Foamy silicone fluid in master cylinder

[hilsideser]

I like that idea. Maybe that's what it is. Thanks!

 

[skucera]

Something else wrong ... even if the fluid did form air bubbles (which I don't believe), they would just push fluid back through the MC into the reservoir, rather than making the brakes drag. You must of had a misadjusted MC, or a sticking pedal, or a broken/weak pedal return spring, or something.

Actually, come to think of it, my brake master cylinder did have a bleed-back valve problem that didn't let the front brakes depressurize, and I flushed the DOT 5 silicone brake fluid out of the system when I rebuilt that master cylinder. The fine bubbles increased over time, however, and I stand by that observation. I've never read that silicone brake fluid is long lasting, but I have read many times over the years that it does absorb air. Many manufacturers now do not recommend DOT 5 brake fluid, including Porsche. The attachments don't bear on this subject. But, yes, the brake master cylinder problem I corrected also explained the dragging brakes I had, but not the soft pedal or increasing air bubbles when the brakes were hot. (And, no, I don't believe I ever got my brakes up to the 500 deg. F service rating for DOT 5 fluid.)

I originally went to DOT 5 fluid because of its high temperature performance. I routinely got my brakes very hot in autocrosses and other races back then, especially since my Midget didn't have vented disks. I didn't like the fade at all with LMA brake fluid, but after the later problems with DOT 5 I decided to live with the problem, especially since I didn't like the soft brake pedal that the DOT 5 fluid gave me.

On the subject of new master and wheel cylinders with dry components, just take extra time bleeding the brakes. I've used a Gunson EZ-bleed for 20 years on my European and British cars, and it works very well for pushing bubbles out. In fact, I bought it specifically to help bleed the Midget's brakes after putting in the DOT 5 fluid, and spending hours hand pumping and hand bleeding, and still not getting the bubbles out of the DOT 5. The EZ-Bleed allowed me to get the bubbles out of the system. If you're rebuilding your own brake components, use a hydraulic seal lubricant. The stuff I have is a synthetic lubricant suitable for natural or butyl rubbers, but brake fluid should work just as well (except for that "squeeky" feeling on your fingers).

Scott

 

[RonMacPherson]

One of the attributes of silicone, being that it does NOT absorb water is long life. It should outlast the tires, several times over.


Sounds like your master cylinder "tip-off" valve may be a problem.

As far as heat resistance Lucas and Ford both make a superhigh temperature resistant fluid. Only thing is it eats paint and is hygroscopic. So it has a limited life span. And the longer it is in the system the more moisture absorbe. The more moisture absorbed the lower the temperature resistance.