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Discussion Starter #1
I was at the track yesterday with a brand new set of front brake pads on my 2005 Elise. The car is track only.. Three sessions into the day, 25 minute sessions, the front pads were all but gone. The rear rotors hardly looked scuffed. I have had an issue with the pedal going soft but not loosing the brakes for a while and I have tried bleeding the brakes, had the Lotus computer on the system to purge the ABS, checked for leaks and done just abut everything I could but its gotten to bad to track. I believe its a bad master cylinder but if anyone has a suggestion I am looking for some thoughts...
 

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Discussion Starter #3
The rear calipers are AP similar to the front but with a smaller piston. they are a year or so old. The Brembos are not on the car. This set up has been working fine until about two months ago...The calipers and lines have been inspected and checked. They are fine. Fresh fluid was done at the same time as the front pad change, two days prior to the track event. I ran a full quart through all four corners to completely flush the system...
 

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Then you have air trapped in one side of the caliper or someplace else... do the calipers have a bleeder on each side of the caliper? If not, then there is air in the side that does not have a bleeder port.
 

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Discussion Starter #5
There are bleeders on both sides. There is no air in the calipers. The rears on both sides looked like the car had been parked for a week after three sessions. There is no pressure applied to the rear on both sides. If it was air it would not be consistent on both sides as it is.
 

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http://www.lotustalk.com/forums/1246664-post81.html

The symptoms being described are a result of the Electronic Brake Distribution (EBD) system operating. This system is also referred to as Dynamic Rear Proportioning (DRP) and is, as the name implies an electronic system which, through the ABS control valve block restricts the line pressure to the rear brakes automatically to a pre-programmed algorithm. You can consider it as an electronically controlled proportioning valve which measures parameters like the rate of deceleration and rate of pedal application and uses this data to anticipate a rear wheel lock-up and then reduces the braking effort at the rear wheels as necessary. If the ABS system is left to do this, it can only react to a wheel as it starts to lock and therefore the car can already start to spin before the ABS can start to work. In extreme circumstances, if the driver brakes very suddenly the EBD system can lock off the pressure to the rear wheels completely;
Sounds like your rear hydraulic circuit may not be working properly due to a possible malfunction of the above.

Do you have adjustable shocks on the car?
It is possible for shock settings to effect the ABS, for instance if you had the rear rebound damping too high, the rear wheels can momentarily lose traction and cause the ABS to do the above.
 

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Discussion Starter #7
I will have to check the ABS sensors. I seem to remember the tech mentioning something about them when we ran the Lotus computer on the car??? I had thought it was a master cylinder issue but this makes perfect sense. Have to get rid of the stock brake system and put in individual cylinders for front and rear. Ditch the ABS not really any value on a track only car!!!
 

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Given that braking effort is ~80% front even with no rear brakes as Vulcan described it seems to me unlikely that you would burn through your fronts in such a short time. What pad/rotor combo are you using and hvee you used it before??
 

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It is possible for the ABS HCU to have a valve issue that would allow fluid pressure for the rears to bleed off. When you manually bleed your brakes do you get good volume from the rears? When your pedal goes soft, is it a result of overheated fluid or is it just a sporadic event? I've seen HCU's on road cars/trucks behave in a similar fashion in that a valve will stick/malfunction in the block and either bleed off pressure or in some cases actually block flow to the calipers. I'm not saying this is your issue but, I would think a base brake system issue would be a consistent failure as opposed to what you described.
 

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Discussion Starter #10
Given that braking effort is ~80% front even with no rear brakes as Vulcan described it seems to me unlikely that you would burn through your fronts in such a short time. What pad/rotor combo are you using and hvee you used it before??
I have used the same rotor/pad combo for years with good luck but the pads do wear fast at this particular track. BOE rotors and CL-8 pads
 

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Discussion Starter #11
http://www.lotustalk.com/forums/1246664-post81.html



Sounds like your rear hydraulic circuit may not be working properly due to a possible malfunction of the above.

Do you have adjustable shocks on the car?
It is possible for shock settings to effect the ABS, for instance if you had the rear rebound damping too high, the rear wheels can momentarily lose traction and cause the ABS to do the above.
Yes I have adjustable shocks but they have been on the car for years without any issues. I does sound like an ABS related issue though and the Lotus computer noted it sensed no sensors at any wheel when we ran the ABS bleed program. The computer did cycle the system though Where is the ABS computer located???

In reading the post you copied in the poster notes the pedal would go hard in this instance which is the opposite of what is happening. I cannot get the pedal hard and where I was perfectly positioned to heel/toe under hard braking I now have to use the side of my driving shoe was my sole is below the gas pedal. This lack of pressure is what caused me to look toward the master cylinder as the cause of the issue...I have one on order.
 

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It sounds like air to me. Did you ever empty your master cylinder? They can be really hard to get the last little bubble out. When we built our race car, we would always bleed and bleed and still have to do the shakedown with a soft pedal and run it a few times bleeding all along the way, and finally it would go from soft to perfect. But I'm sure you tried all this already.

The Dynamic Rear Proportioning system is part of the ABS system? When I autocross at a particular site (known for being sandy) I purposely reach into my rear wheel and pop off an ABS sensor plug to deactivate the system and it makes threshold braking easier (the ABS kicks in on the sandy spots and you can't stop). Would this also deactivate the proportioning aspect of it? It may be worth popping off an ABS sensor and going for a rip just to see if it helps/hurts. It takes about 2 seconds and the car is perfectly safe and drivable like that. Just my 2 cents.
 

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Yes I have adjustable shocks but they have been on the car for years without any issues. I does sound like an ABS related issue though and the Lotus computer noted it sensed no sensors at any wheel when we ran the ABS bleed program. The computer did cycle the system though Where is the ABS computer located???

In reading the post you copied in the poster notes the pedal would go hard in this instance which is the opposite of what is happening. I cannot get the pedal hard and where I was perfectly positioned to heel/toe under hard braking I now have to use the side of my driving shoe was my sole is below the gas pedal. This lack of pressure is what caused me to look toward the master cylinder as the cause of the issue...I have one on order.
I read that as 2 related issues...

1)pedal going soft due to excessive temperatures, from the front brakes doing all the stopping, therefore boiling the fluid at the front calipers.

2)rear calipers not working due to ABS DRP problem, causing front brakes to do all the work.

Any "air" meaning gas from boiled fluid, in the system, will make the pedal get soft, even if the rear hydraulic circuit is blocked off by the DRP.

Does the pedal get hard again after bleeding all the fluid? If so then in that case it would not be the master cylinder. (not that the 2005 Elise pedal will ever truly feel hard, they always feel soft compared to what they should feel.)


Or..........
I suppose it could be that your rear calipers/pads are dragging on the rotor all the time, due to the parking brake adjustment for instance, and generating heat, boiling the rear fluid at the piston. That way the rear circuit would be full of air and not able to exert pressure, hence the lack of wear, and also giving the soft pedal. I have seen that on a car with a badly adjusted parking brake cable, the rear fluid boiled in normal street driving in a few miles and turned black.

What is the color of the fluid that comes out of the rear calipers when you bleed? And what is the color of the fluid that comes out of the front?
 

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Discussion Starter #14
It sounds like air to me. Did you ever empty your master cylinder? They can be really hard to get the last little bubble out. When we built our race car, we would always bleed and bleed and still have to do the shakedown with a soft pedal and run it a few times bleeding all along the way, and finally it would go from soft to perfect. But I'm sure you tried all this already.

The Dynamic Rear Proportioning system is part of the ABS system? When I autocross at a particular site (known for being sandy) I purposely reach into my rear wheel and pop off an ABS sensor plug to deactivate the system and it makes threshold braking easier (the ABS kicks in on the sandy spots and you can't stop). Would this also deactivate the proportioning aspect of it? It may be worth popping off an ABS sensor and going for a rip just to see if it helps/hurts. It takes about 2 seconds and the car is perfectly safe and drivable like that. Just my 2 cents.
The master cylinder has not been off. The pedal is soft at the start of the track day and stays the same all day. No change is pedal feel but it not right. I have been tracking this car for 7 years and do 12-20 events each year, I know the car very well and know it not right...
 

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Discussion Starter #15
I read that as 2 related issues...

1)pedal going soft due to excessive temperatures, from the front brakes doing all the stopping, therefore boiling the fluid at the front calipers.

2)rear calipers not working due to ABS DRP problem, causing front brakes to do all the work.

Any "air" meaning gas from boiled fluid, in the system, will make the pedal get soft, even if the rear hydraulic circuit is blocked off by the DRP.

Does the pedal get hard again after bleeding all the fluid? If so then in that case it would not be the master cylinder. (not that the 2005 Elise pedal will ever truly feel hard, they always feel soft compared to what they should feel.)


Or..........
I suppose it could be that your rear calipers/pads are dragging on the rotor all the time, due to the parking brake adjustment for instance, and generating heat, boiling the rear fluid at the piston. That way the rear circuit would be full of air and not able to exert pressure, hence the lack of wear, and also giving the soft pedal. I have seen that on a car with a badly adjusted parking brake cable, the rear fluid boiled in normal street driving in a few miles and turned black.

What is the color of the fluid that comes out of the rear calipers when you bleed? And what is the color of the fluid that comes out of the front?
The rears cannot be dragging or the rotors would show it.. The rear rotors show a dusty unused look after three sessions. The pedal fee l is soft from the start and stay that way all day. I cannot get a firm pedal back regardless of what I do. I have pushed five containers of Motul through the system over the last several events going slow and taking my time. Starting at the passenger rear and going to the driver rear and then passenger front. I also have bleed them with the help of a Lotus specialty shop just to make sure I was not doing something right. I have done this for seven years so I would think I have it down but I wanted to be certain I took myself and my procedure out of the equation. I am swapping the master cylinder as that is the only thing I can think of that could eliminate the rears from working. If in fact there is a bad seal and the fluid is pushing past the seal under pressure it would eliminate the rears from coming alive...The master cylinder in not too expensive so it seems like the best place to start. I really did not want to go about the repair by replacing component by component. I had hoped to identify a problem and eliminate it but it seems that just is not going to happen...
 

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The rears cannot be dragging or the rotors would show it.. The rear rotors show a dusty unused look after three sessions. The pedal fee l is soft from the start and stay that way all day. I cannot get a firm pedal back regardless of what I do. I have pushed five containers of Motul through the system over the last several events going slow and taking my time. Starting at the passenger rear and going to the driver rear and then passenger front. I also have bleed them with the help of a Lotus specialty shop just to make sure I was not doing something right. I have done this for seven years so I would think I have it down but I wanted to be certain I took myself and my procedure out of the equation. I am swapping the master cylinder as that is the only thing I can think of that could eliminate the rears from working. If in fact there is a bad seal and the fluid is pushing past the seal under pressure it would eliminate the rears from coming alive...The master cylinder in not too expensive so it seems like the best place to start. I really did not want to go about the repair by replacing component by component. I had hoped to identify a problem and eliminate it but it seems that just is not going to happen...
You have completely described the symptoms of an air bubble/pocket. A bad seal would be if you step hard on the pedal and the pedal would creep slowly down to the floor.
 

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Just throwing this out there but if it was a master cyl, why only the rears are not worn?

Just me being a newb but its easy to see if your rear pistons are traveling. Rotate them up and pump a little.

That's what she said!
 

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When did your issue first occur? Had you made any changes just prior? You said that the ABS module is not recognizing any of the wheel speed sensors-is the ABS light on in the cluster? BTW-the ABS ECU is piggybacked on to the HCU. Were you able to retrieve any ABS codes? I also have front type calipers mounted to the rear, although for street use. I can't help thinking that the ABS HCU is at there heart of this. The HCU can bypass or bleed off pressure through a bad valve. The pedal would be long/soft but not pump up (as it would with air in the system). I'll be surprised if it's a master cylinder-keep us informed.
 
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