My 68 is not a daily driver, and I find that the brakes are soft and spongy and need to be bled more often when not driven. Is this common. I have been told to go to the DOT 5 Silicone brake fluid and this will help. Any suggestions.

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I do'nt know if this will help your spongy problem,that problem is probably,rotten brake hoses,or brake lines,the hoses might look perfect out side but jelly inside.unless you know that they have been replaced recently,replace all 4 of them.then check the brake lines,especially where they run along the frame rails,they can be seeping through the rust and running along the frame,and dripping some where else,and do'nt forget the master cylinder,it can be taking a sh-- too. Lastly ,if you do decide to use D.O.T.5,dont mix it with your old fluid,completely flush out the old stuff then start with new.there was a big disscussion on this a little while back,you might want to look back through the old threads ,and check it out ! good luck

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C3VR Lifetime Member #93
 

Silicon brake fluids are good for vehicles that don't get a lot of drive time. It doesn't absorb moisture like regular fluid, so it's good for storage or "garage art"(like mine!) BUT....it will NOT take care of a spongy pedal.Silicon fluids are compressable,so that in itself will give you a slighly spongy pedal.All the big time racers use it, but they change it every race. You may have a warped rotor, which is allowing the calipers to suck air into them while you drive, and then after setting for a while, the pedal will get spongy from the air in the system. I also have had this problem before on my '74, but after turning the rotors and rebuilding the calipers, I haven't had the sponge-bob-brake pedal in over 3 yrs, and I don't put more than 100 miles a year on my car. It's worth checkin' !!!! (ps.I do not use silicon fluid)

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Joel Adams
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I have DOT 5 in my Vette and there are a few drawbacks to it. I've had it in for 12 years now and when I take it over to the Black Hills or the Rockies where the elevation is high, they get real weird. The air bubbles that are still in there swell up and the brake pedal gets spongy. Scares the hell out of you coming down a mountain pass at speed. But when you get back down to lower elevation they come back. I put it in when I did the frame off and changed all the brake parts that had rubber in them, with new because it will not mix with DOT 3. You can change from DOT 5 to DOT 3, but not the other way around. DOT 3 penetrates the rubber and can't be washed out. DOT 5 washes out with soap and water but you have to get all the rubber parts.

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If it's not painted, Chrome it!!!

No leaks in brake system, the master cylinder has been previously replaced. I just have to bled the system every six months or so after a long storage. I will try and replace the rubber and maybe just stay with the DOT 3.

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Adams' Apple said: Silicon brake fluids are good for vehicles that don't get a lot of drive time. It doesn't absorb moisture like regular fluid, so it's good for storage or "garage art"(like mine!) ...

While it is true that DOT 5 will not absorb moisture like the others, it still does not eliminate moisture from what I understand. So there still is the possibility of moisture building up in your system and causing problems.

Another thing to keep in mind is that DOT 5 is a finer fluid (smaller molecules like synthetic engine oils) and will seep/leak past older seals easier than the others. So if you're going to switch it is best to do after a total overhaul of all the brake system components.

I use DOT 3 and flush the entire system every spring (and every other fall). Haven't had any serious problems to date.

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Jim Olson 

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And although DOT 5 does not absorb water, it does like to absorb air. That can make it difficult to bleed. Then there is those mountains.

correct me if im wrong but doesnt DOT5 have a lower boiling point too..

let me correct my self here..

the boiling point is higher but read my post below to see why boiling point is meaningless for comparing dot5 to dot 3/4/5.1

|UPDATED|12/1/2004 1:13:01 AM (AZT)|/UPDATED|

My jug of DOT 5 says a minimum boiling point of 500F and
the DOT 3 says 284F.The DOT 5 also says it is compatable
with DOT 3+4.I've used it for 10 years with no problems.
Steve

At least in the past, the DOT 5 did have a lower boiling point. The spec Steve stated sound odd. When I go into the next parts store it's time to read some updated labels. Silicone is NEVER compatiable with regular brake fluid.

kstyer said: At least in the past, the DOT 5 did have a lower boiling point. The spec Steve stated sound odd. When I go into the next parts store it's time to read some updated labels. Silicone is NEVER compatiable with regular brake fluid.

maybe it means you can use it in dot 3 and 4 applications..

you obviously cannot mix them

Brake fluid boiling point is a very good comparison for evaluating regular or polyglycolether based fluids, "glycol" for short, because it roughly represents the conditions under which glycol turns compressible. On the other hand, it is a very poor comparison for evaluating silicone based fluids because they turn compressible at a lower temperature than that at which they boil.

To add to the confusion, (presuming a properly bled system with negligible air bubbles), non-silicon fluids get compressible only after they boil (with absorbed moisture lowering this temperature), however the silicon fluid is four times more compressible at elevated temps (but before boiling point is reached). This is also not helped by the susceptibility to micro-aeration. This is the crux of the problem when discussing the fluid options.

Glycol based brake fluid is hygroscopic, i.e., it absorbs water from the surrounding atmosphere, and it starts adsorbing water as soon as you put it into your car's brake system. The brake fluid reservoirs on top of the master cylinder are vented to the atmosphere. Moisture can also enter the brake system through the rubber seals and hoses through a 'diffusion' process

from what I have found by looking around this evening..

you use dot 4 or dot 5.1 (which is much more similar to 4 than 5) for a frequently driven performance car.. 5.1 is very nice but it still eats paint. high boiling temp and doesnt compress until it boils, less moisture absorbtion, rubber friendly, low compressability.

5 or silicone fluid is good for sunday drivers and trailer queens as you can leave it in the car for a long time and it doesnt eat paint. it is however 4 times more compressible than dot 4 and leads to spongy pedals in non-ABS vehicles. and while it doesnt boil easily, it more compressible as the temp goes up regardless of whethers it boiling or not. also very prone to micro-aeration..

Ill be using 5.1 in mine

http://www.stoptech.com/whitepapers/brakefluid1a.htm

|UPDATED|12/1/2004 1:18:24 AM (AZT)|/UPDATED|

Good info Ben. Just confirms what I would have said but did not have the specs to back up. I did not know about DOT 5.1 Very interesting. Thanks.