836

To be clear, I have not tried it yet, but it looks the part compared to the the BDR unit.

For reference, I 'll try to attached pics of the BDR unit I took when i received it.
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Last edited by Quentin Z; 09-12-2018 at 03:05 PM..
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Old 09-12-2018, 02:55 PM
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BTW, the link from Spdbrake looks like the original one that I replaced. I believe it is a single bore. The replacement I received from BDR was dual bore, note the first pic, larger diameter in the back than the front. The B/A is also dual bore...

Also note, that the brake lines do enter the new style Master cyl at a slight angle compared to the older style, so there will be some minor bending needed if you have an older car.

Last edited by Quentin Z; 09-12-2018 at 03:10 PM..
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Old 09-12-2018, 04:48 PM
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Thank you for posting that Quentin. The dual bore bore unit would have different pressures it would seem. The larger bore I assume would be for the rear brake circuit (lower pressure).
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Old 09-13-2018, 06:39 AM
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I thought the same. BDR must have revised the part to help with the lockup issue.
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Old 09-13-2018, 05:40 PM
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In case there’s interest, I just pulled out the B/A and took some pics for comparison. The one from BDR comes with the one rear outlet plug:
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Last edited by Quentin Z; 09-13-2018 at 05:44 PM..
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Old 09-13-2018, 05:49 PM
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So it has a M10-1.0 pipe plug on one of the aft ports on the Backdraft MS?
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Old 09-14-2018, 05:52 AM
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Thanks for the info Quentin. I hope to get the car on the lift next week to start checking everything over.
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Old 09-14-2018, 09:01 AM
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Quote:
Originally Posted by spdbrake View Post
So it has a M10-1.0 pipe plug on one of the aft ports on the Backdraft MS?
Yes, you can see it in the second pic of the BDR part if you zoom in. Thatís the only difference I found , except the price!
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