Brake Problems With My 69

[Chuck G.]

Seems like everytime I let the car sit for some time, I lose the brake pedal. When I push it down, the "cherry" on the dash lights up. If I pump the brake, the pedal comes back, but not as high as "normal". After driving the car and using the brake, it seems OK. I drove this car 2000+ miles on the Road Tour to Collinsville last Summer without any brake problems. I have no leaks at any caliper or at the master cylinder. The fluid level stays up. I've bled the brakes many times. I have fairly new SS calipers and new pads and rotors. I can see how an out of round rotor could "pump" air into the system as you drive it, but this happens after the cars sits....this time it has been sitting for 2 1/2 months. Any thoughts ? PB Booster maybe ? Master Cylinder needs rebuilt/resleeved ? Any and all help appreciated. Chuck Gongloff

[Joe L.]

Re: Brake Problems With My 69

Chuck----

I don't think that I can solve your problem, but perhaps I can shed some light on it. In my opinion, C3 brake systems, and particularly 1969 models, are about as enigmatic as anything could be. I've struggled with mine for years, especially involving high-altitude brake loss.

I believe, after much study, that the problem that I have and the problem that you describe, although very different, are a result of the same root cause. That root cause, I believe, is the presence of some quantity of air in the hydraulic system. It is the only thing that could account for a loss of pedal.

In my case, I know that it is not the result of the "air-pumping" phenomenon. I know this since, while my brakes get soft at high altitude, they become rock hard as I descend to low altitudes. With "air pumping", once the air has entered the system, it will not spontaneously evacuate itself; the system must be manually bled. I also know that it is not due to my use of silicone fluid, as some have suggested. That's because I've talked to people who live in high-altitude areas, drive Corvettes, use silicone fluid, and have no problems whatsoever. Also, I've had exactly the same high-altitude problem when I was using glycol-based fluid(DOT 3).

I realize that your problem can't be due to "air-pumping" either, since it occurs when the car is motionless. Nevertheless, I still think that it is due to air trapped somehow in the system. The "somehow" is the part that I've been trying to figure out without success. I've tried pressure bleeding, gravity bleeding, pedal bleeding, and vacuum bleeding. I've even tried bleeding the brakes while the car was at high altitude(+6,000' msl). Nothing has worked, yet.

One other point I'll make is this: when the "brake" light illuminates on the dash (for reasons other than parking brake application), it indicates an "imbalance" in the front to rear braking systems. That's all that it means. There is a switch in the brake distribution block which deflects to the "weak" side when "imbalance" occurs, and causes the light to illuminate. The only reasons that I can see for an "imbalance" to occur is for fluid to be expelled past seals or if air is trapped in the lines. Since the former is obviously not occuring in either of our cases, it must be the latter.

Incidentally, the brake distribution block which I mentioned is often referred to as a "proportioning valve". However, for 1967 to 1977 Corvettes, which use 4 different blocks depending on year, this is NOT a proportioning valve. It is merely a distribution block which includes the "imbalance"-sensing switch. For 1978 to 1982 models, it is a combination valve, which incorporates distribution and proportioning functions. All of the 67-73 distribution blocks have been discontinued by GM.

[John M.]

Re: Brake Problems With My 69

Chuck and Joe. You guys are making me nervous. I bought my 69 in Dec. and won't be able to drive it until late April. Vermont! What should I do about the brake fluid issue. I don't know what's in it, haven't even sniffed it. Calipers are rebuilt; I believe they have GM rebuild stickers on them. Probably not SS. Thanks. (Long winters leave plenty of time to work on Corvettes)

[Joe L.]

Re: Brake Problems With My 69

John------

I don't know if there is anything that you can do with your brakes right now. The fact that they apparently have "stickers" on them might indicate that the system has been relatively recently rebuilt; typical rebuilders paper stickers wouldn't last very long.

GM nevered offerred rebuilt Corvette calipers through the GM parts system. Also, I don't think that they ever did through the Delco parts system, either. However, it is possible that they did. If so, the caliper stickers should say "rebuilt by Delco" or "rebuilt for Delco". This doesn't mean much though; whether or not they carry Delco stickers, these type of parts would be rebuilt by "independent rebuilders" and placed in Delco boxes. Virtually all rebuilt 65-82 Corvette calipers will be stainless steel sleeved; usually the cores would not otherwise be rebuildable.

Flushing the fluid, if it's glycol-based DOT-3 or DOT-4, might not be a bad idea, especially if there is any question as to its age. However, you must use the same type of fluid that was last installed; you cannot change fluid types without completely disassembling the entire brake system and removing ALL old fluid. Also, I know of no definitive means of distinguishing between different fluid types when it's in the brake system. Don't let anyone convince you that you can tell because DOT 5 silicone fluid is purple. It is purple, INITIALLY, but the purple coloration is produced by a dye which rapidly degrades to a straw-colored, yellowish liquid which is virtually indistinguishable from glycol fluids. You could try to differentiate by taking a small sample out of the master cylinder and putting it in a clean, clear glass jar or vial. Add several drops of water and shake. Let it stand for 30 minutes or so and observe the liquid. If there is no "phase seperation", it's probably glycol fluid since water is miscible in glycol. If there is a "phase seperation"(i.e two distinct "layers" of fluid), it's likely silicone based fluid. I've never tried this test, though, so I'm just speculating. To test the test I've suggested, you could perform it first using known samples of glycol and silicone fluids.

If you do decide to flush the system, performing it is quite simple and risk free, PROVIDING THAT YOU DO NOT LET THE MASTER CYLINDER GO DRY DURING ANY PART OF THE PROCEDURE. To do this, remove MOST of the fluid from the master cylinder with a turkey baster or similar suction device and refill with fresh fluid. Then, simply bleed the brake system in the normal manner until the fluid which flows out of the bleeder screws appears clear and fresh. CHECK THE MASTER CYLINDER FREQUENTLY SO THAT IT DOES NOT GO DRY AS YOU PURGE FLUID AT THE CALIPERS. When all calipers bleeder screws are completed, you're done. Bear in mind that rear calipers have TWO bleeders, each.

[Alex Gilmour]

Re: Brake Problems With My 69

Just as an FYI - The Porsche crowd uses a brake fluid that is pretty comparable to the Ford HD Racing stuff (ie its pretty good stuff - better than most brake fluids on the market). I can't remember exactly what it is called, it is something like ATS Super Blue. The good thing about this fluid is that they also have a fluid with the exact same specs, but it is called ATS Super Gold. You can therefore flush your system, alternating the colors, and when you see the new color at the calipers, you know that all of the old fluid has been flushed. This system works pretty well, and I will be using it on my Vette.

Alex

'66 A/C Coupe

[Carl James]

Re: Brake Problems With My 69

Joe,

I have to say that I find your posts very informative. From leaf spring installation to the ZF transmission post on the YL board. This post regarding brakes really got my attention since I'm about to flush the brake fluid on my '79 in the next few weeks. I understand why you wouldn't want to let the master cylinder go dry for allowing air to enter the system, however, if you bleed at the master cylinder, wouldn't it be alright to drain the master cylinder dry to clean it out? The reason why I am asking this is because Corvette Fever magazine showed a brake upgrade on a C4 in a recent issue and they stated to drain the master cylinder completely to be cleaned then add new fluid and go to the corners. They didn't mention anything about bleeding at the master cylinder though. Again, this was on a C4 and not a C3, however I would think that the same principle applies.

You also stated to drain most of the fluid out of the master cylinder. Can you go to far and allow air into the system? I've had my car for about 3 years now and the system hasn't been flushed since I owned it so it probably needs to be done. I don't know too many corvette owners and can't seem to find a decent mechanic with corvette knowledge, so I'm just taking it upon myself to get as much knowledge as possible on the Mako cars. Thanks in advance.

[John]

Re: Brake Problems With My 69

The problem sounds to me like the master cylinder is leaking intermally. The c-3 break system can sometimes be tricky. I had a front caliper (rebuilt) leaking internally that drove me and Four mechanic friends crazy for two days. When replaced with another unit the system was fine. They are cheap and easy to change. Just use bleeder tubes to bleed it either on the car or off the car before connecting the brake lines. Good Luck.

[Joe L.]

Re: Brake Problems With My 69

Carl----

I usually recommend that when "flushing" the brake hydraulic system that the brake fluid be removed from the master cylinder with a suction device such that only a small layer of fluid remains. However, this is only a precaution and is not absolutely necessary. You can, indeed, remove all of the fluid from the master cylinder and clean its resevoirs with a CLEAN, LINT FREE rag. This will not cause any air to get into the sytem, either on a C2, C3 or C4 brake system. However, you have to be ABSOLUTELY CERTAIN that the brake pedal is not depressed or even touched while the master cylinder is empty. After cleaning the resevoirs, immediately refill the master cylinder with fresh brake fluid. Your 79 should not have bleeders at the master cylinder, so the first step of your "flushing" operation should be the right, rear caliper. Remember, the rears have TWO bleeders, each side; the fronts have only one.