Many years ago...and I do mean MANY, I had my calipers sleeved and converted the system to silicone. I don't think it even had a DOT designation back then. Anyway I never liked the pedal feel and at some point probably 10-15 years later I converted to a dual M/C with booster and attempted to flush all the silicone out and use a DOT 3/4 synthetic.

I did NOT remove the calipers and only flushed the system copiously with new fluid but now another 10-15 years after that I believe I'm seeing the downside of not tearing the entire system apart. Over the last few years I've had the occasional leaking piston and each time what I've found is a buildup of some contaminant under the lip seals that allow fluid to seek past under pressure. There's no other trace of it anywhere else in the system other than what's trapped under the seals.

While I can't say for certain what it is, I suspect that flushing a system will not remove all traces of silicone and especially that trapped in such places and over time the result is what I'm experiencing. So to be absolutely safe, I'd advise anyone considering a change from one type of fluid to another, do it the right way and take everything apart and clean each and every piece with an appropriate solvent.

 

If you'd left the Dot 5 in there, likely you wouldn't have that problem.

What you see there looks like corrosion caused by water in the system. Silicone repels water.

What you see is typical of systems using Dot 3-4 fluid.

 

Back in 1986, I installed 4 S/S sleeved calipers and Dot 5 blue silicone fluid. My pedal was and still is rock hard. I have NEVER had a leak or any other problems since I switched the calipers & fluid. Before that, with conventional dot 3 fluid one of my calipers would spring a leak every year or two. I think my good fortune is due to the dot 5 silicone fluid. I haven't driven my car since 2013, and I just went out to my garage and pumped the brake pedal-----Still hard as a rock and right up to the top---where it belongs !!! If your pedal was never high & hard, you never got all the air out.


RON

 

Get that car out and drive it and have some fun.

 

Yes, I realize silicone repels water but this isn't corrosion. What you can't see in the picture is that the buildup on the pistons is a bluish gray translucent material and it will flake off and the aluminum surface underneath is unblemished. The original pistons I replaced 40+ years ago all were corroded in various degrees but this isn't the same.

Perhaps it isn't a compound created by mixing the two fluids, I can't say for sure but I don't have any other explanation for it.

 

It didn't look like rust to me, more like some gasket/thread sealer or an internal "soft" component that the brake fluid attacked over time... You may never know for sure...

 

Shortly after I purchased my '"66 ," had leaking calipers and master cylinder. ..I rebuilt the master and replaced calipers. Flushed system with dot 5 and have never had a problem

 

Please understand I'm not criticizing silicone fluid, I ran it in my car for over 20 years. I'm just saying if someone were to change then be thorough in cleaning the system beforehand.

BTW, I've been running synthetic DOT 4 for close to twenty years without issue until this which I believe is the result of doing a less than complete cleaning before changing over.

 

Could it be that what you see there is the normal sloughing off of lip seal material? The lip seals do wear.

 

I don't think so, the bigger pieces were under the inner surface of the seal and stuck pretty well. There are even impressions in the seal. The seal lips themselves look fine (there's joke in there somewhere, I'm sure) and it's easy to see how the fluid gets past the seal by going under it.

 

Now you have me a little worried. I helped a friend pull their 1964 Austin Healey Sprite out of mothballs last year. It had not run in about 20 years, had brakes but no clutch.
When we installed the new master cylinder I flushed it with Silicone and it has been fine since.
At the same time I pushed them to switch to silicone on the brakes and clutch since the same master cylinder does dual duty and sits in a painted area ripe for Dot 3 or 4 paint damage. Yep, one MC with two pistons in it off one resevoir.
I flushed the dog doo out of that system till we ran clean at all 4 corners. It is still working fine, they drove it last week. But you got me thinking.
I must have run 3 12 oz cans through that system hoping to get a complete flush. Did I make a mistake?
BTW: The Healey was not easy to work on due to size and I see no reason to ever want to drive or own one. Talk about an uncomfortable underpowered Go Cart with a licence plate, that is a Healey.
I was not impressed but it was my friends first car in High School and she really wanted to relive some old memories. She says it makes her happy to putt around the neighborhood with everybody waving at her.

 

While I did say that it would be best to disassemble before switching fluids, enough people have taken the same shortcut as you (and I) without reporting any issues. And keep in mind, it's been nearly 20 years since I flushed mine so do you think they'll be driving that Healey in 20 years?