Brake LIne Pressure Question

[eschaider]

Originally Posted by patrickt

It took me a while to figure out you meant "care" not "car."

Thanks for the heads up, Patrick. I have corrected it.

[DanEC]

One of my Tilton cylinders was bad almost immediately on my car - l believe it was one of the brake masters. I noticed it as the brake cylinder cover had fluid on it. The master had score marks in the bore that led me to believe there was too much angularity in the rod as it was cycled, cocking the piston. I replaced the cylinder and all these years later don’t even remember how I addressed the angularity issue but the problem hasn’t returned. But it does happen and some times after just a few miles.

[Grubby]

I had to shim the bracket that bolted to the block on 755.

The rod was at a terrible angle just bolting it on.

John

[Argess]

I should have the new MC in a few days. Am very anxious to see how it affects the line pressure.

It has occured to me that I may have worn the cylinder egg-shaped early on. When I first assembled the car, the MCs and pedals were already installed, so I didn't pay much attention to them.... for years.

What may have happened is by pressing the brake pedal, the rear master cylinder got up to pressure before the front as the MC pushrods had been set to the same length. This would jam the pushrods against opposite ends of the tube that holds the balance bar bearing. Maybe this would cause the cylinder to wear.

Once I change the front MC, I will measure the bore with a telescoping gauge. According to my brake handbook, a difference of 0.006" is considered to make the cylinder useless.

[Argess]

My new MC showed up yeterday and today I managed to install it. I can now manage 700 psi (plus the width of the needle) vs. 550 psi; so just over 700psi. I might be able to get more, but I need to adjust the seat as my leg is too straight (I had moved the seat back to accomodate the piece of wood I was using to hold the brake pedal down a little bit). Didn't have time for a test drive yet, but wether the brakes can lock the wheels or not, I am very optimistic about getting at least 0.8 Gs while braking hard.

When I look back, hindsight tells me I made too much of this, however there were a lot of factors leading to confusion. First of all, my measured brake line pressure of 550 psi should have yielded over 0.8 Gs, but I only got 0.67. The fact that the front MC would slowly drop was not very noticeable and I didn't realize this was happening for a long time. Nor did I realise the front MC eventually bottomed out disallowing any more pressure increase. Some factors were sort of unkown such as the pad material co-efficient of friction and the pedal to MC geometry and what the brake line pressure normally should be. Additionally the use of DOT 5 came into question as the condition of the seals was unknown. The MC bore wasn't thought to be worn with so little use, but it turns out there was damage.

However presistence seems to have paid off. I'll know for sure after a test drive. I need to do a final bleed first (am leaving it overnight to help air release from the DOT5). I also still intend to change the front HPS pads for Yellowstuff pads, but that can wait until Spring.

Now, as for the old MC bore, I originally stated the bore looked fine. I checked it more thoroughly today. I measured it and there was no wear at all based on the internal diameter measured at locations 90deg apart. There were however some extremely light scoring as per the picture below:



These little scratches don't look like they were made by the piston. My best guess is swarf or grit left in during manufacturing. Or possibly a bit of dirt got in while adding fluid. Of course they may be other damage I couldn't find with the boroscope.

[Argess]

I just got back from a test run and it didn't go as well as I hoped. 0.7 G's was it and I was hoping for 0.8+. I started out cautiously trying to brake hard. A little harder each time. With the 0.7 G number, the right front wheel pulled to the right so there must be an issue with the left front caliper or pads. I stopped testing after that.

Am leaving it for today, but I suspect if I remove the caliper and extend and retract the piston a bit (like I did to the other side), re-sand the pads and re-lube the slider pins it should fix it.

I have to admit, going from 0.61 Gs when I started testing to 0.7 Gs seems like a huge difference when braking hard. Also, now the front end drops so much, the G-Meter goes out of calibration, so I might be getting a little more than 0.7. You calibrate it while on level ground and any variance from that drops the measurement a small amount.

[patrickt]

But if you can lock up all four wheels, aren't you getting the most you can from your brakes, at least at that particular pad temperature? I would think your G forces are more related to the ability of your tires not to brake loose.

[Argess]

I don't think any of them locked up. Mind you, it's quite cold out today so maybe the tires won't squeal in the cold?

Now, a 427 ERA cobra has done 0.9 Gs on the skidpad and even though that's sideways and I'm testing going forwards, the tire traction ought to be somewhat close between the two. Also, ERAs testing that gave time and distance worked out to almost 1 G.

I really need to mess with that left front caliper before I do any more testing or make assumptions based on a brake system that isn't working quite right.

[Argess]

It was raining yesterday so I took that left front brake apart. Couldn't find anything wrong with it, but ran the piston out about half-way and then back in to make sure it was working.

Before I took anything apart, I jacked up the front end, put the brakes on hard and then let off the pedal and then rotated each front tire. The right side (I assume is working fine; and I'll explain why shortly) had a very small amount of brake drag. The left side had none.

After I did the work to the left side, both had close to the same slight drag, with the left a tiny bit less than the right.

Today the rain stopped for the morning, so I did a test drive. 0.72 Gs and I know I didn't press the pedal as hard as I could have, but close. The car did not pull to either side.

I'm at a loss to explain this, but it seems that not all the hydraulic pressure is being transmitted through the caliper pistons to the pads. My only thought is the caliper piston seals are sticking and holding it back a small amount. I tested one of my old calipers and it took 50 lbs pressure to move the piston and up to 100 as the psiton came close to beng all the way in. This seems insignificant to the 5000 lbs of hydraulic force, so I'm at a loss. But if the right side was stuck and let go when the car suddenly veered to the right a few days ago, now somewhat evidenced by the slight brake drag and now that I've moved the left side piston in and out hoping to break any bond between the piston and seal, it's working better.

Hard to believe, so I'm not a believer yet!

Not liking these D52 calipers, I have looked into dual piston calipers, but the SSBC ones have a much smaller total psiton area and I don't want less force, and the Wilwood ones are too bulky and won't fit my 15" rims.

That's my update for today... for all those who may be interested.

[patrickt]

Originally Posted by Argess

That's my update for today... for all those who may be interested.

I'm interested, but I thought you might test the brakes in the rain. If you can't lock the wheels up in the rain, the brakes ain't working right.

[jknich]

What shape are your caliper lines? Braided or rubber?

[Argess]

They're rubber lines, however I've mostly ruled them out. I did try SS braided teflon hoses, but only tested them while monitoring a pressure guage at the caliper. I didn't try driving with them as they were too long and the SS braid would have caused a bad chafing problem on the upper a-arm.

Just as I've run out of ideas, a new thought just occured to me. I may have screwed up. Although the D52 calipers have slide pins, some of the force on the caliper from the spinning rotor appears to be taken up by the caliper in contact with the knuckle. This is an area where sliding occurs when pushing the pedal. It never occured to me to grease that area. I must look again to confirm this and try some brake grease.

Although the D52 calipers have slide pins, the actual force on the caliper from the spinning rotor is taken up by the caliper in contact with the knuckle. This is an area where sliding occurs when pushing the pedal. It never occured to me to grease that area. I must look again to confirm this and try some brake grease.

[DanEC]

I’m kind of surprised Bob hasn’t weighed in more on this - at least to the point of if your expectations are reasonable or we’re missing something in the details. Are these “G” meters all that accurate?

[Argess]

I'm not surprised. I'm sure Bob realizes that I have a combination of problems from parts to installation issues. Also, he has stated many times he has hundreds of cars out there with the standard brake package that work fine. As far as my expectations go, they are based on ERA test data and I'm not quite there yet. All on me really to deal with but as two (or maybe more) heads are better than one, I like to ask in the forums. Sometimes when I think about what I want to write, something clicks and I make progress.

My G-Tech meter is an old one that relies solely on an internal accelerometer. The new ones rely on GPS. The Instataneous G setting has a stated accuracy of +/- 0.02 Gs.

I originally bought it to measure horsepower but unfortnately that is not that easy in our cars. First of all they are a standard and second there's too much engine torque so you need to use high gear while testing.

For Hp, you input the weight and the accelerometer part monitors the acceleration. Because our cars need to be put in high gear, we wind up going too fast. All the meter measures is the acceleration of the car and converts to Hp, however it doesn't take much speed before air resistance comes into play and that takes Hp not measured by the meter.

I do know the meter works accurately for Hp as it calculated the correct Hp for my Toyota RAV4 (automatic). It's rear wheel Hp and I had to convert for an AWD system. Bang on, exactly!

[cycleguy55]

Originally Posted by Argess

Not liking these D52 calipers, I have looked into dual piston calipers, but the SSBC ones have a much smaller total psiton area and I don't want less force, and the Wilwood ones are too bulky and won't fit my 15" rims.

I don't know which Wilwood calipers you looked at or the 15" wheels you're running, but I have Wilwood Dynalite calipers on mine with 15" American Racing Torq-Thrust D wheels and there's no issue with clearance.

Wilwood Classic Series Dynalite Front Brake Kit
Prod #: 140-14271
11.03" x 0.88” rotor and hub assembly
FDL-M four piston forged aluminum Dynalite calipers
Piston bore: 1.75”
Piston area: 9.62 in2 (2.4 in2 x 4)

[Argess]

Now that led to good information for me. T-Y I wanted to use the Wilwood 120-10937 dual piston D52 caliper but their drawing showed it wouldn't fit.... for 12" rotors. I thought I had measured mine and got 12", but the ERA site says they are 11" so they will fit!

Note: I used a tape measure and as the darn knock-off hub was in the way; no doubt my measurement was incorrect.

As I'm trying to get more braking power, I didn't want to go with the Dynalites as the total piston area was much smaller than the Wilwood D52 or my GM D52s. Also it's nice for me to use D52 pads which fit the same in the GM calipers or Wilwoods.

and I assume yours work fine?

EDIT: There is one slight problem with the Wilwood D52 units. They use a 7/16 x 20 banjo bolt so my existing flex lines would need to replaced or reworked. Not a big deal I suppose.

[cycleguy55]

Originally Posted by Argess

As I'm trying to get more braking power, I didn't want to go with the Dynalites as the total piston area was much smaller than the Wilwood D52 or my GM D52s. Also it's nice for me to use D52 pads which fit the same in the GM calipers or Wilwoods.

and I assume yours work fine?

EDIT: There is one slight problem with the Wilwood D52 units. They use a 7/16 x 20 banjo bolt so my existing flex lines would need to replaced or reworked. Not a big deal I suppose.

Yes, mine work fine.

As I noted in my comments, the total piston area of a Dynalite caliper is 9.62 in2 (2.4 in2 x 4). Wilwood's website shows them as 4.8 in2, but if you do the math you're realize it's actually double that, as I've calculated. BTW, I have an email thread with Wilwood about this, and my calculations are correct. I have no idea why they haven't corrected their website.

With 4 x 1.75" pistons the Dynalites actually have 53% greater piston area (9.62 in2 / 6.28 in2 = 1.53) than the Wilwood D52 calipers (2 x 2" pistons).

As to brake hoses, they're relatively cheap, even the SS ones. Pick your brakes, then go from there.

[Argess]

I don't think you should have used the fact it's 4 pistons. Calculations should be one side of the caliper only, so base it on two.

Anyway, it's good to know yours work fine.

[sunman]

The single piston D52s are available in a range of sizes from 2 3/8” to 2 15/16” factory and I think down to 2” aftermarket.
Just a FYI thing I haven’t seen mentioned.
Ive always had 1 1/8” master cylinder with power when using the big calipers.
Using a 1 1/8” Corvette disc/disc mc with power and big D52s on the CR.

[Argess]

Good point. I did change out my D52 from 2.75 (1975 Camaro) to 2,15/16 (1974 Malibu) in an effort to get more pad force. Oddly, the larger calipers took a 10mm Banjo bolt which still worked fine with the flex hose made for a 3/8" banjo bolt. No telling when you get remanufactured units I guess. One of them was even re-worked for a larger bleed nipple.