Spongy Pedal on new Wilwood Setup

[Tim Kay]

Frustrating chain of events always lead me back to you guys with results that usually have me scratching my head saying "why didn’t I think of that".
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I recently installed a new set of rear calipers & rotors to my already upgraded set of front calipers and rotors. In addition, I did away with the brake boosters and installed a new pedal box with top mount bias pedal - dual m\c (with 7:1 pedal ratio)
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I have no pedal resistance first third of pedal stroke and "spongy" resistance on remaining pedal stroke (at bottom of stroke it gets a bit firmer).
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What I have done:

1. Bench Bleed – more than once
Final bench bleed I capped the outlet and get a rock hard pedal immediately upon very short pedal travel (leads me to believe there is no air in the m\c)

2. Numerous caliper bleeds (nary an air bubble).
Typical procedure: two person, pedal pump, open bleed screw, ect.
Vacuum bleed
Power bleed at the m\c, out through the caliper bleed screws

3. Isolated the two m\c to operate one at a time.
Went through the bleed process
Tried to at least get one set of brakes to get a hard pedal, to no avail.

4. Residual valve installed on front brakes only.
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Below are the caliper piston sizes if anyone can do a quick calculation:
Front caliper (6) piston diameters are - 1.62/1.12/1.12
Back caliper (4) piston diameters are - 1.38/1.38

Are my ¾” m\c too small? Do I need more volume from the m\c for the piston size I have? Any suggestion?

 

[Scott]

Tim,
I had the same problem when upgrading the master cylinder/brakes on one of my Camaros. You basically answered your own question about increasing the bore size. That was what I had to do as well. I started out with a 7/8ths bore, and had to go to a 1" bore before it felt "right". The smaller bore supposedly gives you more leverage but feels spongy?!?! (that is the way the tech at wilwood explained it)

I would suggest giving them a call and see what their recc. is, Scott

 

[Fran Hall RCR]

Ryan Ellis, driver of the SL-C 01 car likes a very soft production car feel to his pedal and we use smaller master cylinders...we still get 1500/1000lb line pressure but with a very soft feeling pedal...I originally set the car up with a rock solid pedal ...normal to me. I personally dont like the soft feel and neither do other drivers that have driven the car but Ryan prefers it...

Next year we are moving back to a hard pedal and Ryan is going to "man up" and drive with a manly pedal feel.

 

[Tim Kay]

I don't mind the soft pedal but the pressure is not there. Car moving, I can barely lock the brakes up if I intentionally try. And more specific, it's not there early enough in the pedal stroke.

 

[Mick Ridley]

Hi Tim

Here is a chart that will help you find the correct master-cylinder size for your calipers

Its been on this forum for a few years now

Put your values in the yellow columns, and it will tell you what you need in the blue columns

View attachment Claytons brakes.xls

Mick

 

[Tim Kay]

Thanks Mick, That's the same spread sheet I used and it's a great calculator. From that I got the results to go 3\4".

 

[Mick Ridley]

Tim,
Im no expert on the subject, but I did play around a lot with that chart, and chatted to some experianced guys on this forum, including Clayton, who posted the chart

And I would have thought that 3/4 master cylinders, would have been a bit small, I know there are many vairable factors, but if you are useing 6 and 4 pot calipers then, as Scott says 7/8" to 1" would be a more suitable size,

mick

 

[Tim Kay]

Spoke to Wilwood tech and the immediate suggestion was because the use of two m\c you need to bleed right side calipers simultaneously then the left side simultaneously.
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The one procedure (which I had read about) I did not try because I thought by isolating one master cylinder at a time (disconnected from the pedal assembly) would essentially be the same as bleeding front and rear simultaneously.
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Give me a couple hours and I’ll be doing this procedure then I’ll let you guys know the results.
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BTW, Wilwood Tech also confirmed the m\c size should be ok.<o></o>

 

[EGLITOM]

Dear Tim

Although you should be able to achieve a good pressure point with the described setup, I think a pedal ratio of 7:1 in itself is generating a to big of a lever. Usualy they are more like 5 - 5,5:1.
I have modified the excel file with your caliper sizes. ( i did use the file for myself with the exact same datas ,except caliper size and pedal ratio, with good result).
I realy think 3/4 is to small and also having the same m/c front and rear with the very different piston surface (20%) will result in off brake balance at a centered balance bar adjustment. In my opinion one want to have a almost ideal brake balancing in centered position and then go from there.
Check "---TIM.xls" which is your current configuration, see what the brake balance graph shows and pedal travel and force.
I did a optimised version "---TIMOPT.xls" which i think is a more reasonable set up.

TOM

 

[Tim Kay]

Under further inspection and comparison of the Calculators provided by Tom & Mick (thank you guys very much) I discovered a peculiar result that mine produced vs all the others. On line 19, “effective piston area”, my calculator divides the final result by two! Whereas on the other’s the result is not divided. When I do not divide by 2 then my result matches the other Calculators. This could be the culprit in my original calcs and subsequent purchase of m\c’s that may be too small. A fix similar to what Scott, above, described may be in order.

First though, I am off to the garage to test the “simultaneous” bleeding.

 

[EGLITOM]

TIM

line 19 is the calc of the total piston surface described as :
area = #pistons * calliperType * pi * (pistonDiameter/2)^2
i did this on a side calc, not using the original formula, because the front pistons do have different diameters,

TOM

 

[Tim Kay]

I did a dual m\c bleed procedure as described above and did not get any significant change, little if any. Leads me to believe you guys are correct in advising the use of a larger master cylinder.

I am going to go with Tom's recommendation, per his calculator, and install a 1" m\c on the front and see if the issue gets better.

My sincere thanks to all and I'll let you know my results as soon as the install is done.

Can't believe my calculator had the one critical formula wrong. Retrieved off the internet and by all appearances it was identical to both Tom and Mick's. In addition, I had reviewed my final calcs with a couple people who agreed it was correct, go figure!

 

[Tim Kay]

Changed my M\C's up from 3/4" to 7/8" front 13/16" rear and although I get a firm pedal earlier in the pedal stroke it takes an extreme amount of pedal pressure to even get the car to slow down (if this is what "man up" means Fran, your a stud!) and to lockup the brakes is impossible.

I wanted to go incremental in M/C size so I didn't jump up to the 1" M\C's as Tom's calcs and Mick have suggested but at this point it may be my next move.

Tom, your comment:

I think a pedal ratio of 7:1 in itself is generating a to big of a lever. Usualy they are more like 5 - 5,5:1.

You have me thinking. My understanding may be incorrect but I was to believe since I did away with my boosters the 7:1 ratio would be necessary. Wouldn't the additional leverage offset the pedal pressure necessary to get clamping force as compared to a 5:1 ratio?

One other object in the mix is my Wilwood Bias Pedal. Maybe my settngs on the bais and piston clevis adjusters are creating part of the issue for all I know. Suggestions here may be of some help also.

 

[Jonathan Strain]

I am no expert but shouldn't you run a residual valve for the rears as well? I thought they were recommended anytime the masters were mounted lower than the slave cylinders to minimize pedal travel. Just be sure to use the 2 psi valve for disc brakes.

Also have you checked to see if your rotors are warped? If they are they can cause pedal kick back.

 

[Mick Ridley]

Tim.
Rob Broomfiled, in our club upgraded from 4 x 35 mm willwoods to the biggest 4 pot willwoods, he could fit to his car. Initally he was dissapointed with the brakes, but the last time I emailled him, he said the brakes were really good, almost overbraked.

If he is a the club meeting tomorrow night, ill ask him what he did, to improve the brakes..

mick

 

[Tim Kay]

Jonathon, a residual is not necessary because my masters are higher than the calipers so I removed it for the time being. The only reason I had one on my old setup was to prevent "knock back". Rotors are ok too. Regardless, if I had either issue that required a residual valve one of the symptoms is the need to pump the pedal once or twice to get a good pedal. I don't have that problem.

 

[Tim Kay]

Tim.
Rob Broomfiled, in our club upgraded from 4 x 35 mm willwoods .....
If he is a the club meeting tomorrow night, ill ask him what he did, to improve the brakes..

mick

That would be great, thanks.

 

[Mick Ridley]

Tim,

Ive been thinking about this...and I might be wrong...and now doubt someone will soon say if I am wrong........

The true, pedal ratio, will only be 7:1 when your leg is at 90 degrees to the foot pedal, and the top part of the pedal, is at 90 degrees to the mastercylinder

Your set up should be that,.....

The angle at the top of the pedal box, should be less than 90 deg to start with, and when your foot pedal has stopped moving, it should be at 90 deg, If you go over 90 deg, then your pedal ratio, will be less than 7:1, So in a way, the further you push the pedal, the less pressure you have on the pistons



Have a look at the pedal box, and see what the angle is...get someone to push the pedal as far as they can, and see what the angle is at the top ....... If its goes over 90 deg and is less than 90 deg, then thats where your problem is, and a larger M/C is what you need

 

[Tim Kay]

Mick, your making some sense. The pedal gets firm before 90 deg but pushes just past 90 deg to stop the car. It was exagerated with the smaller m\c. So maybe the next step up to 1" will have the car stopping at 90 deg pedal position.

Also, I spoke with Wilwood and he suggested adjusting the bias to see what different result, if any, I get. He seemed to think the m\c was ok.

 

[Mick Ridley]

Tim,

What i said here isnt strickly correct


........."The true, pedal ratio, will only be 7:1 when your leg is at 90 degrees to the foot pedal, and the top part of the pedal, is at 90 degrees to the mastercylinder".....

The pedal Ratio will allways be 7:1, weither you have a 90 deg or a 45 deg angle, But maximum pressure can only be achieved when you have a 90 deg angle