brake fluid synthetic vs silicone

[Ted K.]

I recently rebuilt a caliper on my 66. When topping off the brake fluid I used Preston Synthetic Dot 3, supposedly compatible with all brake fluids. When I topped off the fluid I noticed the synthetic Dot 3 did not mix with what was already in the master cylinder,possible silicone fluid.
Question will the mixture of Dot 3 and I assume Silicone brake fluid damage the system. Does the entire system need to be flushed out? What about residue? What is the easiest and best way to correct this situation?
Thanks,
Ted

[Steven S.]

Re: brake fluid synthetic vs silicone

If you just added it and noticed the problem, I would just use a turkey baster (sp?) or something similar to "suck out" the master cylinder as completely as you can. If you used the brakes since and think there was any possiblility of the mixture getting into the brakes lines than yes, you would need to flush the entire system. Silicone and Dot 3 mixed will cause the seals to swell throughout the system.

[William G.]

Re: brake fluid synthetic vs silicone

Ted
Just bought Dot 5 silicone and this is what it said on the bottle.

If the system is filled with Dot 3 or Dot Brake Fluid completely drain
and flush entire systen before replacing with silicone Dot 5 Brake Fluid.
Replacement of seals and boots is recommended. This product is not
compatible with DOT 3 OR DOT 4 Brake Fluid
GOOD LUCK
Bill

[Ted K.]

Re: brake fluid synthetic vs silicone

Flushing? Flush with what? How does one get all of the old fluid out of the calipers? The bleeder is at the top the the caliper and it seems the fluid will just pass thru to the bleeder and not clean out the caliper.
Ted

[Michael F.]

Re: brake fluid synthetic vs silicone

disassemble system and flush all lines, calipers, master with alcohol.

[Cathy S.]

Re: brake fluid synthetic vs silicone

Wow there has to be an easier way

[Terry M.]

Re: brake fluid synthetic vs silicone

Get the fluid (which ever kind it is) out of the master cylinder. Fill the MC with DOT5 and flush the new DOT5 through the system. You will need to use pressure on the master cylinder to get the new DOT5 through the system. Vacuum at the bleeder screws will only draw the brake pistons into the calipers. I have been there and got that T-shirt.

The SAE papers on mixing DOT5 with the glycol-based fluids (DOT3, 4, & 5.1) only describe a black sludge build-up from mixing the different types of fluids. There is no mention of swollen seals. If you put petroleum-based fluid (as in power steering fluid or ATF) into the brake system the seals will swell up.

[Terry M.]

Re: brake fluid synthetic vs silicone

That is the safe way and probably what a shop would do. After all it isn't their money, and they have a liability on the line.
BTW: Alcohol will not "cut" DOT5 -- and I don't know what will -- except perhaps the old-style alcohol free "Brake Clean" that has 1,1,1-trichloroethane in it.

[Timothy B.]

Re: brake fluid synthetic vs silicone

When I changed my 67 to dot 5, my procedure was as Terry mentioned.

I cleaned the master cylinder of dot 3 then overhauled the calipers and assembled them with dot 5 and just flushed the silicone fluid through the system until the color was pure silicone. Not a big deal and the pedal is hard with no problems six years later..

My memory tells me I let the open lines drip for a few days while I overhauled the seals in the calipers then flushed until I was satisfied..

[Terry M.]

Re: brake fluid synthetic vs silicone

Tim,
When I flushed my DOT5 some years back I used the gravity method you describe. I couldn't use it initially because my lines were empty of fluid and so there was nothing to "pull" the fresh fluid through. The gravity method is a bit like a siphon in that you have to have the fluid pulling on the downhill side of the system for the fluid to begin to flow. DOT5 is a little more viscous than glycol-type brake fluid, so it has to have a little more force to get it moving.

[Craig B.]

Re: brake fluid synthetic vs silicone

If you open your brake bleeders starting at the right rear, left rear, right front, and finally the left front the fluid will pass through rather quickly unless you have a kink or a plugged bleeder valve. This method has worked on everything from Indy cars to my 92 Chevrolet pickup whether silicone or synthetic ( Indy cars require silicone fore the very high brake temperatures). I still bleed the brakes after I have fluid passing through all the bleeders to expell any residual air left in your system particuliarly with the use of silicone brake fluid. Bleeding the brakes insures a firm brake pedal.