Brake LIne Pressure Question

[patrickt]

I have absolutely no idea what the problem is with your brakes, but I will tell you that with my four piston Sierras up front, which are pretty similar to the four piston Wilwoods, and the outboard braked rear behind me that uses the floating PBR calipers that went on Corvettes and the performance Camaro/Firebird builds, that I can lock my wheels up without too much trouble. Now, I did switch to Hawk street pads to eliminate the squeal/groan of whatever pads were on there to begin with. Wish I could contribute more, but I just don't know what the problem is. Do you have any inclination to move up to what ERA calls the "big brake" option? In my experience, when you can't figure out what the problem is, if you just start replacing everything in sight with bigger and better stuff you will eventually stumble on whatever it was that was causing the problem and you will have fixed it.

[Argess]

Yes Patrick, it was because of you I installed Hawk pads a few years back and there was noticeable improvement. Since then I installed larger diameter piston calipers and EBC Yellostuff pads in the back. With every change, the brakes got better and really, they arn't horrible like they once were.

One thing I wonder about is all this business on bedding in and glazing. Somewhere it states that bedding in the brakes causes a small amount of pad material to deposit on the rotors which is good, yet "they" talk about glazing and that's a bad thing.

Not sure about mine, but the rotors look like brand new. Shiny and no rust and they're 26 years old. Something must be coating them as I live near the coast, salt air.

So there's more that I can try. One thing is, judging by the number of threads on braking, it appears I'm not alone. Wouldn't surprise me if we all have the same problem.... and to confuse things, most don't have an issue, or at least have never stated they do.

[patrickt]

You know, the big rotors on my Jeeps literally develop surface rust spots overnight when the weather is just right -- I've seen it a hundred times. Yet the rotors on my Cobra never have even a hint of anything. Yes, she does stay in a comfy warm garage, but other parts of her have developed a little surface rust from time to time, but never the rotors. So maybe you're on to something.

[Argess]

Maybe so, my RAV4 develops rust on the rotors overnight if it's been raining. You can hear it the first time or so you apply the brakes.

By the way, I meant to mention in a previous post I once found some sort of military publication about ergonomics, and for a seated position they recommended a max. of 125 lbs at one foot. So 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. I think that 150 lbs at the pedal is not all that reasonable, or maybe achievable is a better word.

[Argess]

Update: I moved the balance bar a significant amount (1/4") to favour the front brakes. The pressure at the caliper did not increase. This seems to imply a problem with the master cylinder, however it does not leak (external leak) nor does the pedal sink to the floor (internal leak).

The pedal does not hit anything that would stop it. Bracing the pedal with a piece of wood against the seat to eliminate the free play, the MC rod only moves 5/16" so the rod is not bottoming out as the full rod stroke is 1".

I think my best move is to get two new cylinders or rebuild kits.

Thoughts?

[Argess]

I forgot about my G-Tech meter, so I went for a test drive today. All I could achieve was 0.64 Gs, which works out to 187 ft to stop from 60 mph.

ERA states they got 135 feet from 60 mph. That works out to about 0.89 Gs based on 2800 lbs for car, driver and fuel.

As an aside, I also tested my RAV4 and only got 0.67 Gs.

As mentioned before, I intend to replace/rebuild both master cylinders. I didn't think they showed any sign of internal leakage, but now I see there might be a little. Stand on the brakes long enough and the pedal does depress ever so slowly. So slowly I've never noticed it driving or doing pressure tests. So slowly it shouldn't stop peak pressure from building, but it would bleed down over a significant amount of time.

[Argess]

A little update. I decided to bleed the brakes once again and this time I removed the pads and re-surfaced them with sandpaper. I also scuffed up the rotors a little bit. A good improvement after bedding in the pads again. The nose of the car would now squat down significantly under heavy braking. The 0.64 G's I previously had mentioned was a one time thing. The repeatable number was 0.61 G's. Now I get 0.67 G's and it is repeatable.

Still curious, I mounted a tape measure under the car to measure the front master cyllinder stroke and found that it was bottoming out. I bought a Tilton rebuild kit and changed it out. The old piston and seal showed no signs of wear and the cylinder bore was immaculate. No difference after doing this.

Thinking something was yeilding anyway, I installed SS teflon lines. No difference after that. So it's air. Now I've bled the brakes both with the vacuum method and reverse bleeding. The last time, I removed the calipers and held them up so the flex line was vertical and tapped the caliper to loosen any air bubbles. The net result was 550 psi where before it was a little over 500, maybe even 540. Hard to be acccurate with such a small guage.

So, there's one fact I didn't ever mention, and now it's time to fess up. I'm using DOT 5 fluid which is known to hold air and can make brake bleedng difficult. Mind you the rear brakes seem to work OK. The rear tires actually locked up during one test as I hit a few tiny pebbles on the asphalt and I could hear the tires skipping on them. The rear brake cylinder only moves a small amount when activated, so the DOT 5 is working well with them.

Also, it should be noted that the front master cylinder seals showed no signs of being affected by 25 years bathing in silicone brake fluid. They were the same softness as the new seals and did not show wear. Neither the piston nor bore showed any signs of wear either; (only 9000 miles on the car, so not a lot of braking).

I am at a loss. I guess I could swap out the whole master cylinder, but I doubt very much anything will change. I could swap out the DOT 5 to DOT 4 or something. I hope I don't have to do that. I also bought some Yellowstuff front pads to obtain the best braking I can get with what I have, but until I can figure out how to keep the master cylinder from bottoming out, it makes me uneasy to rely on those pads.

[patrickt]

Originally Posted by Argess

So, there's one fact I didn't ever mention, and now it's time to fess up. I'm using DOT 5 fluid which is known to hold air and can make brake bleedng difficult. Mind you the rear brakes seem to work OK.

Well I wish I could be of more help. But I do remember that when my car was being built at ERA Doug told me that if I ever put DOT 5 fluid in it that he and Bob would fly down and take my car away from me and give me no refund. So I believed him, and I never did.

[Argess]

Well Patrick... I think they know something I don't know, or maybe am slowly finding out. You may remember this excerpt from my original manual (which I'm sure was based on the best information available at the time):



Of course they changed it to this:



I speculated that it was due to Tilton warnings about seals, but maybe they had trouble like I'm having too.

[patrickt]

Hmmm, I believe that is the Gutenberg Version of the ERA Manual. Oh well, live and learn I always say. We've had any number of threads on going back to DOT 3/4 after toe-dipping in DOT 5. It apparently is not as catastrophic as we all feared.

[Tommy]

Just today I was reading an article on C2 Corvette brakes by a guy recognized as a genuine authority. He commented that silicone fluid is prone to making and holding tiny bubbles when agitated. They will remain in the fluid and adhere to the metal surfaces so they will not migrate to the bleed valve. For that reason he recommended a very specific process for adding silicone fluid to the system. I'm including a link to his article.

I still think 550 PSI is too low and the fact that your MC stroke is bottoming out likely means one of two things. Brake fluid is not compressible, so if there is no air in it your system the MC must have too little volume for the calipers. If your MC is properly sized for your calipers, there must be air in it that is compressing to a smaller volume before the stroke of your MC can go above 550 PSI.

CRG Technical Article - Master Cylinder Rebuild

[DanEC]

Originally Posted by Argess

Well Patrick... I think they know something I don't know, or maybe am slowly finding out. You may remember this excerpt from my original manual (which I'm sure was based on the best information available at the time):



Of course they changed it to this:



I speculated that it was due to Tilton warnings about seals, but maybe they had trouble like I'm having too.

I think it does stem from warnings by Tilton and possibly other cylinder manufactures. I think it may be in Tiltons literature. But - I’m not convinced it’s a real issue.

DOT 5 has been fine for many years in my 66 Corvette - which incidentally also takes a great amount of leg effort to motivate the brakes. Old time GM Engineers on the Corvette Forum told us - that’s how we designed them way back then.

[strictlypersonl]

Bring up something that I wrote 30 years ago. ;>)

Even I can learn from experience.

[Argess]

No offense intended Bob. I merely wanted Patrick to know why I chose DOT 5.

Didn't have much time this morning, but I may have made some headway. Reading up on DOT 5 it turns out many people have had trouble bleeding their brakes. One guy "tickled the pedal" and re-bled every day for two weeks before he was satisfied with the pedal. Another took 2 months playing with his on and off.

A lot of speculation has been presented stating that the fluid is very tenacious keeping air bubbles attached to it.

So I wondered where this air mught be. Suspension most likely, but then I thought of the space around the caliper piston. Even at 0.005" gap, a 2,15/16" piston could have enough air around it (incredibly unlikely) to take up 3/4" of MC travel.

Now all vacuum bleeding does is pull fluid from the reservoir to the caliper bleeder. I wanted more of a bell jar test. So I put a piece of wood between the seat and the pedal... just enough for the piston seal to pass the compensation/fill hole thereby isolating the front brakes from the reservoir. Loosened the bleeder and applied vacuum. Yes siree... air started coming out!

Next step is to take the caliper off and lay it out flat with the piston bottom facing skyward and repeat the procedure. Then install it and pull any air that crept out of the piston/bore clearance.

[6TNCRZY]

in theory a small amount of air in the system should have no impact on ultimate pressure produced as long as the piston does not bottom out or linkage is binding. it would have an impact on feel and the amount of travel. I would check to make sure the master cylinder push rods are adjusted so that when the brakes are pushed hard the brake bias adjuster is not hitting the inside of the pedal tube, if it is touching it could limit maximum pressure.

[DanEC]

I’ve had a lot of issues trying to vacuum bleed brakes due to air being sucked in around loose bleeder screw threads and right back out the bleeder port to the jar. I like to never got the GM calipers on the ERAs front brakes bleed out because of this. I now use speed bleeders or the wife in the driver seat pressing on the brake pedal in rhythm to commands from under the car.

[Argess]

Thanks guys.

Dan: I discovered this a while back and now use teflon tape on the threads. Grease didn't work for me.

6TN: Yes, I was puzzled for a while why the pressure wouldn't increase until I found the front MC was bottoming out. As far as the balance bar goes, I've also found out that at extreme angles the balance tube can limit the motion of the pushrod, however it's adjusted for the largest motion of the front MC with respect to the rear MC and it's only being limited by the front MC bottoming out.

I'm rather optimistic about my method outlined in my last post. Blocking off the reservoir allows me to keep fluid from flowing while enlarging the air bubbles so they break free. Tapping the calipers while doing this also helps. Mind you, too much vacuum would allow fluid past the MC seal as they are only meant to hold pressure in one direction. I find 8 to 10 in/Hg seems to work well.

It's still sort of experimental, however I hope to find out this weekend if I make gains with this endeavor.

[Argess]

Well... before I start I must admit to a lot of confusion with trying to diagnose my brake issue.... and so the saga continues.

I tried my method of using a piece of wood to hold the brake pedal slightly on, blocking the compensation/fill port and then using my Mityvac applied a vacuum to the caliper. I took the calipers off the steering knuckles and manipulated them around so the enlarging air bubbles could rise. I also tapped the calipers with a hammer to loosen bubbles. I also did something else I'll get to shortly.

So it worked. I got more air out, but not very much. Mind you, it did make a difference. The pedal felt firm right away.... like it should.

So now the "something else". One thing I did to help release trapped air, was to remove the calipers and pump the pedal a bit to get the caliper pistons to come out a bit hoping this might help release trapped bubbles between teh bore and the piston.

And here's the rub.... I used my caliper piston resetting tool to push the piston and back in and it seemed to take more force than I thought it should although it was going back in. Then I remembered I had left that piece of wood between the pedal and the seat. I removed it and the piston went back easily.

Hmmm.... now why did the piston go back at all if the port was closed off? It must be fluid seeping past the MC seal... but I just put a new seal in it....

So another test. I monitored the reservoir level and pushed on the brake pedal. Sure enough, the level stayed constant until the pedal started to sink and then the fluid started getting past the seal and caused the reservoir level to rise. This confirms the seal is passing fluid.

Despite the bore looking perfect (although I didn't measure it) and all new guts installed, something is still wrong, so I ordered a whole new MC. I hope this will resolve the issue, but I have doubts. With only 9000 miles driven and the old piston and seal looking perfect, what would a new MC do for me... unless the original MC had a manufacturing defect?

Once I replace the entire MC and still have the same problem, all that's left is the fluid (maybe because viscosity of DOT 5 is lower than 3,4 or 5.1), which is the last thing I want to change (imagine the work needed to deal with the rear inboard brakes).

It will probably take some time to get this done. Summit was out of stock on the MC so I had to go with E-bay. I expect at least 2 weeks delivery time.

[eschaider]

Take care not to get one of the Chinese knock-offs that so often find their way into eBay

[patrickt]

Originally Posted by eschaider

Take car not to get one of the Chinese knock-offs that so often find their way into eBay

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