Brake LIne Pressure Question

[Argess]

After doing all I could to the existing brake system, I finally decided to measure the brake line pressure. With 120 lbs on the pedal, I calculated 500 psi in the front lines. I measaured it today and got 500 psi with my best guess at 120 lbs. I then pressed it as hard as would be possible when driving and it went up to almost 550 psi. Everything I've read recommends 900 - 1100 psi for non-assisted brakes.

Now Bob has stated in another thread how ERA has hundreds of these cars with the stock brakes that all work fine, however over the years there has been some threads complaining about the brakes. It is often suggested that people used ot power brakes are simply having an "experience" problem, but it appears to be more than that.

I will admit my brakes are probabbly glazed and that will be a next step looking for improvement, however they've never been good since I built the car. Now that I've made improvements I may get wbat I want with deglazing.

I was thinking of trying a 3/4" front master vs. the 7/8" unit, but Bob seems very sure the stock system should work fine.

So, does 500 psi seem OK, or low for line pressure? If it's low, any ideas what could be wrong?

[eschaider]

If you have the room to change your pedal ratio (giving your foot more leverage) without the pedal bottoming out against something, increase it, and you will substantially improve your line pressure. Alternatively, you can go to a ¾" master cylinder.

One of the challenges you want to be sensitive to, if you choose the ¾" master cylinder solution, is the potential to run out of master cylinder volume if you have big brakes with big pistons. This situation produces a problem known as no brakes ...

The ⅞" master is usually a good choice in most applications. You need to get your pedal leverage up to snuff, so it is not necessary to bust your gut to stop the car. Proper brake pedal mechanical advantage (leverage) will allow you to easily execute a stop w/o an extraordinary amount of pedal pressure.

If you have room in your chassis, and if you have sufficient vacuum at idle, you might want to try one of the old-style vacuum brake boosters, like the one below from a 1970 mustang:

Alternatively, you could use one of Ford's HydroBoost systems. However, they will require a power steering pump to operate. This is what a HydroBoost adapter for a '79 - 93 Fox Body mustang looks like from the side:



Same Hydroboost from the front:



As you can see from the comparison to the ⅜" fasteners this unit uses, it is vanishingly small compared to the older style vacuum-assisted model. The pictured unit uses an aluminum adapter to change the master cylinder bolt pattern from a 12 o'clock / 6 o'clock positioning to a 9 o'clock / 3 o'clock positioning. It does, however, require the use of a power steering pump. That said, a low manifold vacuum at idle has no impact on its performance.

Four different solutions to your dilemma. One is correcting your mechanical pedal advantage, one selects a different master cylinder, and two use different booster assists to achieve essentially the same end. One of the four will fit your needs better than the other three. Only you know which one it is.

I hope this helps.

[Tom Wells]

So, does 500 psi seem OK, or low for line pressure? If it's low, any ideas what could be wrong?

Argess,

I agree with Ed's answer - it seems your brake line pressure is below "standard." Here's a typical article on pressure: https://info.mpbrakes.com/faqs

If you DuckDuckGo brake line pressure this is kinda typical of the result.

It appears 750-1000 psi is a much more common number for brake line pressure. A bit higher for panic stops.

Measuring your present master cylinder's actual diameter seems like a good place to start.

Tom

[Tommy]

Argess -
500 PSI seems low to me. It's about what I'd expect from a power brake car with the power assist not working.

Does your car use Tilton style master cylinders or something else? If Tilton, they can diagnose your problem using data you provide about the various components. I used them when I encountered the same symptom as you on my Cheetah tribute (that uses C4 Corvette calipers). They had me drop my front MC diameter down one size and the problem was fixed.

[jknich]

What is your pedal ratio? Manual brakes are usually 6 to 1 and power brakes around 4.5 to 1. Maybe try a 13/16" master?

[strictlypersonl]

The pedal ratio is 6:1. My spreadsheet " brake balance calculator" shows that 150 lbs at the pedal will result in about 1G deceleration. That is a little high for people who are accustomed to power brakes. I'm working on some slightly smaller bore masters, but I am really careful about having enough volume for every circumstance.

[Argess]

Pedal Ratio is 5:1. No room for a booster. Those hydraulic pressure increase units don't appear to offer any more than replacing my front master cylinder with a smaller diameter unit.

Anyway, my plans are (if no-one has any different ideas):

1/ deglaze the pads and rotors, or instead install EBC pads as they have a deglazing top coating

2/ play with the balance bar adjustment. The pedal arm is more or less centered between the two cylinders now. I hanv't made any measurements yet, but if the distance between centers is 2" and I move the center of applied force 1/4" towards the front cylinder, I can expect 625 psi vs. 500 for the front brakes. The rear brakes will suffer, however as the front discs are so much larger, I may get an overall improvement.

3/ Adjust the pedal height so the force is at 90 egrees to the cylinders and hope it's ergonomically OK.

4/ Install a 3/4" front master cylinder instead of the original 7/8" one. No money wasted as if it doesn't work out, it can be a replacement for the rear brake cylinder when needed.

A couple of odd things:

One member did actually post his front caliper pressure and got 760. Not sure how that was posiible.

Also, another member once complained one of his front wheels would lock up before the other one. I say this is odd as I can't lock up any wheels at all. Not that one would want that, but it's an indication the brakes can work to the maximum.

After all this, I must admit my brakes are more or less satisfactory for normal driving. It's a panic stop situation I worry about.

[Argess]

Oops. Bob posted while I was writing. So it's 6:1. My rough measurement gave me 4.5:1 and someone else posted 5:1 from actually measuring.

So if it's 6:1 and I"m getting the same pressure as I predicted from a 5:1 calculation, I may get an improvement with getting the pedal arm force at 90 degrees to the cylinder rods as some of the force may be vectored off.

[Tommy]

Argess,
My method for setting the balance bar is to find a safe dry place where I can do an emergency stop from about 30 MPH. My goal is to lock either the front or rear tires. It can be helpful to have someone watch from a safe distance to see which end locks up first. If the rear tires lock first, I put in more front bias and try again. Repeat until the fronts lock up just before the rears. . . If the fronts lock up first, put in more rear bias until the rears lock up first, then take out enough so the fronts lock up first.

The goal is to get full braking from the front tires before the rear tires lock up. This will maximize stopping force and make it easier to control the car as the rear will not be trying to step out and spin the car.

[jknich]

Pedal ratio from 4.5 to 6 to one will increase pressure about 25%. 3/4" master from 7/8" will increase pressure at the caliper and decrease leg pressure needed but increase travel at the pedal. You want around 1200 psi with 150 lb leg pressure.

http://wallaceracing.com/brake-master-cyl-calc1.php

[Morris]

Or you could balance the brakes like us Racers do with temperature paint....paint the rotor and run a session then read the rotors

[url=http://www.clubcobra.com/photopost/showphoto.php/photo/90611][/u

[Morris]

Or you could balance the brakes like us Racers do with temperature paint....paint the rotor and run a session then read the rotors

[url=http://www.clubcobra.com/photopost/showphoto.php/photo/90611][/u

[6TNCRZY]

I believe the standard pad for ERA cars with Sierra or Wildwood Calipers are BP-10s. I had these pads on my car (ERA2158) and braking took a lot of effort. I just replaced the pads with BP-40s and the difference is amazing. Much less effort to stop the car and it now feels like the fronts are really working.

[eschaider]

Originally Posted by Argess

Pedal Ratio is 5:1. No room for a booster. Those hydraulic pressure increase units don't appear to offer any more than replacing my front master cylinder with a smaller diameter unit.

You want to be careful not to estimate what your brake system is or is not doing. Your 4.5:1 pedal ratio for a pedal with a 6:1 ratio is an excellent example of this. Estimation errors in assessing your current system can be a recipe for failure and braking system failures are more often than not, disastrous. You don't need to go there.

As you begin whatever modifications you determine are necessary, measure for certainty before moving forward. It will prevent disasters.

Originally Posted by Argess

Anyway, my plans are (if no-one has any different ideas):

1/ deglaze the pads and rotors, or instead install EBC pads as they have a deglazing top coating.

This is not a bad idea but has absolutely no effect on the line pressure your braking system sees at a given pedal pressure. Your current system, at a 500 or 600 psi operating pressure is woefully inadequate.

Originally Posted by Argess

2/ play with the balance bar adjustment. The pedal arm is more or less centered between the two cylinders now. I hanv't made any measurements yet, but if the distance between centers is 2" and I move the center of applied force 1/4" towards the front cylinder, I can expect 625 psi vs. 500 for the front brakes. The rear brakes will suffer, however as the front discs are so much larger, I may get an overall improvement.

The balance bar is quite useful in, not surprisingly, balancing the braking capacity between your front and rear wheels. You do not want the rears to lock up before the fronts — bad things happen. This bar is the adjustment you use to prevent that.

More to the point, this bar will not increase line pressure from your current levels to where it ought to be. It will allow you to, as its name implies, balance the car's braking capacity front to rear.

Originally Posted by Argess

3/ Adjust the pedal height so the force is at 90 egrees to the cylinders and hope it's ergonomically OK.

Unless you have a very significant misalignment between your master cylinder (M/C) bore and the brake M/C pushrod that applies pressure to the M/C piston, this amounts to little more than rearranging the deck chairs on the Titanic. Here is a graphical representation of the possible change in applied pressure. The graph on the left is the equivalent of the pedal force applied and the various pushrod offsets. The right graph is the change in force seen at the M/C piston for a zero, 5˚, 7.5˚ and 10˚ misalignment.



Although the misalignment units are metric the message is clear, small changes in axial alignment offer only small changes in applied pressure and therefore line pressure. It is improbable you have a significant misalignment.

Originally Posted by Argess

4/ Install a 3/4" front master cylinder instead of the original 7/8" one. No money wasted as if it doesn't work out, it can be a replacement for the rear brake cylinder when needed.

THIS will change your braking system line pressure. Again you want to proceed with caution. As Bob pointed out he is doing his own testing to look for situations where the smaller diameter M/C may underperform or fail. You need to do the same thing.

An example of potential braking system failure would occur as the pads and discs wear and the pistons need to protrude further from the caliper to clamp the disc with sufficient force to stop the car.

If the volume of brake fluid used in this scenario exceeds the capacity of the ¾ inch bore in the M/C, braking pressure will be inadequate and the car will not stop as anticipated.

Originally Posted by Argess

A couple of odd things:

One member did actually post his front caliper pressure and got 760. Not sure how that was posiible.

His pedal ratio and M/C bore along with how hard he presses on the brake pedal determine this number.

Originally Posted by Argess

Also, another member once complained one of his front wheels would lock up before the other one. I say this is odd as I can't lock up any wheels at all. Not that one would want that, but it's an indication the brakes can work to the maximum.

When a single front or rear wheel locks up before it's twin, the braking system is telling you it needs maintenance.

Originally Posted by Argess

After all this, I must admit my brakes are more or less satisfactory for normal driving. It's a panic stop situation I worry about.

The challenge you are dealing with is similar to attempting to explain to someone who has been blind from birth what the color blue looks like. The blind person simply has no frame of reference.

Same thing when attempting to assess the efficacy of your brake system from the seat of your pants while driving the car. It is very difficult, if possible at all. Because you have no frame of reference (and other reasons) is why the industry settles on standards for the myriad of systems like brake system line pressure that we have in these cars.

The standard for hi-po street driven cars brake system line pressure is the 750 to 1000 psi number Tom Wells was speaking about. If your line pressure is correct, your friction material (brake pads) are correct and your disc diameter is adequate your car's stopping experience will be as it should be and very likely much different than it is right now.

If you want to absolutely nail it then the there is the racer approach Morris shared with you using Tempilaq® temperature indicating liquids. You have multiple paths to a happy ending available to you. All that remains is for you to choose the one you wish to use.

You already have the right discs and calipers and very likely the right or close enough to right brake pads. The only thing you are missing is the proper line pressure.

As the secretary in the mission impossible series always informs Mr. Hunt, "You mission Mr. Hunt, should you choose to accept it ..." In your situation, your mission, and you have no good alternative other than to accept it, is to get your brake system line pressure up to normal levels.

[Argess]

Just got power back after Hurricane Fiona raged through, so I'm a bit late reponding to your posts, but T-Y. I appreciate the replies.

So what's going on? I did some measurements today and I get what I once did before for pedal ratio; 4.5:1. I measured with the pedal arm in the car and got 9" from pedal face pivot to the arm pivot and 2" from the arm pivot to the balance bar shaft. So it's not 6 as far as I can tell. The 5:1 was from a thread on this site where it appears the poster measured more accurately most likely due to having the arm on the workbench.

Here's my calculations for front caliper pressure:

Say 150 lbs pedal force, which I might have achieved when I tried extra super hard, times the ratio of 4.5:1 = 675 lbs.

Next divide by 2 as the force is split between the front and rear cylinders by the centered balance bar and we get 337.5 lbs. As the pedal arm currently is off 14 degrees from vertical, we multiple by the Cos (14) and that reduces the force to 327.5 lbs for the front master cylinder.

So the pressure at the caliper is that force divided by the cylinder area which is Pi * (r^^2), or 0.6 sq.in for a 7/8" cylinder, and we get 544.9 psi which is exactly what I measured within a pound or two.

OK, so we all agree the pressure is too low, but it matches the system theory.

Now, as mentioned above, the offset angle is only 14 degrees, so not much to gain from adjusting it.

The balance bar is not like what's shown in the current manual as mine is an older ERA. It uses tubes for spacers vs. washers, so new tubes of different lengths would have to be made. For all intents and purposes, the exisitng tubes are the same length (one does appear to be 1/32" longer).

So, I appear to be getting the maximum out of the design but it seems low compared to quoted recommendations, so trying a smaller dia. cylinder seems worthwhile. Removing glaze seems to be a good idea, but I agree, it won't change the pressure.

So, I'm a little stuck on all this..... more experimenting I guess.

ps: On the brake temp indicators, I have no idea hot brakes should get... no baseline, so that's out.

[Whodeeny]

My brakes on ERA #821 are not very impressive either. I am unable to lock the wheels up in a panic stop and it seems as if the pedal goes almost to full stroke in a panic stop.

[jknich]

3/4" front master would be a good bet.

[Argess]

Yes, a 3/4" MC is likely the next best thing to try. All in all, increasing the size of the caliper piston will also do the same thing.

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.

This is one of the things I've already done and it did contribute to better braking. So did the installation of Hawk HPS front pads and EBC YellowStuff rear pads. BUT, I still want more! LOL.

[eschaider]

Originally Posted by strictlypersonl

The pedal ratio is 6:1, ... I'm working on some slightly smaller bore masters, but I am really careful about having enough volume for every circumstance.

Just as Bob is, you should also give careful thought to these modifications, in particular if you are using OEM calipers with their larger diameter pistons and larger required fluid volumes both for braking and also to compensate as friction surfaces wear. A lack of attention to these kinds of design / engineering considerations can result in a crash or worse, loss of life for someone who is not at all involved — and that would be on you! If you do not know how to do the design / engineering required then you should buy the professional services of someone who does.

[Argess]

My apologies to Bob. I did make a mistake about the pedal ratio. Knowing he's an excellent engineer, I doubted myself, re-measured the pedal and found my 2" measurement was off. It's about 1.5" so with the other part at 9", that is indeed a 6:1 pedal ratio.

It's tricky to measure with the pedal installed, but I still shouldn't have messed up that badly. But I did. Sorry Bob.

So now a new problem. Why did my math with the wrong pedal ratio yield the same results as the pressure test? It's all because of one unknown... my foot pressure on the pedal. I think I have an solution for that. I have a valve spring pressure gauge I may be able to put between the pedal and my foot. As I won't be able to see it, I'll make a video and.... oh-oh... I need the video camera to monitor the guage on the caliper too!

Looks like I'll be recruiting a helper.