C1 - Silicone Brake Fluid Story

[Brad K.]

I just completed a silicone brake fluid (DOT 5) installation on my '59. (which WILL be completed by July 4th after a multi-year ground-up restoration !) I thought I'd tell you a short story about the installation and then ask a couple questions.

I read in this discussion board, that silicone is not the best fluid available but I decided on it because of long garage times through the winter and its resistance to moisture absorption. My resto included an entirely new brake system so I thought it would be fairly easy. I knew that you should not shake the container and it should sit for a while so bubbles would not be present. You should be careful filling the master cylinder as not to create bubbles and you should not pump the pedal but press it slowly down for 5 seconds and let it come up for over 5 seconds (all intended to mimimize bubbles).

I ended up bleeding the brakes (starting at the farthest point) a total of 5
different times! Each time bled out fewer and fewer bubbles until at last
it bled clear. I used a clear bleed line into a partially filled bleed cup
with my wife (properly trained!) slowly depressing/releasing the pedal.

Now for my questions....Now I have a firm pedal, but it doesn't get firm
until I've depressed the pedal about 4"..... or about 3" from the
floorboard. It's been so long since I've driven the car I can't remember
if this is OK or not. Is it? Or do I need to let the car sit for a
while and then bleed again?

Second question....Is it normal to have such a long, drawn out series of
bubbles when I worked so hard to prevent them?

[Dennis A.]

Re: C1 - Silicone Brake Fluid Story

Brad...

Your story sounds normal to my experiences with solid axle vette brakes. Dealing with a single master cylinder and drum brake takes you back in time. I do recommend you drive the car for a stop & go few miles, Then check for leaks all over the place and bleed one more time.
P.S. using a power bleeder is easier on the wife and will usally get the bubbles out in one cycle of the four wheels.

[Joe L.]

Re: C1 - Silicone Brake Fluid Story

Brad-----

1) Some of the air bubbles that you experienced may have been due to air being drawn around the bleeder valve during the bleeding operation. To prevent this, I apply some silicone grease to the bleeder valve threads as a "sealant";

2) Despite what folks say, including some automotive "experts", I have not found silicone brake fluid to be any more susceptible to entrained air than regular glycol-based fluid. You can prove it to yourself. Take 2 glass vials or small glass jars. Put silicone fluid in one and DOT 3 or 4 glycol in the other. Shake them both vigorously. Set them on a counter an observe. You will note that it takes just about the same amount of time for both of them to become completely clear (indicating the release of all entrained air). In my mind, this demonstrates, unequivocally, that the notion of silicone brake fluid retaining entrained air for a long period is just another "myth";

3) There are LOTS of other possible causes for your difficulty in bleeding the brakes which you would likely have suffered whether you had used silicone fluid or conventional DOT-3 or 4. Since you didn't use the latter, you're assuming that it would have gone easier if you did. It very well might not have, though. By the way, did you bench bleed the master cylinder? Failure to do that is a common cause of brake bleeding problems.

[Brad K.]

Re: C1 - Silicone Brake Fluid Story

Thanks, Joe....You're exactly right....I did not bench bleed the MC!

[Justin N.]

Brake Fluid Story

Brad,
While finishing the resto on my 68 GT Rustang, I installed a complete new brake system with lines, Power/Master, and rotors. I gravity bled the thing overnight with great success! This simply involved jacking the front end as high as possible with stands and opening the right/rear bleeder screw only and making sure the master was full with cap off. The next morning, about 6 to 7 hours later, some fluid had dribbled into the catch I set up. Next was the left/rear, then right/front and finally left/front. The car has been stored undriven for over a year and I still get stiff pedal with a lopey cam and power brakes. Here's a dark shot of where I started, simply because we share the rear drums system on our cars. Good Luck!
Justin #41362

[Rob Dame]

Re: C1 - Silicone Brake Fluid Story

As long as the pedal does not pump up, that is get higher/firmer as you pump the brake pedal i believe you've gotten the air out of the system. Now that you have centered the shoes adjusting the brakes should raise the pedal and make it firm up near the top not 4" down. I think the ST12 calls for adjusting the star wheel untill the wheel/tire stops turning then back off seven clicks. Rob

[Patrick T.]

Re: C1 - Silicone Brake Fluid Story

A few years ago, I put silicone DOT 5 brake fluid in my '57 with no problems. Before you pour the fluid into the M/C, you need to heat it up on the stove to get the emulsified air that you can't see. Get a large clean coffee can and pour a quart of silicone in and put it on medium heat on the stove. As it heats up you will see small bubbles of air come up to the surface. Keep the heat on until there are no more bubbles.

The boiling temperature of silicone is 500 degrees, but do not bring it to a boil. After all the air is out, let it cool for a couple hours to room temperature. Then without agitating the silicone, pour it carefully into the M/C and then gravity bleed the entire system and you will have a rock hard pedal. PT

[Joe L.]

Re: C1 - Silicone Brake Fluid Story

Brad-----

This surprises me. I'll have to test it out sometime. Generally speaking, if not always, entrained air in a fluid will cause a cloudiness, even if there are not obvious small bubbles of air. I can't see how there could be entrained air in a fluid that was absolutely crystal clear and "transparent" with no cloudiness, whatsoever. When I've tested silicone brake fluid in a clear glass vial, it was absolutely clear with no cloudiness, at all.

[Christopher R.]

Re: C1 - Silicone Brake Fluid Story

I did the exact same thing as you, and had the exact same results. I lived with it for some years because I thought that was normal. Last Winter, while having some other work done at a professional Corvette shop, I had them check and adjust the brakes. The pedal came up to the top. I'm still getting used to it.

They didn't need to bleed the brakes. They simply adjusted them. They adjusted the shoes closer to the drums. Now the brakes apply much earlier in the stroke of the pedal.

Your condition may not be a problem with bleeding. May be adjustment.

[Brad K.]

Re: C1 - Silicone Brake Fluid Story

It's amazing how much you forget when it's been 25 years since you have done something! My pedal now becomes firm 1 1/2" down from the top (5 1/2" from the bottom). My problem was I statically adjusted the brakes before I filled the system (and I didn't adjust them tight enough) After re-adjusting the brakes (just like Chris said) the pedal came right back up.

Incidently, the bubbles that I experienced during bleeding were fairly large (1/16 to 1/8") and dissipated fairly quickly

What did I learn from this silicone experience?
-With a new system....bench bleed the MC
-Keep the bubbles to a minimum
-Statically adjust the brakes afterward (like Chris said!)

Thanks a lot for the comments!