1967 Brake issues
I am having brake issues on a 67 L79 with power brakes. The car has stainless steel brakes and has been restored put possibly not driven much. I have owned the car a few weeks and have had two issues to date. The first was the brakes almost completely locked up after a two mile drive and I could barely get the car back home as a result. After they cooled down they freed up and I could push the car. Several days later i thought I would drive it again and the brake pedal was soft but worked fine after warming up. DOT 5 brake fluid was put in when I purchased the car. Could the issues I am having come from not completely getting DOT 3 or 4 out of the system before adding the DOT 5. I did not put in the fluid myself and am unclear what the original fluid was. If this is not the issue any thoughts on what the problem could be?
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Re: 1967 Brake issues
John------
Mixing brake fluid types is not a good thing. However, I don't think it would result in the type of problem you've experienced. A more likely cause of that sort of problem would be the master cylinder or, perhaps, the rubber brake lines between the calipers and the steel brake lines.
As far as the brake fluid goes, you could do a rudimentary test for brake fluid type by flushing a little of the fluid out at each wheel bleeder valve. Put samples in small glass jars or vials. Then, observe for phase separation. In other words, observe to see if you get an "oil-on-water" type of effect since glycol-based DOT 3 or 4 is not miscible or soluble in silicone-based DOT 5. However, this test will not absolutely confirm that you don't have mixed fluid types; it will only tell you if you do. If you have mixed fluid types, there's a LOT of work involved in getting it right. You would have to remove and disassemble all of the calipers, remove all of the brake fluid from each, remove the master cylinder, disassemble it, and clean out all of the fluid from it, and then flush out all of the fluid from the car's brake lines. It's a major job.In Appreciation of John Hinckley -
Re: 1967 Brake issues
All 4 brakes lock? How old are the brake hoses? You may be able to see a date on the hoses if they were replaced at one time as they all have date codes.
Another possibility, and I had a recent experience in relation to a '67 Power Brake system. The vacuum booster push rod length is critical where it enters the master cylinder. If the booster pushrod extends into the MC and applies pressure internally, at rest, it can cause the brakes to lock up. The reason is that the MC piston gets partially driven and the internal seal can cover the compensation port within the MC bore. Because fluid cannot return after a brake application, the brakes remain applied. It's possible that the elevated temperature of the engine bay could exacerbate this condition.
A simple check would be to simply loosen the 2 fastening nuts of the MC several turns and release it from the booster when the brakes lock up. If the brakes are then freed, you've located the issue. If you remove the MC completely from the booster, ensure that when it is installed that there is no pressure of the booster rod to the MC piston.
I recently had a new booster and I rebuilt the original MC using a new kit. The rod length of the new booster was too long, and the bore of the new MC kit piston was too shallow, and applied MC pressure prematurely. I recall is was substantial, maybe 2 to 3 tenths of a inch.The old booster rod and MC piston didn't have this issue. I had to either shorten the rod, bore the MC piston deeper, or add a spacer between the booster and MC. I fabricated a spacer.Comment
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Re: 1967 Brake issues
Thanks Joe. It sounds like you feel there are other problems than mixed brake fluid types. Assuming that is the case, it sounds like I should stick with DOT 5 while looking for other issues unless I disassemble everything, clean it out before putting back together.Comment
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Re: 1967 Brake issues
John,
In a pinch one day I had to add DOT 5 over Dot 3 in my tug because it had a leak in the system and the pedal went to the floor. I figured I would have to eventually fix the leak and I would deal with it then because I needed to use the tug. That was about 5 years ago and no more leak!
DomComment
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Re: 1967 Brake issues
Allow me to put forth another possibility: The rear rotors are out of spec and wobble, pumping air into the brake system creating the soft pedal at the start and locking up the brakes as more air is introduced into the system whilst driving. This is a not uncommon problem when people unfamiliar with the Corvette fixed caliper brake system work on it. However, usually the brakes will not resume normal function without bleeding some of the air out of the rear brakes. For them to return to functionality with no human intervention is not typical for this malfunction. It is, however, rather easy for a person with even rudimentary tools to check the run-out of the rear rotors. See the 1967 Chassis Service Manual for instructions on how to check the run-out. A dial indicator, which you can probably borrow from your FLAPS, is helpful, but not essential. Any fixed base pointed tool can tell you if there is rotor run-out, just not how much it is.
Mixing the types of brake fluid will cause a black residue (GUNK is the technical term) over time -- as in a year or more, but should cause no additional harm. You will lose some of the advantages of DOT5 in that the resulting fluid could remove paint if spilled on the body, and the resulting fluid will absorb moisture. So not thoroughly flushing the polyglycol based brake fluid will give you the worst of situations.TerryComment
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Re: 1967 Brake issues
I had a similar problem with the brakes on my 67. After driving a few miles the brakes would lock up to the point that the car would not move. Allowed the car to sit for about an hour and it was good to go....for a few more miles. The problem was in the Master Cylinder. I had replaced the M/C with a rebuilt replacement. (Refer to page 5-19 in your service manual). The M/C had been set up for a car with drum brakes. M/C was interchangeable with standard cars. There are check valves located in the M/C lines that provide a back pressure on the standard drum brake shoes. These valves caused the brake pads to be somewhat "applied", which over time (miles) the brake fluid would heat up and expand causing more pressure on the pads, eventually locking up the brakes. As the system would cool down the pads would release enough to allow the wheels to turn, until things heated up again. After removing the check valves from the M/C all was well.
Because you have recently purchased the car and do not have a history on the brakes or its components, this would be a good place to start your troubleshooting......
Let us know the solution when you get it fixed....
Good luckComment
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Re: 1967 Brake issues
Thanks everyone for your replies on my brake issues. There were multiple problems I had to resolve. The first is the bearings in the front were bad and needed to be replaced. There also was a bent caliper bracket causing the caliper to not line up right with the rotor. Both of these issues were causing the front wheels/brakes to heat up and the brakes to lock up after driving a while. The rear brakes had run out beyond .005 and needed to be shimmed. All have been corrected and are working much better.Comment
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Re: 1967 Brake issues (continued problems)
I am still having brake issues and need advice please. I continue to get air in the brakes and a spongy feel when applying the brakes. As I mentioned above, I had rotor runout in the rear and they were shimmed. How can you tell the source of air getting into the brakes? Where would you go next the calipers or the MC? What parts are numbered (TF car)? Where would you go to buy parts? As a reminder they are power brakes. Thanks again for your replies.Comment
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Re: 1967 Brake issues (continued problems)
Is the air just in the rear calipers? If so, rotor runout or bearing play is probably still an issue.Comment
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Re: 1967 Brake issues (continued problems)
John,
First I don't recommend shimming the rear rotors to correct the problem you are having. Take those trailing arms off and send them to a competent re-builder such as Bair's or Lone Star Caliper and get them done right.
Then go and chase your other problems. IF you continue to have problems after the rebuild.
JRComment
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Re: 1967 Brake issues (continued problems)
Runout out of spec is a major contributor to air intrusion.
What's is both front rotors runout? Was it also checked after replacing the bearings?
Are you sing DOT 3,4,5.1 glycol based or DOT 5 silicone? I use DOT 5 exclusively and it is more sensitive to air intrusion and line joint weakness.
Carefully check ALL line joints, brass blocks, hoses and bleeders for any signs of seepage. Get in the car and hold the pedal down hard with 2 feet for at least a minute. Any further travel towards the floor will tell you if there's a leak. If so, a dry finger at each junction will tell you where.Comment
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Re: 1967 Brake issues
Hi John,
After reading your first post again and not wanting to send you on a wild goose chase I would first make sure the master cylinder is bench bled and no air at that high point. The next area is to identify which wheel locked up and look for air at that caliper. If air gets pushed in the caliper and can't escape (air goes up toward the bleeder) that caliper will not release.
If foe some reason it's all four wheels locking up I would disassemble the master cylinder and look for the correct parts.Comment
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Re: 1967 Brake issues
I too just reread the first post.
What was found which solved the lock-up condition?Comment
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