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  #1 (permalink)  
Old 08-22-2020, 07:51 AM
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Default Front brake caliper pressure

Hi, I have a ERA 427 and my brakes just aren't what they should be. I installed a new Master cylinder and checked the line pressure. I'm getting aprox 760 lbs at the caliper. Is that normal? I bled and flushed the system several times and I'm using Dot 4 brake fluid the pedal is hard and positively no air in the line. Does all this make sense?
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Old 08-22-2020, 11:21 AM
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It depends on how hard you press down, but for 128 lbs of pedal force, 64 lbs presses the 7/8" dia. front MC to achieve approx. 479 psi with a 4.5:1 pedal ratio... if that is indeed the ratio.

I too have never been satisfied with my brakes. I've made some relatively inexpensive changes that have helped a lot.

First I installed Hawk HPS pads in the front.

Then I found the balance bar was out of adjustment and was locking up due to a severe angle between the bar and the Heim joints. The bar should be perpendicular to the cyl rods WHEN the brakes are applied.

Then I changed the front calipers. Turns out GM made their D52 calipers with quite a variety of piston diameters. For example, the ones used on a 1974 Malibu are 2,15/16" in diameter compared to my original 2,3/4" pistons from a 75 Camaro. This applies more force to the pads with the same line pressure. A fairly cheap and easy mod.

Lastly I installed EBC Yellowstuff pads in the Jag rear calipers. Hawk doesn't make pads for the Jag.

So I decided to calculate the foot pedal force for threshold breaking. For the first assumption, I’m going to assume that 0.9 Gs is around where the tires would lock up. The manufacturer of my car quotes a test value of 135 ft for 60-0 braking. This works out to 0.896 Gs so that helps support the 0.9 assumption. Also, they have achieved 0.9 Gs on the skid-pad, so although lateral skidding maybe not be the same as longitudinal skidding, they are probably close and once again the 0.9 G figure is supported.

So I went through some long (although not very complicated) calculations to calculate the pedal force required to get 0.9 Gs of braking. (I’m not going to bother showing these calculations here unless asked. Don’t want everyone falling asleep….)

With my original set-up, it takes 128 lbs of force on the pedal to achieve 0.9 Gs.

Once upgraded to the larger front calliper, it will take 117 lbs of force.

Although I found nothing so far for the 427 version, I did find two different specs on brake pedal pressure vs. stopping rate for the early 289 Cobras. One states 120 lbs for 0.93 Gs and the other yields 116 lbs for 0.93 Gs, the latter being an interpolated value from a table presented in the article.

By the way, I wanted to see how much 125 lbs of force felt like, so I braced myself against the bathtub and used my left foot to press a bathroom scale against the wall. It took a lot more effort than I thought and I have quite strong legs.

Finally I must admit that operating manual brakes takes some getting used to. You press quite hard and don't think it's braking well BECAUSE you are used to servo-assisted braking in your regular car.

All in all I'm satisfied with my brakes now, although I don't think I can lock the tires, it feels like it's on the threshold, which is good.
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Last edited by Argess; 08-22-2020 at 12:15 PM..
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Old 08-23-2020, 05:04 AM
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Excellent write up. Manual brakes take leg pressure. I have an old C2 Corvette and it has good disc brakes but you really have to stand on them to haul it down and the old Corvette design engineers on the Corvette Forum says that was the way they were designed - for lots of leg pressure if you really need to stop. I have the pretty much run of the mill brakes on my ERA and they are a bit easier on leg pressure for normal stopping than my 66 Corvette so Bob obviously put some good effort into the set up and pedal ratio/master cylinder sizing.
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Old 08-23-2020, 08:30 AM
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Quote:
Originally Posted by DanEC View Post
Excellent write up. Manual brakes take leg pressure. I have an old C2 Corvette and it has good disc brakes but you really have to stand on them to haul it down and the old Corvette design engineers on the Corvette Forum says that was the way they were designed - for lots of leg pressure if you really need to stop. I have the pretty much run of the mill brakes on my ERA and they are a bit easier on leg pressure for normal stopping than my 66 Corvette so Bob obviously put some good effort into the set up and pedal ratio/master cylinder sizing.
Totally agree with this. The Cobra brakes definitely need to be learned and appreciated. You have to drive it like an old race car I guess, power assisted nothing! I takes effort to steer and stop and I have learned to actually like it that way. Thank goodness the clutch is not the same way as all of my old muscle cars way back when. I find the Cobra clutch system to be almost effortless....unlike the brakes. Like Dan said, if I have to really stand on the brakes, the cars stops great. It is just a little unnerving to have to put that much effort into it because that is not what we are used to in our everyday modern cars
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Old 08-23-2020, 09:30 AM
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Default Here's how I test it...

I just came back from a ride and I thought I would test the brakes out as I have no idea what the PSI is on the brakes when you're standing on them. The test I've always used is that, after everything is nicely warmed, say after a half hour or more of spirited driving, you should be able to lock the brakes up so you hear and feel a little tire noise, and you should be able to do that without a mountain of muscle in your leg and while still retaining reasonable leg modulation. That means I can take the braking up to the skid point, by pushing harder, and then back off when I just begin to hear or feel a little skidding noise, and then hold it there or below. If you lock everything up and just skid completely you're not going to stop as quickly as if you do it properly. I have 12.2" rotors, Sierra dual piston calipers up front, with Hawk #HB101F.800 pads on them, and on the back I have the ERA outboard braked rear, which uses the old PBR calipers that were on the 1993-97 Camaro, Firebird, and some Corvettes. The pads are nothing special, just Bendix #SBM413. Fresh DOT 4 ATE Super Blue fluid (and it's really blue, not amber). This is how I would test your car's brakes if I was behind the wheel.
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Old 08-24-2020, 12:53 PM
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Yes Patrick, your braking system is much envied by those of us with a stock system, yet I feel bad for you as you're not getting the full Cobra experience.

Seriously, I don't even think those options were available when I ordered my kit back in 93, and even if they were I wouldn't have realized that I might need them. But for me, all is not lost because I enjoy tinkering and modifying my car more than driving it. In fact, most of my driving has been test runs... well a significant percentage at any rate. But that's probably just me.
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Old 08-24-2020, 01:31 PM
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I have owned an early ERA with stock brakes and a late one with the optional front brakes and ERA rear.

They both had similar brake feel and effort. The optional brakes could take more high speed stops before brake fade.

John
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Old 08-24-2020, 01:40 PM
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Quote:
Originally Posted by Argess View Post
... yet I feel bad for you as you're not getting the full Cobra experience.
I think I could achieve that by putting some grease on my brake rotors, loosening the hose clamps ever so slightly on the fuel lines and radiator hoses to induce a periodic fuel drip and coolant steam leak, and maybe lightly wiggling a plug wire or two so they have an intermittent connection when hot. Oh, and of course wiggle loose the wires on the fans so they only operate sporadically and the car overheats in traffic.
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Old 08-24-2020, 03:00 PM
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No, no, no; not talking about neglect requiring maintenance. It's about modifying to make things better.. and doing it yourself because either ERA didn't offer the upgraded system, or it wasn't chosen as the benefits weren't apparent to me as I had never driven one stock, or at all for that matter. It's very satisfying to improve the car, especially when it was well designed from the start. (a compliment to Bob P. here).

Back to the brakes. Maybe I can lock the wheels, but it doesn't feel like I should. Back in the day ERA recommended DOT 5 which I still use. So instead of a rock hard pedal, I get one that feels like if I press harder I'm bending something. It's just that DOT 5 is slightly compressible. No air in the system either. But it's a weird feeling that's hard to get used to.

They no longer recommend it, but I will say I havn't had any seal problems with it.

There was a thread, about 10 years ago or more, where the OP was worried because one front wheel locked before the other. So he could lock the wheels, but I'm not sure what braking system he had. I was surprised about his problem though.
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Old 08-24-2020, 03:09 PM
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Quote:
Originally Posted by Argess View Post
Back in the day ERA recommended DOT 5 which I still use...
Jeez, are you sure about that?
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Old 08-24-2020, 04:15 PM
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Quote:
Originally Posted by patrickt View Post
Jeez, are you sure about that?
Yes. Here's a scan from the manual I got in 1993 when I placed my order:



Here's a scan from a newer manual I got a year or so later after I had already added DOT5:

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Old 08-24-2020, 04:26 PM
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Well, dang. They likely did not change the brake calipers and master cylinders in between those two manuals.
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Old 08-24-2020, 04:39 PM
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I have no doubt Bob P. gave the best advice available at the time. Really, after all these years, my hydraulics are OK.

I did replace the clutch slave cylinder a few years ago because it had a very slight leak, but the seal was still firm. Just normal wear I think, possibly accelerated by a poor pushrod angle I later corrected.

The brake system is still original and has never leaked even though Tilton says to stay away from DOT 5.

Edit: Now Patrick, you know I like you and think highly of you, but I came back to edit with a little teasing: I thought all you lawyers knew never to ask a question unless you already knew the answer!

Last edited by Argess; 08-24-2020 at 04:42 PM..
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Old 08-24-2020, 04:50 PM
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Quote:
Originally Posted by Argess View Post
Edit: Now Patrick, you know I like you and think highly of you, but I came back to edit with a little teasing: I thought all you lawyers knew never to ask a question unless you already knew the answer!
That's only when you're asking them for the benefit of a judge or jury that is watching and listening. During depositions and the like most of the questions you ask you don't know what the answer will be. On the fluid issue though, every couple of years we have a thread where somebody asks what you have to do to switch back from DOT 5 to DOT 3/4 -- the last time it was discussed in depth in seemed like it was a huge PITA.
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Old 08-24-2020, 04:58 PM
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DOT 5 was popular in the early 90's or so for show cars. It isn't corrosive to the paint like DOT 3 or 4. It obviously has other issues and it isn't as popular as it once was.

John
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Old 08-24-2020, 05:33 PM
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Yes Patrick... I knew the question thing was a courtroom thing, not an internet forum thing, and that was part of the tease!

About a month ago or so I did an experiment by adding a few ounces of DOT 3 to the same amount of DOT 5. I shook the bottle up and waited. In only a few minutes, the two had separated with the DOT5 on the bottom. I should make a video and post it.

I've repeated the shaking a few times since then and never did I observe any of the frequently reported goopiness. They separated like water and gas, but took a few minutes instead of being almost instant.

It really makes me wonder if a lot of the problem switching from one to the other is generated by people repeating internet speculations.

I really can't see from my little test how a few drops of DOT 5 left in a system could cause any trouble when refilling with DOT3 or 4.
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Old 08-24-2020, 05:46 PM
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You're probably right. There's not that many people on the forums that have actually done the switch over. There's a ton of car related crap that is just plain wrong that gets regurgitated over and over on the 'net, like your oil temp having to be 212 degrees for water to be boiled off. But, unless you've actually done the job yourself (or measured the results yourself) all you really know is the old tales you've heard since you were a kid or have read on the 'net. Like, if somebody asked me "Patrick, how should I weld my gas tank?" I'd tell them that you have to fill it with water first. If they then asked me "have you ever done that yourself?" Well, no. "Have you ever personally seen it done with your own two eyes?" Well, no. Then WTF are you giving advice for? Well, that's a good question....
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Old 08-24-2020, 06:37 PM
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So what's the downside of Dot 5? Slight compressibility...anything else?

A good friend has it in his TR3A and 912 and also in his MGTC; he's used it for decades, swears by it, and wouldn't use anything else.

So....why is it no longer recommended? (I have to say, I've never used it myself)


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Old 08-24-2020, 07:13 PM
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Originally Posted by xb-60 View Post
So....why is it no longer recommended? (I have to say, I've never used it myself)
I tried to find a performance brake manufacturer that recommends DOT 5 and couldn't - but there's bound to be somebody out there that does, I just couldn't find one. Most of them say about the same thing that RDA does below (they were the last ones I checked).

Quote:
DOT 5 - Silicone based and must have a minimum boiling point of 265 degrees Celsius dry and 180 degrees wet. Being silicone based this type of fluid flows more easily through the pressurized braking system Giving greater braking performance and thereby reducing heat build up. The disadvantage is that by itís nature being more compressible it allows more room for air to be present within the fluid (air becomes trapped within its molecular structure).
Source: RDA Brakes, The Big Brakes Company. | Brake Fluid Information
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Old 08-24-2020, 08:02 PM
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My C2 corvette used DOT 5 as per their recommendation when I swapped out the calipers for ones that were stainless steel sleeved. Corrosion was a problem in the originals.
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