Mixing D.O.T. 3 or 4 with Silicone break fluid?

[oil pan 4]

I was told if you mixed regular break fluid and Silicone based fluid that the Silicone would eat the rubber seals in the break system?
Now where I work we have bomb lifts they use three sets of 2 piston caliper breaks with standard car or truck looking on the inside) master cylinder. Every thing on the inside of the caliper and master looks like regualr car and truck break parts. Now some times we get regular break fluid (dot 3), other times we get silicone based. We mix the two with out reguard. So far it has not caused any problems.
And some times we pull those break calipers off the little bomb lifts and put them on the big ones. Only thing is the big one use their hydraulic (ATF) system to directly run the breaks. That is you bleed them and red ATF comes out. Still the never leak.
If you should use some other break seals besides rubber with Si fluid what is?

 

[Tosh]

It isn't the mixing per se, but that silicone based (DOT 5.1) brake fluids are not compatible with the rubber seals in a regular braking system.
Perhaps the problems are waiting for that long downhill section before they show up...

There are rubber manufacturer tables for fluid compatibility available (but I can't imagine anyone but a race car builder replacing every seal on a car).

 

[Tosh]

Why not run an experiment on fluid compatibility yourself? Find some new, dry rubber seals and weigh and measure them accurately. Then keep them immersed in varying concentrations of different brake fluids and see what kind of swelling you observe after a certain time. (It may take hours to months for changes to show.) Then post back with your findings.

 

[onion]

I can't say much about silicon break fluid, as I've never used the stuff.

But I've seen the results of the 'wrong' brake fluid firsthand many times- Case backhoes switched back and forth between brake fluid and hydraulic fluid more than once over the years. When some rocket scientist tops off the master cylinder with the wrong stuff, it ain't pretty. The brake fluid will 'mix' with the hydraulic fluid forming a brownish sludge. The seals and rubber hoses swell and come apart... and distribute throughout the brake system. It isn't much fun, and it's EASY to avoid.

So just based on that experience... I wouldn't go mixing fluids unneccesarily.

But if you're in the military... then who cares? My taxes will pick up the tab.

 

[bulwnkl]

Quote:

---

...but that silicone based (DOT 5.1) brake fluids...

---

DOT 5.1 is NOT silicon-based. DOT 5 is silicon-based. DOT 5.1 is the same type as DOT 3 and DOT 4. Personally, I think that the naming convention is monumentally stupid because of the difference between 5 and 5.1, but I didn't get to make the names.

 

[oil pan 4]

After 2 years of going back and forth with the 2 fluids nothing abnormal happened. We all ways had calipers stick and master cylinders go out. Nothing changed as far as I could tell after we started useing both.
Dot 5 and Dot 3-4 fluids do mix right together.

".....brownish sludge. The seals and rubber hoses swell and come apart... and distribute throughout the brake system."
I have never seen or even heard of any thing like that.

 

[Big Jim]

Quote:

---

After 2 years of going back and forth with the 2 fluids nothing abnormal happened. We all ways had calipers stick and master cylinders go out. Nothing changed as far as I could tell after we started useing both.
Dot 5 and Dot 3-4 fluids do mix right together.

".....brownish sludge. The seals and rubber hoses swell and come apart... and distribute throughout the brake system."
I have never seen or even heard of any thing like that.

---

I have. When someone tops off a master cylinder with motor oil or when a hydro-boost system leaks power steering fluid into the brake system. The last time I saw it, the car was towed to a wrecking yard instead of spending the money to fix it.

 

[Tosh]

Quote:

---

DOT 5.1 is NOT silicon-based. DOT 5 is silicon-based. DOT 5.1 is the same type as DOT 3 and DOT 4. Personally, I think that the naming convention is monumentally stupid because of the difference between 5 and 5.1, but I didn't get to make the names.

---

You're absolutely right!