I’ve had DOT 5 silicone fluid in my 300G since 1978, when I bought the car and went through the brake system using standard replacement wheel cylinders and a rebuilt master. I drive the car about 1,000 mile per year on average, and did nothing on the brake system other than adjustments for 18 years (1996) until one day after a long slumber took it out for checkout ride and found the front brakes pulling to the left. Pulled the front drums and noticed seepage at the lower front left wheel cylinder.
I inspected the leaking lower cylinder and found corrosion in the bore. Removed all my wheel cylinders and sent them off to be sleeved with brass. Reinstalled all and flushed and filled with DOT 5 again. No brake work has been done since except I now flush the system every 7-10 years. My pedal is firm, but it will be spongy if you are not careful handling the fluid and you get air in the system. I had a ’51 Chrysler I also used DOT 5 in, and had no problems once in service, but did notice a spongy pedal due to improper handling of the fluid when first installed. Just keep bleeding the system until the fluid comes out clear and no bubbles.
If you are willing to bleed your brakes every 2-4 years and no more, then DOT 3&4 are fine. I can’t keep up with that regimen, and in collector cars they sit for long periods, and the system does not get heated up often enough to drive out the water built up in the system as do daily drivers. DOT 3&4 are hygroscopic, that means they have an affinity to attract and hold moisture, that’s why you bleed them. DOT 5 is inert and does not attract or mix with water. Yes water can get into the system, depending on use and humidity and other factors beyond my knowledge but it can happen. That is why I’ve found if DOT 5 is used, and the car is driven, it should be bled out every 10 years or so. For me, using silicon fluid and having brass or stainless steel sleeves in the cylinders are a “belt and suspenders” approach. We only have one pot in these masters, and safety is paramount in braking systems, so the two combined reduces maintenance and assures they will work when needed.
Another important benefit is that DOT 5 will never remove your paint if it ever spills or drips on it. Also, if you do use your brakes hard (mountain driving) and they heat, up DOT 5 is good up to 500 degrees, whereas DOT 3&4 has a lower boiling point and goes lower as the fluid ages and absorbs more water. My 300G is a late (July) 1961 build car with the brake light switch activated by mechanical pedal movement, not fluid movement, so I cannot comment on the switch failures others mentioned. As I write this, my fluid was last bled in 2011 so I’m due..
Posted by: "Bob Jasinski" <rpjasin@xxxxxxxxxxx>
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