Rear brakes 1969 Superior Cadillac

D

Doug Lemon

I need to do the rear brakes,and I remember Paul saying something about the drums had to be machined to fit the shoes.Paul could you chime in here and set me straight.
 
Some, but not all replacement brake shoes are lined with an over sized lining. This is because the brake shoe manufacturers want the brake drum to be machined to make sure that the surface is true and round. If you decide that you want to go this route, then make sure that you tell the machine operator that you only want a "skim" cut, and not to go after any deep scratches. The idea is to just true up the drum to the point where there is no "egg" shape to the drum, but not take so much material off that it might render the drum unserviceable. The commercial car drums are heavier and wider than the normal Cadillac drums, so they are not as easy to find. The good news is that there are enough cars in salvage yards that finding a serviceable drum isn't impossible in the states. Down under, it might be a little more difficult.
If you don't have the drums "turned", and they are egg shaped, you will get a pulsation in the brake pedal as you apply the brakes. Also, brake drums that are not turned, will have a work hardened surface that will prevent the brake lining to produce the friction that is required for stopping. This is especially important with the newer brake lining materials that contain no asbestos or metal particles. You want to make certain that the brake lining material is riveted to the brake shoes, not bonded with glue. The purpose of the brake linings being riveted is to transfer heat from the lining material to the brake shoes.
You will also find that the return springs on the commercial chassis cars are much heavier than the springs used on the standard cars. I haven't found a replacement for these, and I have reused the ones on the car. One way to know if a brake spring is good, is to drop it on the cement floor, and if it lands with a dull thud, it is good to use. If it has a "springy bong" sound to it, then it has lost its temper, and should be replaced.
Make sure that you check the wheel cylinders for leaking, and if in doubt, I suggest that you remove them and rebuild them. I don't recommend replacing them, since today all you can get are foreign manufactured parts that are of dubious quality. If they are badly pitted, then check the recommended vendor website for the name of the company that will bore them and install brass sleeves. Once this is done, they will last the life of the car. If doing a complete brake job, with replacement of all parts, including the hydraulics, then I recommend the use of silicone brake fluid. Only time that I wouldn't recommend the silicone brake fluid is if you were in a mountain area, where the brake would be subject to extreme braking temperatures. For normal driving, the silicone brake fluid will be a lifetime use fluid. If you are going to use the mineral based brake fluids, then I would recommend you read this article about brake fluids. I don't agree with the assessment of the DOT 5 (silicone based) fluids, and have been using them successfully for over 20 years. I have never had a problem with silicone brake fluid and moisture.
 
trim the drums is always a good move. a good shop will fit the shoes to the drum also. each drum is stamped on the lip with the max diameter. if it's turned to the max or beyond the shoes will only contact on the tips. you don't get a lot of effect that way. the only way to get a good brake is to cut the shoes to match the contour. not good but necessary it you want to stop.

the last brake fluid I got was synthetic dot 3. they said it would mix.
 
John ED Renstrom said:
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the last brake fluid I got was synthetic dot 3. they said it would mix.
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I wouldn't mix brake fluids on the advise of some counter man that may be repeating misinformation that he received somewhere. I would only trust what it says on the container. Everything that I have read, says that synthetic is not compatible with the DOT 3 (or 4) brake fluids. Here is a great article on the subject.

Battle of the DOTs

DOT 3-4 Verses DOT 5. Which brake fluid should I use?

From Oak Okleshen #35 "With regards to the DOT 3-4 verses DOT 5 brake fluid controversy, here is an article sent to me by Mr. Steve Wall. It is one of the most professional treatments I have seen on the subject".
[I had to condense this article from 6 pages to 1 due to space limitations -ed]
Brake Fluid Facts
by Steve Wall
As a former materials engineering supervisor at a major automotive brake system supplier, I feel both qualified and obligated to inject some material science facts into the murky debate about DOT 5 verses DOT 3-4 brake fluids. The important technical issues governing the use of a particular specification brake fluid are as follows:
  1. Fluid compatibility with the brake system rubber, plastic and metal components.
  2. Water absorption and corrosion.
  3. Fluid boiling point and other physical characteristics.
  4. Brake system contamination and sludging.
Additionally, some technical comments will be made about the new brake fluid formulations appearing on the scene.
First of all, it's important to understand the chemical nature of brake fluid. DOT 3 brake fluids are mixtures of glycols and glycol ethers. DOT 4 contains borate esters in addition to what is contained in DOT 3. These brake fluids are somewhat similar to automotive anti-freeze (ethylene glycol) and are not, as Dr. Curve implies, a petroleum fluid. DOT 5 is silicone chemistry.
Fluid Compatibility

Brake system materials must be compatible with the system fluid. Compatibility is determined by chemistry, and no amount of advertising, wishful thinking or rationalizing can change the science of chemical compatibility. Both DOT 3-4 and DOT 5 fluids are compatible with most brake system materials except in the case some silicone rubber external components such as caliper piston boots, which are attacked by silicon fluids and greases.
Water absorption and corrosion

The big bugaboo with DOT 3-4 fluids always cited by silicone fluid advocates is water absorption. DOT 3-4 glycol based fluids, just like ethylene glycol antifreezes, are readily miscible with water. Long term brake system water content tends to reach a maximum of about 3%, which is readily handled by the corrosion inhibitors in the brake fluid formulation. Since the inhibitors are gradually depleted as they do their job, glycol brake fluid, just like anti-freeze, needs to be changed periodically. Follow BMW's recommendations. DOT 5 fluids, not being water miscible, must rely on the silicone (with some corrosion inhibitors) as a barrier film to control corrosion. Water is not absorbed by silicone as in the case of DOT 3-4 fluids, and will remain as a separate globule sinking to the lowest point in the brake system, since it is more dense.
Fluid boiling point

DOT 4 glycol based fluid has a higher boiling point (446F) than DOT 3 (401F), and both fluids will exhibit a reduced boiling point as water content increases. DOT 5 in its pure state offers a higher boiling point (500F) however if water got into the system, and a big globule found its way into a caliper, the water would start to boil at 212F causing a vapor lock condition [possible brake failure -ed.]. By contrast, DOT 3 fluid with 3% water content would still exhibit a boiling point of 300F. Silicone fluids also exhibit a 3 times greater propensity to dissolve air and other gasses which can lead to a "spongy pedal" and reduced braking at high altitudes.
DOT 3 and DOT 4 fluids are mutually compatible, the major disadvantage of such a mix being a lowered boiling point. In an emergency, it'll do. Silicone fluid will not mix, but will float on top. From a lubricity standpoint, neither fluids are outstanding, though silicones will exhibit a more stable viscosity index in extreme temperatures, which is why the US Army likes silicone fluids. Since few of us ride at temperatures very much below freezing, let alone at 40 below zero, silicone's low temperature advantage won't be apparent. Neither fluids will reduce stopping distances.
With the advent of ABS systems, the limitations of existing brake fluids have been recognized and the brake fluid manufacturers have been working on formulations with enhanced properties. However, the chosen direction has not been silicone. The only major user of silicone is the US Army. It has recently asked the SAE about a procedure for converting from silicon back to DOT 3-4. If they ever decide to switch, silicone brake fluid will go the way of leaded gas.
Brake system contamination

The single most common brake system failure caused by a contaminant is swelling of the rubber components (piston seals etc.) due to the introduction of petroleum based products (motor oil, power steering fluid, mineral oil etc.) A small amount is enough to do major damage. Flushing with mineral spirits is enough to cause a complete system failure in a short time. I suspect this is what has happened when some BMW owners changed to DOT 5 (and then assumed that silicone caused the problem). Flushing with alcohol also causes problems. BMW brake systems should be flushed only with DOT 3 or 4.
If silicone is introduced into an older brake system, the silicone will latch unto the sludge generated by gradual component deterioration and create a gelatin like goop which will attract more crud and eventually plug up metering orifices or cause pistons to stick. If you have already changed to DOT 5, don't compound your initial mistake and change back. Silicone is very tenacious stuff and you will never get it all out of your system. Just change the fluid regularly. For those who race using silicone fluid, I recommend that you crack the bleed screws before each racing session to insure that there is no water in the calipers.
New developments

Since DOT 4 fluids were developed, it was recognized that borate ester based fluids offered the potential for boiling points beyond the 446F requirement, thus came the Super DOT 4 fluids - some covered by the DOT 5.1 designation - which exhibit a minimum dry boiling point of 500F (same as silicone, but different chemistry).
Additionally, a new fluid type based on silicon ester chemistry (not the same as silicon) has been developed that exhibits a minimum dry boiling point of 590F. It is miscible with DOT 3-4 fluids but has yet to see commercial usage.
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Brake drums

Another item to throw into the mix. Several years back, I discovered that 76 CDV drums fit 69 M&M commercial cars (and other Cad. cars).;) I didn't know if that was common knowledge. I was stripping the CDV when I had a drum shatter on my combo.:poster_oops::eek:
 
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