This is a continuation of several threads discussing the reasons why Lone Star and other Corvette brake suppliers warn against the the use of DOT 5 silicone brake fluid in their calipers and master cylinders. This policy created a particular problem for those of us who wanted to use DOT 5 fluid for its advantages of preventing rust and avoiding risk to our very expensive paint jobs with the o-ring type brake pistons that are only available from the specialty Corvette brake suppliers who warn against DOT 5 use.
The reason given by Lone Star for this policy was that DOT 5 fluid had changed over the years and was now incompatible with brake seals. Tests conducted by TDB members recently demonstrate that this theory is false. DOT 5 fluid has not changed. Mark Lincoln had a number of new and old DOT 5 fluids tested under FTIR Spectroscopy. I have attached that study as a pdf file (I hope it actually attaches). It shows the Spectroscopy signatures of the various fluids to be almost identical. I have also reported compatibility tests of EPDM rubber o-rings and two kinds of silicone brake fluid, a preproduction Dow Corning fluid made in 1977 and a current DOT 5 fluid sold under the Carquest brand. The EPDM o-rings passed the material compatibility test with both the new and very old silicone brake fluids. This test is on the TBB under the thread name: Final Test Results of Brake Caliper O-rings with Silicone Brake Fluid.
So, if the DOT 5 brake fluid did not really change, why are the brake suppliers worried their products will fail if we use silicone brake fluid? The only reason left is that the rubber in the seals is different. I have Lone Star calipers with o-ring pistons on my 67 Corvette and my friend has CSSB o-ring pistons (sold by Zip) on his 65 Corvette. We used my supply of the old Dow Corning fluid on both cars about 5 years ago, and we have not experienced problems. These cars are not driven very much and certainly not in a way that heats the brakes a lot. I had assumed that the warnings were out of an abundance of caution because the suppliers did not know what kind of rubber the o-rings were, but that they were probably some type of EPDM because they seemed OK in cars I knew had DOT 5 fluid in them. Peroxide cured EPDM is the best material for brake seals, but the much cheaper sulfur cured EPDM also performed ok in the compatibility I ran. So maybe the o-rings were the cheaper EPDM?
The material compatibility test involves submerging the rubber samples in the DOT 5 brake fluid at 248 degrees F for 72 hours. It is an accelerated test and the failure mode of the rubber is swelling and softening. Jim Schwering, who is active on the TDB, sent me front and rear o-rings of unknown rubber from Lone Star rebuild kits so they could be tested in the same manor as the known EPDM o-rings I tested previously. Much to my surprise, the Lone Star o-rings failed miserably. Both had a section width of 0.210". The section width is easy to measure with accuracy before and after the test. These o-rings were highly affected by the exposure to hot DOT 5 fluid. They failed the test by expanding the section width from 0.210" to 0.240", and becoming much softer. The first picture shows the front and rear o-rings that have gotten "fat" along with a normal size front o-ring for comparison.
Gary Beaupre, another active member of the TDB, gave me parts from a 66 Corvette single master cylinder rebuilt by Lone Star. One part was a lip seal cup and the other was lip seal ring that I was able to remove from the piston without damage. I printed (after several tips from Gary) a series of circles increasing in .005" diameter increments that Gary had sent me for estimating the growth in diameter of the parts from the exposure test. The lips were slightly different between the two pieces, with the cup overlaying the 1.047 circle pretest and the ring overlaying the 1.052 circle. The picture to the right shows the parts after the exposure test. The cup lies within the 1.052 circle and the ring seal lies easily within the 1.057 circle. It is very hard to measure precisely a soft surface like a lip seal, but the circles indicated to me that the seals had expanded by less than 0.005" and perhaps not at all. Also I do not think any change in hardness occurred during the test. My conclusion is that these pieces pass the test, and that they are probably EPDM rubber either peroxide cured or sulfur cured.
This result seems to say that the master cylinder parts used by Lone Star came from a normal brake parts manufacturer who used some kind of EPDM rubber which should be universal in that industry. But the o-rings must come from some manufacturer who is not in the regular brake parts industry and does not know the business. I do not know if the o-ring piston kits from Lone Star and CSSB are supplied by the same manufacturer but they use the same size o-rings and the pistons appear to be identical in their machining. If they are from the same manufacturer, that company has not even used a chemical compatibility chart such as the one on the website of theoringstore.com. The o-rings I have seen are not even recommended for DOT 3 conventional fluid. The extra cost "high temperature" CSSB kit I bought from Zip used silicone o-rings. The chemical compatibility chart predicts that silicone o-rings would swell at least 20% in DOT 3 conventional fluid. The only o-rings recommended by the compatibility chart for even DOT 3 conventional brake fluid are EPDM and Simiz FFKM. The later is so exotic that a single o-ring costs an unbelievable $138.08, compared to $1.67 for the same size peroxide cured EDPM o-ring. We know the Lone Star o-rings are not EPDM because they failed the compatibility test for DOT 5 silicone brake fluid (which both flavors of EPDM o-rings passed), so they are made of something not even recommended for DOT 3 by the compatibility chart. It does not appear that even the most basic diligence was used in the choice of o-rings for the o-ring piston kits. The material is not what would be recommended for DOT 3 or DOT 5 fluid.
I am now concerned about my car and really all others using the Lone Star and CSSB o-rings. The o-rings in my car have not yet failed. It has been suggested that maybe the o-rings used by Lone Star 5 years ago were really EPDM but they were different in the kits Jim Schwering bought recently. I don't think that is likely. A manufacturer who understood that EPDM was really the only o-ring material recommended for any brake fluid would not have changed to something else. Therefore, we know that we are using o-rings that will fail an accelerated test in 3 days. The only question is how long they take to fail under the benign conditions of our lightly driven mostly parked restored cars, and this includes cars using DOT 3. When I get home from Florida, I am going to take at least one caliper apart to check its state of failure and replace the o-rings with peroxide cured EPDM that I know are compatible with DOT 5 (and DOT 3). If the o-rings I remove still look good, I will give them the accelerated test with DOT 5 to see if they are EPDM (unlikely IMHO).
The reason given by Lone Star for this policy was that DOT 5 fluid had changed over the years and was now incompatible with brake seals. Tests conducted by TDB members recently demonstrate that this theory is false. DOT 5 fluid has not changed. Mark Lincoln had a number of new and old DOT 5 fluids tested under FTIR Spectroscopy. I have attached that study as a pdf file (I hope it actually attaches). It shows the Spectroscopy signatures of the various fluids to be almost identical. I have also reported compatibility tests of EPDM rubber o-rings and two kinds of silicone brake fluid, a preproduction Dow Corning fluid made in 1977 and a current DOT 5 fluid sold under the Carquest brand. The EPDM o-rings passed the material compatibility test with both the new and very old silicone brake fluids. This test is on the TBB under the thread name: Final Test Results of Brake Caliper O-rings with Silicone Brake Fluid.
So, if the DOT 5 brake fluid did not really change, why are the brake suppliers worried their products will fail if we use silicone brake fluid? The only reason left is that the rubber in the seals is different. I have Lone Star calipers with o-ring pistons on my 67 Corvette and my friend has CSSB o-ring pistons (sold by Zip) on his 65 Corvette. We used my supply of the old Dow Corning fluid on both cars about 5 years ago, and we have not experienced problems. These cars are not driven very much and certainly not in a way that heats the brakes a lot. I had assumed that the warnings were out of an abundance of caution because the suppliers did not know what kind of rubber the o-rings were, but that they were probably some type of EPDM because they seemed OK in cars I knew had DOT 5 fluid in them. Peroxide cured EPDM is the best material for brake seals, but the much cheaper sulfur cured EPDM also performed ok in the compatibility I ran. So maybe the o-rings were the cheaper EPDM?
The material compatibility test involves submerging the rubber samples in the DOT 5 brake fluid at 248 degrees F for 72 hours. It is an accelerated test and the failure mode of the rubber is swelling and softening. Jim Schwering, who is active on the TDB, sent me front and rear o-rings of unknown rubber from Lone Star rebuild kits so they could be tested in the same manor as the known EPDM o-rings I tested previously. Much to my surprise, the Lone Star o-rings failed miserably. Both had a section width of 0.210". The section width is easy to measure with accuracy before and after the test. These o-rings were highly affected by the exposure to hot DOT 5 fluid. They failed the test by expanding the section width from 0.210" to 0.240", and becoming much softer. The first picture shows the front and rear o-rings that have gotten "fat" along with a normal size front o-ring for comparison.
Gary Beaupre, another active member of the TDB, gave me parts from a 66 Corvette single master cylinder rebuilt by Lone Star. One part was a lip seal cup and the other was lip seal ring that I was able to remove from the piston without damage. I printed (after several tips from Gary) a series of circles increasing in .005" diameter increments that Gary had sent me for estimating the growth in diameter of the parts from the exposure test. The lips were slightly different between the two pieces, with the cup overlaying the 1.047 circle pretest and the ring overlaying the 1.052 circle. The picture to the right shows the parts after the exposure test. The cup lies within the 1.052 circle and the ring seal lies easily within the 1.057 circle. It is very hard to measure precisely a soft surface like a lip seal, but the circles indicated to me that the seals had expanded by less than 0.005" and perhaps not at all. Also I do not think any change in hardness occurred during the test. My conclusion is that these pieces pass the test, and that they are probably EPDM rubber either peroxide cured or sulfur cured.
This result seems to say that the master cylinder parts used by Lone Star came from a normal brake parts manufacturer who used some kind of EPDM rubber which should be universal in that industry. But the o-rings must come from some manufacturer who is not in the regular brake parts industry and does not know the business. I do not know if the o-ring piston kits from Lone Star and CSSB are supplied by the same manufacturer but they use the same size o-rings and the pistons appear to be identical in their machining. If they are from the same manufacturer, that company has not even used a chemical compatibility chart such as the one on the website of theoringstore.com. The o-rings I have seen are not even recommended for DOT 3 conventional fluid. The extra cost "high temperature" CSSB kit I bought from Zip used silicone o-rings. The chemical compatibility chart predicts that silicone o-rings would swell at least 20% in DOT 3 conventional fluid. The only o-rings recommended by the compatibility chart for even DOT 3 conventional brake fluid are EPDM and Simiz FFKM. The later is so exotic that a single o-ring costs an unbelievable $138.08, compared to $1.67 for the same size peroxide cured EDPM o-ring. We know the Lone Star o-rings are not EPDM because they failed the compatibility test for DOT 5 silicone brake fluid (which both flavors of EPDM o-rings passed), so they are made of something not even recommended for DOT 3 by the compatibility chart. It does not appear that even the most basic diligence was used in the choice of o-rings for the o-ring piston kits. The material is not what would be recommended for DOT 3 or DOT 5 fluid.
I am now concerned about my car and really all others using the Lone Star and CSSB o-rings. The o-rings in my car have not yet failed. It has been suggested that maybe the o-rings used by Lone Star 5 years ago were really EPDM but they were different in the kits Jim Schwering bought recently. I don't think that is likely. A manufacturer who understood that EPDM was really the only o-ring material recommended for any brake fluid would not have changed to something else. Therefore, we know that we are using o-rings that will fail an accelerated test in 3 days. The only question is how long they take to fail under the benign conditions of our lightly driven mostly parked restored cars, and this includes cars using DOT 3. When I get home from Florida, I am going to take at least one caliper apart to check its state of failure and replace the o-rings with peroxide cured EPDM that I know are compatible with DOT 5 (and DOT 3). If the o-rings I remove still look good, I will give them the accelerated test with DOT 5 to see if they are EPDM (unlikely IMHO).
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