Help with '75 brake diagnosis

[rpounds]

Here's the story. I went for a little cruise today. Took the freeway about 12 miles down to the ocean. I'm on vacation for a couple of weeks and had nothing better to do.

Anyhow, got down there, got off the freeway and everything was fine. Diddled around for about an hour and headed back home. Still, everything is fine. About half way home on the freeway, traffic started to slow, so I hit the brakes. Darn pedal went all the way to the floor with very little stopping power. Of course, this sent a bit of a chill up my spine. I got off at the next off ramp and pulled into a parking lot. I opened the master cylinder to check the brake fluid level. Right up where it should be. Considering that I had some stopping power left, I cautiously made my way home.

I yanked the wheels and looked at the calipers and brake pads. Everything looks normal. The calipers are only about 2 years old, so no leaks and everything else seems to be fine.

I took the cover off the master cylinder again and the fluid in the rear cell looks milky as though it has been aerated. At first I thought that the calipers may have been sucking some air back up into the lines from a rotor running eccentric. I'm going to try to bleed the brakes a little later to see if this may be the case.

However, with the milky look to the fluid in the master cylinder, I'm leaning towards having lost the seals on the primary side.

Any suggestions or thoughts on where to start dianosing?

Sorry for the long post.

Ron

 

[JohnZ]

Sounds like the classic symptoms of "air-pumping" in the rear calipers due to excessive lateral runout in the rotors; it always happens during a freeway/open road drive. If you have a dial indicator and rear rotor runout looks OK (less than .005" TIR), I'd look up under the dash and see if there's any brake fluid showing where the master cylinder comes through (rear seal leak); if you have power brakes, pull the master cylinder forward a bit off the booster and see if there's brake fluid on the seal where the master attaches to the booster. If a thorough bleeding cures it and it returns again on a highway drive, it's most likely the rear rotors.

 

[resto75]

When was the last time you changed the brake fluid?. Maybe you just need to do a complete pumpout and replacement of the fluid.

 

[Stallion]

Sounds like the classic symptoms of "air-pumping" in the rear calipers due to excessive lateral runout in the rotors; it always happens during a freeway/open road drive. If you have a dial indicator and rear rotor runout looks OK (less than .005" TIR), I'd look up under the dash and see if there's any brake fluid showing where the master cylinder comes through (rear seal leak); if you have power brakes, pull the master cylinder forward a bit off the booster and see if there's brake fluid on the seal where the master attaches to the booster. If a thorough bleeding cures it and it returns again on a highway drive, it's most likely the rear rotors.

John, it's funny you bring this up. I was just reading about runout of the calipers and it would cuase air in the brake system. But, I don't understand how this could be a problem. And what exactly is meant by "runout" of the caliper? Would that mean just being loose? Or no? And how does that put too much air in the braking system?

Thanks!

TR

 

[rpounds]

TR,

Caliper runout means that the caliper does not turn true to the spindle. In other words, if you were looking down from the top, as the caliper turned it would move side to side. As JohnZ said, it doesn't take a lot of 'runout' to cause the caliper to 'pump', which sucks air up into the hydraulic cylinders on the caliper, thereby introducing air into the system - a real no no in a hydraulic system.

John, I initially thought the same thing, except that it has never done this before. It's all of a sudden after about 2 years on a brake rebuild, including rebuilt calipers. I will pop the master cylinder off of the power brake booster tomorrow and see if it is leaking past the rear seal. Any thoughts as to why the rear cell in the master cylinder would look aerated?

Resto75,

Yes, it is probably time to flush the entire system out, since I'm dinkin' around with it anyway.

Thanks for the responses everyone.

Ron

 

[Stallion]

Okay, I understand what you mean now. Thanks!

TR

 

[Nut]

Ron..... I feel your pain. Trust me. I've had this problem a couple times on both front rotors since get the bulk of the braking (heating). Each time I had them turned on the bearing assemblies and it cured the problem. I'm now considering High perf vented rotors this winter. I can't deal with never knowing when they're going to go again. That chill is something I never have forgotten. Good luck.

............ Nut

 

[rpounds]

Okay guys, I finally got back out to the garage to check this little problem out. Turns out that the inner side of the left rear caliper was absolutely full of air. So, I have been bitten by the old Corvette brake caliper sucking air nemisis. What has me truly baffled is that this came on so suddenly. The brakes are about two years old, with no problems until now. It wasn't gradual. Just BINGO, no brakes.

Guess I get to true up the rotors again. Geez, I hate to whine, but this thing only has about 7,000 miles on it in the last two years.

It's either do another brake job, or take it out back and

Thanks again for the input guys.

Ron

 

[sscam69]

Well look on the bright side. You caught the problem.

 

[rpounds]

Yes. I think I'm going to start looking for a caliper upgrade. I have tried to keep things as original looking as possible, but I need to avert any disaster which may come from losing my brakes again at an inopportune time. Although I have tried to keep most everything original (although the engine is not), the fact is that this car will never be an NCRS Top Flight car. So, in the interest of safety (mine, not to mention whoever I might plow into), I'm looking for recommendations for a better caliper. Any suggestions?

 

[sscam69]

I was looking into some SSBC aluminum calipers myself as a future upgrade.

I have heard they are pretty good. I know they have some for vettes but not to sure about C3's. The website is www.ssbrakes.com. I think they shave off like 7lbs a piece. I was looking at them for weight savings. After the restoration I am shooting for a 3000lb car with creature comforts.

Then there is Baer and Willwood. I have heard Willwoods performance is lacking. Don't forget VB&P's stuff.

Hey Ron I didn't know you were trying to keep it original. I thought you kept it that way because of the smog Nazis.

Frank

 

[Vette/Berlina-coupe]

-Wow!!! -Identical brake-failure thing just happened to me on my trip to the ocean today, so John Z is dead-wrong, --its obviously a salt-air problem! --and having narrowly missed plowing into several other vehicles on the highway, there is still another problem--- what's the best way to "dry-out the driver's-seat"!!! :L This is my first experience of this C3/tweeked/rear-rotors syndrome, and the hand-brake seems about worthless, as yanking it on only mildly slows the car!! Then as I turned for home the red/brake-light stayed-on, and the brake-pedal goes to the floor even after pumping-it! I also immediately checked the mastercyl.-reservoir, -and found the front-portion 80%-low (even though topped-it just a week-ago). So if one or both of the rear-rotors is somehow tweeked, and the pistons are thus pumping-in air as JohnZ describes while you are blissfully driving along (yes, the pedal did become spongy-feeling), -it sounds like some sort of inferior rotor-material or maybe just the seals are just old (no longer resilient enough to prevent sucking-air). --Geeesssh, yah gotta have brakes that you can rely upon (wonder if there is a viable up-grade for this problem, like stainless-steel rotors?). This '80-Vette has less than 15k-miles, but apparently needs a thorough quality-overhaul of the entire braking-system; --am taking this "wake-up call" very seriously. Will be installing braded-steel Flex-hoses, Silicoln/brake-fluid, and all new seals and Pads (what brand Pad is best for the C3?). So any further comments are most welcomed. For example, is the C3's handbrake really only good for parking, --shouldn't it be able to lock-up the rearwheels at say 45mph ???
-B.B.
:confused ;help

 

[GWHITE75]

RPOUNDS,

Sorry to hear of your misfortunes with the brakes. The rear calipers have two bleed valves, as you know and you really need to make sure you get the air totally out of both sides. Brake failures have been a constant and popular subject on every Corvette forum I have read.

I too have been a victim and have read several ideas. I went to the stainless steel calipers about 10 years ago and my brakes were fine for a while. Mainly due to the fact that I turned the rotors and replaced my bearings to reduce the run out.

Recently, I started experiencing some pedal fade so I decided to take a chance and experiment with the "O" ring type pistons that VB&P sells. They are made to specifically eliminate the "air pumping" problem with our type cailipers. After cleaning up the calipers and ensuring no excessive wear or grooves in the calipers, I installed the pistons. I have had good pedal ever since.

I have also heard of people installing Willwood calipers on the front. Several owners have converted to the Hydraboost system originally out of diesel trucks, but now kits are made for Corvettes for about $600.00. They claim excellent pedal pressure, never before felt in their vehicles.

My next step is to install the hydraboost system and stay with my current calipers. I want the stronger pedal, as my system has never given me the warm fuzzies my other vehicles do.

I recommend you speak with the people at Vette Brakes and Products about the "O" ring calipers or pistons. You will certainly remove the aspect of air pumping into your calipers. I know this first hand.

 

[sscam69]

Several owners have converted to the Hydraboost system

Whats this?

 

[GWHITE75]

Hydratech Braking System

sscam69,

Go to this web site to find out more about the Hydroboost system:

http://hydroboost.com/multimedia/

 

[smurfvet]

Somebody had comments on this problem a few months ago. I seem to remember comments that the o-ring sealed calipers were a fix for this, according to someone who had similar issues.

I had the same problem last spring changing the rear pads. Lucky I found the soft pedal in the garage rather than the road.

 

[rpounds]

Hmmm . . . I like the idea of being able to upgrade my calipers with o-ring seals. I'll check into that.

You know, I've never had much in the way of complaints with the pedal feel. If anything, I would describe my brakes as being a little on the 'touchy' side.

I have not put a dial indicator on the rotors yet. I'll probably get around to that in the next week or so. I'll keep everyone posted what I find out. JohnZ mentioned T.I.R. at .005" max, so I'll check that out and let everyone know.

Ron

 

[hotrodd]

This is just my expereince, I have G.M. rotor's on the front and never had a problem.I have Auto Zone on the back and after 10,000 mile's they pumped air like a Weinand 6-71 Blower. Are the rear more prone to air ?

 

[resto75]

VBP O'rings

I recently purchased the VBP o'ring conversion kit and am very satisfied with it. It included all 16 new pistons o'rings and seals and was only 120.00. If you price out just new standard pistons it will cost more then 120 bucks.

If you see their cut away caliper you will see why the o'rings are much better design. As long as you already have stainless inserts this is the way to go.

Roy

 

[Wills79Vette]

With the restoration of my 79 Vette, I have replaced the front brake calipers with ac delco rebuilt, the stopping is great now. When I took off the front calipers, the were leaking bad. Now that I have replaced the front brake calipers, the rear calipers seem to get hung up. When I release the brake, it seems to take a few for the them to release. Gives a jerky feel, like ABS is activated on them. Has anyone else run into this problem, and what is a possible cause for this?