Manual Brake System Trouble Shooting: Expected Line Pressure?
 

Hey guys, I converted from a power booster w/ front disk and rear drum to manual brakes running Z06 6-piston fronts and still have the rear drums (for now).

Under panic braking I can't lock up the fronts and I feel a little scared at how much pedal input is required to slow down. Not sure if I have air in my MCs or if my proportioning valve is reducing the pressure.

Here is the setup I have:

Wilwood 6.25:1 pedals
Wilwood High Volume 3/4" bore (front)
Wilwood High Volume 7/8" bore (rear)
Wilwood adjustable proportioning valve

My balance bar is set equal front to rear and the proportioning valve is full open, with 57% reduction to the incoming line pressure. I think the pedal is too hard for being a 3/4" bore and have had others drivers give me the same feedback. Next action I'm going to take is to check the line pressure at the front calipers, does anyone know how much pressure is expected?

I can't tell you exactly what line pressure is expected, and don't want to throw out a number that is wrong, but I might be able to help with the pedal setup.

I'm running dual master cylinders for the brakes, and I have Wilwood W6/W4 calipers. The W6s, (if I recall correctly) have a similar piston bore volume to the C6 Z06 calipers, but I don't know the rear Wilwood calipers would compare to the drum brakes.

I am using 3/4" front and 7/8" rear master cylinders and I have the balance bar set significantly towards the front brakes, and no proportioning valve. From my experience the 7/8" seems to move enough volume in relation to pedal travel that you need to make sure the pedal is pushing a lot more towards the 3/4" master cylinder. At the last RTTC I had the bar centered, or maybe even towards the rear after some tinkering the previous week, and I could not get the car to slow down at all!

As for the pedal feel. The pedal in my car is pretty firm. It isn't what I'd call rock hard, but it doesn't feel anything like manual brakes with a single tandem more master cylinder. There isn't a ton of pedal travel, and the pedal isn't what I would describe as "springy" like something you'd find in a new car.

If you have any other questions, or a better or more specific description I would be happy to try and help further.

Wllwoods site has some great tech advice for this topic as well. I have manual brakes with a willwood master and i had horrible pedal feel, u needed both feet to stop the car and i couldnt lock the wheels, i looked on wilwoods site and i was reading on the ratio you need for your brake pedal ,mine was way off,i redrilled and relocated the push rod and i got a really good pedal feel and stopping power. worth looking into.

The reason you run a twin master is to
 

correctly size the masters to the calipers. May be hard in your case with drum rear brakes. Personally I would not run the proportioning valve.

Most calipers do not want more than 1200 psi. You can get some caliper pressure gauges from Longacre for about $100 for 2 gauges and a cool metal case. For C5 sized brakes front and rear I was getting 1100 psi front and 850-900 rear with about 100 lbs leg pressure. I ended up with a 3/4 front and 3/4 rear on mine, but the rear sizing was due to 335/19 tire that was 27 inches tall. It could take more rear brake and still be ballanced.

I am going to guess that your problem lies in what brake pad you have up front. If you are running a street pad, I would change to something more agressive. I tried the Wilwood BP10's and could not get my car to stop no matter what I tried. Went to a Carbotech Ax6 (autocross pad with good cold bite) and it was a night and day difference. They will be dusty and will make some noise. Mine never made any noise, but I have heard other people say they do.

I have not tried but heard good things about the Hawk HP+ as well.

I was also going to say pads, but our red 69 has 14" proplus front and rear with a 1" wildwood master and it is awesome with the pads supplied by Baer. my nova has 3/4 f and 7/8 rear with 14" proplus and it to is good but I do run a wildwood auto x pad

I should also mention I've run BP20 (street) E (autocross, okay cold. needs a tiny bit of heat to work best), and B pads (road course, terrible cold) and could lock all of them up during a panic stop. The B pads took a ton of effort though.

It doesn't take much to dial out too much front brake on the balance bar. With rear drums I bet it would be down right scary trying to stop if out of adjustment.

I would check with the caliper manufacturer, however 3/4 bore master seems small for the two front calipers. I typically recommend 3/4 bores for clutch only. Looking at the last few posts on clutch issues, it's beginning to look like 3/4 is too small for some of them also.
Simply put, if the master cylinder does not have enough volume,(more than the volume of both of the calipers) it will not fully engage the calipers.
This is the second post today I have replied to with this suspected "volume" problem. Best of luck and hope this helps. Check the link below, same issue, just with a clutch master cylinder.

https://lateral-g.net/forums/showthread.php4?t=40386

Josh, are you still running the HPS 1-piece pads you purchased with your kit? I know you were going back and forth between those and the HP+, but didn't want the increased dust/noise/wear associated with the more aggressive pads. You basically have three ways to reduce pedal effort, increase the pedal ratio, decrease the MC bore size and/or increase the pad CoF. The pedal ratio on your Wilwood pedals is fixed at 6.25:1 and I don't know whether a 5/8" bore MC will have the necessary volume to supply the front calipers without excessive pedal travel, leaving the pads as your best option IMO.

What bore size wheel cylinders are you running? If I'm not mistaken, there are some larger bore wheel cylinders that are a direct swap which might help get your balance beam adjustment a little more centered.

Tobin
KORE3

Payton, how do you estimate 100 lbs of leg pressure? I've read that one can exert up to 300 lbs in panic situations, but is there a way to truly know how much pedal input one is exerting?

@Tobin

Yes, I'm running the HPS pads, should have gone with the HP+ pads. Forget the noise and dust, I want higher CoF!

I'll be updating my brakes in the coming weeks to Wilwood W6A in the front and moving my Z06 PBRs to the rear. Trying to decide if I buy 4 sets of line pressure gauges or use a clamping force pressure tester. Has anyone used this tool?

That's a cute tool, but I don't know anyone who has one or who has even used one, so I can't comment on its worth in a toolbox for troubleshooting. It kinda reminds me of a valve spring tester to be honest. As for caliper pressure gauges, you should only need two since you only have two brake circuits, front and rear, and they should provide the same pressure to both calipers on the same circuit per conventional hydraulic principles. Alternatively, they make the brake pressure gauges intended to be mounted in the dash to provide feedback to the driver when adjusting in-cab detent proportioning valves and/or balance beam position with the remote adjusters.

As for pads, any "GG" friction rated pad is going to be a step in the right direction given a manual track application, so Hawk HP+, EBC Yellowstuff, Carbotech AX6, or something similar should provide significantly higher brake torques with less pedal efforts.

Tobin
KORE3

I found the HP Plus to be the best all around pad for a pro-touring car and manual brakes. All out road course performance would be their weakness.

Ron

I have a dumb question.

The figure at the bottom is total braking force of the front brakes when they are doing 70% of the work or is it an estimation of the total including the 30% contributed from the rear brakes? :headscratch:

I have W6A front calipers with a 7/8" master cylinder and I have a 6 to 1 pedal ratio. So I read 3141 lbs of force with this set up. Is that 3141 for the front and 4487 total or 3141 total?

Rob,

Looking at the math, I think the answer is that the Brake Torque line represents the amount of work that the front brakes on an application would be doing while the Total Braking Torque if 70% line means that if 70% of your total car braking is done in the front, and 30% in the back, then you have a combined braking torque equal to that last line. Thus, if you multiply the value in the bottom line by 70%, you get the number in the Brake Torque line (at least for your W6A caliper):

3141 * 0.7 = 2198.7 (2199 rounded)

Now, exactly what the Total Braking Torque number actually means, I'm not sure. I think it refers back to Ron's Guideline, meaning that the 3141 total would fall between the 3000# (Performance Production Car) and the 3500# (Good street & track braking system) areas. Ron, could you please confirm that I'm understanding what that row of the table is referring to?

(I've said it before, and I'm saying it again... I love all this tech you are providing!!! THANX! )

Bryan,

You have it correct. You can look at my post answering Rob for more detail.

.

Ron

As always thanks for the detailed explanation. Looking at Brian's math I see it is obvious.

Brian you obviously have a better skill set with calculations than as a food critic. :action-smiley-027:

On my set up in the rear I have a 14" rotor and a Wilwood FNSL 4 piston caliper 120-11782-BK.

:cheers:

Checking brake specs after a night out with friends.:happy23: What else?:popcorn2:

:cheers:

Hmm, 25 years as an engineer vs. 45 years of eating, what's wrong with this picture? :catfight:

Ron,

Do you happen to have the piston size numbers for the Baer 6P caliper? My guess is that it's very similar to or the same as the 6S. I'm running the 6P front and rear on my car with 14" rotors.

Given your definitions, I would consider my setup to be more conventional, not a high-travel setup. So I would be leaning towards the 65/35 setup. I'm using the DSE booster and master cylinder, which is obviously a power brake setup instead of a manual setup...

Ron, thanks for the charts! I've been studying and pondering many questions. It is great to see a line up of all the calipers. Interesting that the GM single piston on and 11.88" rotors have almost as much clamping force as the Wilwood massive TC6R. Can this TC6R caliper be used on a 13" rotor, the smallest size rotor i saw offered is 16", reading the caliper drawing shows it may accomodate a 14" rotor. With a 16" shouldn't this lead to a dramatic brake torque increase? This got me thinking it might be an alternative to the W6A (5.4) on a 14" rotor since it provides 6.9 piston area!!! Key downside is the availability for more tack oriented pads, looks like the highest CoF is .40, comparable to the HP+ pads.

I was also thinking of the benefits of adapting the GM single piston caliper to a larger rotor. Not very sexy, but effective. Quite disappointing to see the Z06 6 pistons have such low clamping force.

Another thread made even better by our main man Mr. Sutton. :thumbsup:

Ron, do you happen to have a chart for the AP6000 fronts and AP6050 rears?

:cheers:

Oops! I had the Baer caliper model listed as 6S ... but those specs are for the 6P ... as Ron in SoCal's post reminded me. I have updated the two charts to properly reflect the caliper model. No details change.


So the specs in the charts ARE of your 6P. But I need the rest of your system specs to calculate braking force & bias.

If you will provide me with front & rear:
Brake piston sizes (all)
Rotor diameters
pedal ratio
Master cylinder bores
Brake pad brand & compound name

I'd be happy to calculate your braking system bias & total braking force.

BUT ... I do not have experience factoring in the hydraulic booster. So the bias will be accurate, but the total braking force will be lower than you actually have, as long as you have booster assist.

Maybe Baer or Tobin can provide us with a correction factor based on the amount of assist from your specific booster ... or at a minimum, a guideline.

Bryan - I've given RS the 6P piston bores w 14" rotors so I'm sure he'll post as soon as he wakes up :D. Post up your MC size? IMHO I also think you could achieve a high travel set up with the proper springs/sway bars.

Line pressures
 

Another thing I noticed on your charts is an assumed static line pressure of 499psi for 7/8" MC and 679psi for 3/4" MC, can you speak to the what is an expected maximum line pressure in a manual setup. Some guys have mentioned measuring pressures of 800-1000psi. I assume this is primarily achieved by a greater input to the pedal rather than reducing MC size. Do you have numbers for the following driving styles: normal (non fatiguing pedal input), semi-aggressive pedal input (spirited driving / auto cross), aggressive pedal input (high speed track braking and panic stop situations).

As always thanks for your wealth of knowledge and willingness to educate us!

Yes very clear this am.:thumbsup: Something was making them a little fuzzier to review last night:happy23: Not certain what that was:headscratch:

Formal education has helped you on the engineering front. Your eating habits are Feral. :stirthepot:

:cheers:



The chart assumes a 100lbs of driver force on the pedal resulting in a 300lb force on the master.

So I guess the question boils down to why the 100 lb number is used and how do we relate the figure to an amount of pedal pressure that is familiar?

Hi Josh,

No worries.

Good morning Rob,

Read my post to Josh & that should provide clarity.

Hey Josh,

Again, nothing wrong with the PBR/Z06 caliper. I think your application may need more aggressive pads & more rear braking force to be a good system. Also nothing wrong with upgrading it as you've mentioned doing.

Please go test the proportioning valve adjustment to confirm that is not a part of the problem.

Hi Dave,

I have worked with AP calipers a LOT ... but I am not familiar with those part #s, other than knowing they're 6-piston calipers.

Two questions:
1. What is/was their intended design purpose?
2. Can you measure the piston sizes & we'll run some calcs?

.

Guilty as charged.:shakehead:

You see a lot of oversized rotors in the PT world because of the aesthetic value of filling the wheel. Many of us know better and still commit the crime.

I fell victim but reading this reminds me why I really wanted to run smaller rotors out back.


Technical questions for Flash, this should be good.:popcorn2: :relax:

I have found the solution for all of our braking concerns! And it's cheap too!!!

Brembo Red Disc Brake Caliper Covers

:trophy-1302:

That is hilarious. :rofl:

I'm sure they're licensed by Brembo ... NOT. :sarcasm_smiley:

.

This is great stuff guys. :popcorn2:

I think I'm just going to add a "Thanks Ron" to my sig. :)

I got these from James Shipka as they were used on his One Lap Camaro a couple years before he switched. Their intended purpose is dual street/track. I have the slotted-only version (not drilled). Here are the links he sent me:

http://www.brake-pros.com/product_fi...0S%7Ebroch.pdf
http://www.brake-pros.com/product_fi...0S%7Ebroch.pdf

Piston sizes (Hey Rob, look at that!) we measured to be as follows:

1.05 / 1.05 in the rear and 1.5 / 1.25 / 1.05 in the front.

They also came with some Mintex Xtreme and Raybestos pads. Not sure which we will try out first.

After some discussions with a couple people I was planning on starting with 3/4 / 3/4 MC bores.

Thanks Ron

Yep. Nothing looks worse than an 11" rotor inside a 20" wheel. :bitchslap:

But then a lot of this talk is really helping to validate that form can be trumped by function and then form can actually be aesthetically appealing because of it.


hey Roberto..... see this post by your new Crew Chief below. He has posted this more than once.... FYI. :poke:

:thumbsup:

Thanks Ron.

Yes, 6:1 on the pedal ratio and 14" front and rear rotors.

I need to see if I can get an exact number/model # of the brake pads I got. I was also considering going with Carbotech XP-12 and/or XP-10 pads. I used them on my previous Wilwood setup (XP-10 front / XP-8 rear) and liked them.

Any experience with them?