Bb Conversion Now Very Soft Pedal

Hi All,

I recentyl put the EP BB conversion on the front, and the AP 2 pots on the rear, and now there is virtually no feel through the pedal.  With the vacuum connected to the servo, the fronts will lock up with the lightest of brushes, but with it removed, the brake pedal is so hard there is no feel the other way!

I found this post, to try and reduce the vacuum effect on the servo http://www.vx220.org...e-servo-effect/ but I can't find anywhere to buy one of these valves.  With the national coming up I really want to try and get this fixed.  Changing the pivot point on the brake pedal and removing the servo altogether would work also, however i feel this would be rather involved. 

Anyone else out there suffered this problem with the BB conversion and how did you overcome it?

TIA

VXed

Read about recommendation to change out the master cylinder for a bigger bore version... (Not sure which type.)

Big Brake 

They don't bother mentioning that your brake bias and master cylinder ratios will be buggered on their website do they?!  Just the usual emphasis on selling you something for lots of money without mentioning the real world problems it will create.

Anyway, to adjust/reduce the vacuum being delivered to your servo, you need an adjustable bleed valve that retains X In Hg of vacuum. I have something similar in Nipper, but it's an adjustable vacuum switch (to electrically trigger my vacuum pump). so I can adjust the vacuum from anywhere between 2in Hg to 28in Hg.

You will need something similar to this, that simply bleeds vacuum and retains a more modest level of vacuum in the pipework. The company I used is called Applications Engineering Limited, best to ring them up, they have 1001 specialist switches that do this sort of thing, more than likely to have the part you need.

Bear in mind, if you are bleeding vacuum at idle, then your idle may be affected. If your car uses a closed loop lambda sensor for this then it will regulate your mixture and revs correctly and hopefully you won't get any kangarooing of revs.

Regards.

Edited by Nev, 14 May 2019 - 06:38 AM.

https://www.elisepar...aster-cylinder/

They don't bother mentioning that your brake bias and master cylinder ratios will be buggered on their website do they?!  Just the usual emphasis on selling you something for lots of money without mentioning the real world problems it will create.

 

Anyway, to adjust/reduce the vacuum being delivered to your servo, you need an adjustable bleed valve that retains X In Hg of vacuum. I have something similar in Nipper, but it's an adjustable vacuum switch (to electrically trigger my vacuum pump). so I can adjust the vacuum from anywhere between 2in Hg to 28in Hg.

 

You will need something similar to this, that simply bleeds vacuum and retains a more modest level of vacuum in the pipework. The company I used is called Applications Engineering Limited, best to ring them up, they have 1001 specialist switches that do this sort of thing, more than likely to have the part you need.

 

Bear in mind, if you are bleeding vacuum at idle, then your idle may be affected. If your car uses a closed loop lambda sensor for this then it will regulate your mixture and revs correctly and hopefully you won't get any kangarooing of revs.

 

Regards.

Thanks Nev, useful information.

I guess this would still work when the vacuum is being taken directly from the inlet manifold, rather than using a vacuum pump?

Pretty sure I have a closed loop lambda so that shouldn't be an issue.

Ideally i am looking for an adjustable mechanical release value to modulate the amount of vacuum in the servo.  I am also liaising with EP on this as it is their calipers, so they may know of a solution already.

Thanks!

https://www.elisepar...aster-cylinder/

The size of the Master cylinder isn't an issue at present I don't believe.  It may come to that though  

Waiting on advice from EP and Hofmanns

 

https://www.elisepar...aster-cylinder/

 

The size of the Master cylinder isn't an issue at present I don't believe.  It may come to that though

 

Waiting on advice from EP and Hofmanns

 

You increased the surface/volume of your brake cylinders, so the (master cylinder) pedal travel increases with very little feel.

Up the master cylinder surface and the pedal travel will become normal again and the needed pedal force (feel) will increasei...

Read that most track BB conversions delete the servo and ABS completely and prefer the hard pedal feel. You could modify the pedal hinge point to get more lever ratio...

I would not bleed air in the vacuum hose, as that creates an unmetered air leak into your engine. (high idle) But you can try a (adjustable) spring loaded one-way valve in your vacuum line. Say that has a crack pressure of 0.3 bar, it will substract that from your servo vacuum. (Assume max vacuum on engine braking is -0.7 bar, with the 0.3 bar valve in the line, the servo will only see -0.4 bar vacuum...

Not the most accurate devices, but it might work...

Edited by Exmantaa, 15 May 2019 - 04:05 PM.

 

 

https://www.elisepar...aster-cylinder/

 

The size of the Master cylinder isn't an issue at present I don't believe.  It may come to that though

 

Waiting on advice from EP and Hofmanns

 

 

You increased the surface/volume of your brake cylinders, so the (master cylinder) pedal travel increases with very little feel.

Up the master cylinder surface and the pedal travel will become normal again and the needed pedal force (feel) will increasei...

 

Read that most track BB conversions delete the servo and ABS completely and prefer the hard pedal feel. You could modify the pedal hinge point to get more lever ratio...

 

I would not bleed air in the vacuum hose, as that creates an unmetered air leak into your engine. (high idle) But you can try a (adjustable) spring loaded one-way valve in your vacuum line. Say that has a crack pressure of 0.3 bar, it will substract that from your servo vacuum. (Assume max vacuum on engine braking is -0.7 bar, with the 0.3 bar valve in the line, the servo will only see -0.4 bar vacuum...

Not the most accurate devices, but it might work...

 

Thanks, so you think replacing the master cylinder with no further mods would cure my issue and return the feel to sort of normal?  The lack of feel is p*ssing me off quite frankly.  Any idea where I could get one of these valves from to try that first?  Alternatively looks like I might have to go for the larger master cylinder and no servo...

I hate brakes.  What have other people done with the big brake conversion?  From looking on here, it appears people have just "plug and played" them....  Can't find anything on modifying any other parts of the braking system other than maybe a bias adjuster.

 

 

 

https://www.elisepar...aster-cylinder/

 

The size of the Master cylinder isn't an issue at present I don't believe.  It may come to that though

 

Waiting on advice from EP and Hofmanns

 

 

You increased the surface/volume of your brake cylinders, so the (master cylinder) pedal travel increases with very little feel.

Up the master cylinder surface and the pedal travel will become normal again and the needed pedal force (feel) will increasei...

 

Read that most track BB conversions delete the servo and ABS completely and prefer the hard pedal feel. You could modify the pedal hinge point to get more lever ratio...

 

I would not bleed air in the vacuum hose, as that creates an unmetered air leak into your engine. (high idle) But you can try a (adjustable) spring loaded one-way valve in your vacuum line. Say that has a crack pressure of 0.3 bar, it will substract that from your servo vacuum. (Assume max vacuum on engine braking is -0.7 bar, with the 0.3 bar valve in the line, the servo will only see -0.4 bar vacuum...

Not the most accurate devices, but it might work...

 

 

Thanks, so you think replacing the master cylinder with no further mods would cure my issue and return the feel to sort of normal?  The lack of feel is p*ssing me off quite frankly.  Any idea where I could get one of these valves from to try that first?  Alternatively looks like I might have to go for the larger master cylinder and no servo...

 

Big brakes + servo = too sensitive/no feel => fit bigger bore master cylinder...

Big brakes + No servo = too much leg force needed => fit smaller bore master cylinder...

Incase anybody searches this thread, I found what I think the issue is with the pedal travel.... The piston area of the original rears are 2036mmsq, however of the original front APs is 6194mmsq... so roughly I now need to move twice as much fluid to squeeze the disc, and yet the area of the new 4 pots on the front is 9099mmsq, which isn't the same increase ratio (approx 1.46:1 on the front and 3:1 on the rear).  I think this is causing the major issue, so, I am going to try reducing the fronts to the 34/36mm EPs rather than the 36/40 I have on there now, and revert back to the standard brembos on the rear.

Alternatively I guess I could put a brake bias valve in to send more fluid to the rear.... 

What's the best option here?

Incase anybody searches this thread, I found what I think the issue is with the pedal travel.... The piston area of the original rears are 2036mmsq, however of the original front APs is 6194mmsq... so roughly I now need to move twice as much fluid to squeeze the disc, and yet the area of the new 4 pots on the front is 9099mmsq, which isn't the same increase ratio (approx 1.46:1 on the front and 3:1 on the rear).  I think this is causing the major issue, so, I am going to try reducing the fronts to the 34/36mm EPs rather than the 36/40 I have on there now, and revert back to the standard brembos on the rear.

 

Alternatively I guess I could put a brake bias valve in to send more fluid to the rear.... 

 

What's the best option here?

??
Original AP front pistons are 44mm => 1521mmsq      (60%)

Original Brembo rears 36mm pistons => 1018mmsq   (40%)

You changed to:

Front 36mm +40mm = 2275mmsq  (60%)

Rears 44mm => 1521mmsq  (40%)

So you have the exact same brake bias as before (The VX can use a bit more rear bias on track), but your main issue is that you have changed from total 2539mmsq piston area to a total of 3796mmsq, which is 50% more!