Brake experts, advice please.

I have put together a brake system for my old muscle car which is set up for track use. I down sized the pistons a bit on the calipers to help with feel, and the results are pretty good, but i have run into an issue, I think i might have made the pistons too small.

It takes a lot of effort to push the brake pedal, but I found that about 3/4th's through the pedal stroke, i reach a plateau of stopping force. If i continue to apply force and press the pedal down to the floor, the brakes do not apply any more force. I figure the pressure is becoming so high in the system that the system itself is beginning to flex and thats where the added leg force goes.

I am using Performance Friction 01 pads, and I believe these pads should be plenty for a car like mine. I could step up to a more aggressive pad, but I really like the 01's because they are so rotor friendly.

But, I'm wondering if my analysis of the situation is correct. I'm running very affordable wilwood calipers, and for a little bit more than the price of new pads, i can get different sized piston calipers. But I want to be sure that this is indeed the issue and that i haven't somehow exceeded the pad's operating range.
 

Randy V

Admin
Lifetime Supporter
John, if you can make that pedal go to the floor, you have air in the lines or your flex hoses are turning into baloons...

I used to run Performance Friction 92's in my American Sedan Camaro. I don't know how that equates to today's compounds but the 92's would work very well with mild rotor wear...
 
I have braided lines all around and its taking a ton of force to do what im doing, I really have to stand on it to get it to the floor. I recall air having a spongy and inconsistant feel, this is rock hard from the moment I get on the pedal and very consistent. I'm wondering whats going on. Could be caliper flex?

I wonder if the pad just doesnt produce enough torque for my application. Its a fairly heavy car, 3300lbs, but im on sticky street tires, not race tires. It doesnt sound right that the brakes cant overcome a hot street tire...
 

Randy V

Admin
Lifetime Supporter
You might get a PBR caliper to flex, but I seriously doubt you will flex a Wilwood.. Pull the pads and if the backing plate is still flat, you can rule out cailper related problems.

There is no way you will get the pedal to compress pure brake fluid unless you are boiling it - which will put air into the lines. My A/S car weighed about the same as yours. I ran DOT BF Goodrich gForce, Kumho v700 tires. I could not out-brake a Porsche GT Cup car, but I could deal with Vettes and Vipers all day long..
 

Howard Jones

Supporter
Here's the calculator I use. Very good and a lot of help. Start with feeding in your piston sizes, rotor diameters, weight, etc.

Then compare your line pressures to a known system like a new mustang GT or a Corvette. Better yet would be a friend's car that you know to be working well. A 65 mustang track car or something of similar layout and weight, running somewhat similar tires really works best.

Dual Bias Calc.

I don't think your pressures are too high. I think you are not generating ENOUGH pressure. Are you saying you can't stop (lock up) the tire under full brake effort? Either end of the car? That's really bad and you should stop driving it until you get it sorted.
 
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Here's the calculator I use. Very good and a lot of help. Start with feeding in your piston sizes, rotor diameters, weight, etc.

Then compare your line pressures to a known system like a new mustang GT or a Corvette. Better yet would be a friend's car that you know to be working well. A 65 mustang track car or something of similar layout and weight, running somewhat similar tires really works best.

Dual Bias Calc.

I don't think your pressures are too high. I think you are not generating ENOUGH pressure. Are you saying you can't stop (lock up) the tire under full brake effort? Either end of the car? That's really bad and you should stop driving it until you get it sorted.
If i adjust my proportioning valve more to the rear, i can lock those up with no problem. But I cannot lock the fronts when the tires are hot.

This calculator look good. I'm not sure if i can get some of these values. I hate how PFC doesnt publish their friction values, I guess I will have to estimate with that.

My brakes are vacuum boosted, im not sure what i should enter for 'leg force' considering the added power of the booster.

I am running a 1" MC bore and my front calipers have a 3 sq/in piston area. These are quite small. I'm thinking that my system is not efficient enough to generate the required pressure in the stroke that i have. In an ideal situation, there shouldnt be a problem, but, i must be applying well over 100lbs to the pedal and things are flexing.
 

Doug S.

The protoplasm may be 70, but the spirit is 32!
Lifetime Supporter
It seemed counterintuitive to me when my MC on the Cobra went south, but if you need more line pressure you need a MC with a smaller bore. I went with 15/16" bore and it was like changing over to power brakes!

Cheers!

Doug
 

Ian Anderson

Lifetime Supporter
My guess here

You get to a pressure of x and at that time your master cylinder starts to no longer hold pressure and fluid seeps past the seals

By seeping you wil gradually get your pedal to drop to the floor

Swap out your master cylinder as a quick and reasonably cheap check

Ian
 

Clayton

Supporter
My 2 cents worth
If your brake pedal is travelling at or over 3/4 of the stroke, you must have flex somewhere in the system.
I assume you run the std master on the firewall ?? As you have come down in size with the front calipers, you'll require more pedal pressure for the same pad clamping force. This maybe flexing the firewall area giving a longer pedal.
I have also seen some of the Wilwood calipers act like a hyd hinge (flex due to no bridge bolt and / or the housing bolts are well above the piston centreline)
Easy to check with someone cycling the brake pedal and you watching the caliper.

Could you lock the fronts with the old larger piston calipers ??
Did you use the PF01's in the larger calipers ??
How much piston size difference did you go down ??

Clayton
 
Which Willwood calipers are you trying? I once used the Billet Dynalites and got the piston size wrong( a little small) and used their street pad that gives off no dust( and as a tradeoff, not much stopping friction). I could almost bounce the pedal off the floor and it wouldn't stop any better. When I had a friend stomp on the pedal as I watched the caliper, I flipped!! The caliper flexed like a balloon! Superlites and Dynalites are better left for use on a dirt race car. Calipers like the SL series with the external rib are much stiffer. After that project I went with either Brembo or Alcon calipers, more expensive, but much better feel( when sized correctly).
 
I just spoke with someone who has set up a lot of these cars, I will check for myself when I have the time, but he was saying the firewalls really flex a lot on these cars when you run a high effort setup like I have. He went even as far tie the master cylinder to his strut tower brace, and he said the entire brace then flexed. I think I'm going to get calipers with bigger pistons to sort this out, might tie it to the strut brace as well.
 

Doug S.

The protoplasm may be 70, but the spirit is 32!
Lifetime Supporter
My guess here

You get to a pressure of x and at that time your master cylinder starts to no longer hold pressure and fluid seeps past the seals

By seeping you will gradually get your pedal to drop to the floor

Swap out your master cylinder as a quick and reasonably cheap check

Ian
That's exactly what was happening on my Cobra, Ian....with the result that the "leaking" brake fluid was escaping the master cylinder's body inside the cabin where the rod from the brake pedal enters the rear of the master cylinder, running down the inside of the firewall and ruining carpet (and shoes and the cuffs on pants and....well, you get the idea).

It'll be pretty evident, and particularly the smell would be, in any enclosed cabin such as an American made muscle car.

What I wonder is if Jim might have a leaking O-ring on one of the pistons in the caliper? That would result in a loss of brake fluid and that might be difficult to trace...except for the fact that the wheel and the exterior of the inner wall of the tire ought to be wet with brake fluid if it's leaking at the caliper.

Cheers!

Doug
 
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Terry Oxandale

Skinny Man
I'm going to deviate a bit and suggest the more aggressive pads. I've got the Outlaw 4000 series on mine, manual brakes, 3/4" MC with 1-3/4" calipers, solidly mounted with good lines. For the street I was using the Hawk HPS pads, which required a high degree of pressure (inordinate amount) to lock up the wheels. It almost felt like power assisted brakes with the engine turned off.

So I swapped to the DTC-60 pads, and once they were bedded in, the braking immediately felt "normal" in that much less pressure was needed to lock the tires, and the torque control was much more linear and lighter. The dusting is much worse though, but the control is so much more improved. I'm now thinking about backing that off to the DTS-30 pads instead to find a happy medium between the HPS and the DTC-60 pads (a little more effort, less dusting).
 
Terry, I also thought of putting in different pads. If you are running hawk pads, try PFC, you will be amazed in the difference. PFC makes compound 13 which is supposed to be substantially more aggressive. The price of the pads is around 200 bucks... or, i can get get new bigger piston calipers for 350 bucks. For that price, I might as well get the calipers and be able to run the awesome pad that i do.

Your MC being 3/4" and your calipers having 1.75 pistons (4 pistons i assume?) would give you a over 2x more leverage than what I have if i did my calculations correctly (and assuming our pedal leverages are the same).

My MC is 1" and pistons in the calipers are 1.38" (4 piston). I had these calipers laying around and thought I could get them to work. I think the pistons are just to small to work with my MC.
 

Terry Oxandale

Skinny Man
Wow. I'm amazed you are not having to stand on the pedal in order to stop (unless you have some kind of power assist). My calcs show a disadvantage of almost 3X on your set-up (Need 178% more pressure on the pedal with only 61% of comparable pressure at the calipers as compared to mine). I'm using the Tilton brake pedal assembly, in a 2000 lb car. Your "muscle" car may be heavier by at least 50%?

If your analysis of the "3/4th's through the pedal stroke" is correct, then making changes to improve the clamping pressure will result very quickly in exceeding the amount of pedal movement available. You'd need another "3/4" more pedal to reach a reasonable goal? Perhaps some other issues are also contributing to your problem (as noted by other contributors to this string).
 
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My brakes are boosted. The car weighs in at 3300lbs wet. Calipers I'musing in the front are Superlite's.

Am I calculating this wrong? I got your master cylinder at about 55% smaller area than mine and your front calipers having about 60% more area, for a total of 115% more area (and leverage) in your legs favor?

I think its hard to calculate pedal travel because we do not take the losses of flex (lines, calipers, firewall) into account. Its possible the pedal travel may be similar if i install larger pistons because the brakes will start working at a lower pressure resulting in less system flex/expansion. If there was no flex in the system, I would not be able to put the pedal to the floor.

Stock caliper area was 4.81...

Heres the car in action yesterday. This is my first run of the season and early in the day. I was a little rusty and feeling out the car after doing so much work to it. My runs latter that day were much smoother, but my gopro only lasted 2 sessions. The brakes never overheat, I could just not use all the traction the tire had...
https://www.youtube.com/watch?v=PukVEcnnaHI
 
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Ian Anderson

Lifetime Supporter
Do you have ABS on your car?
I have found that in most abs cars you can get the pedal to the floor as you stand on the pedal.
Ian
 
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