Hi Larry , assuming that question is to me , I have never drained a silicone fill to exchange it . why ? have had no problems either .
I was fortunate to find gallon cans of it 30 years ago as mil surplus still living off that .
Related to that 30 years — Another aspect of glycol not mentioned is a can of it on shelf goes to hell once opened .. Silicone does not . In fact dissolved air micro bubbles dissipates , easier bleeding . Unlike another post here , i think it takes 6 months to fully deaerate .a can that is shaken . Complaints of spongy pedal can line up here …
water gets into glycol at master storage surface , it slowly soaks it in by design , after many years it is all through the brake system , until it can hold no more , so it is everywhere in the glycol but not as free water . It is still working ok . Boiling point does drop , obviously . But I think a non issue with drum brakes , huge issue with discs .
Why old style glycol was fine , back to 30’s wheel cylinders are thermally removed from brake shoe compared to disc pads .
Saturation point or water holding ability is usually temp dependent .
so on a cold day it may come out as liquid water if saturated and you had not exchanged it , now you have stratified water and glycol , water sinks to bottom ends up in lower wheel cylinder , rust starts . I have seen this rust pit so often any denial it happens is pointless .
It gets in over many years at surface of pool in master cylinder as air pressure rises and falls, bringing in humidity . And the pit may be there already even if cleaned and converted .
Silicone simply does not do that . it drives off water , used for example in modern high voltage insulators on poles , anti hygroscopic design , it beads up water , probably in rain x too ! ( silicone rubber but same deal ) .So unlike glycol it will not carry water in solution to wheel cylinders . But if there was any water in system or saturated gycol left inside lines it would probably sink to bottom under your new silicone ( i am not sure of SG of each) and make it to wheel cylinders due to motion of fluid in lines .
above paragraph is conjecture on my part ,if junk left behind , but total resistance to any significant water mixed into silicone is not conjecture . Does not happen.
Perhaps conversion is slightly risky to some degree unless all new rubber parts and blowing out metal lines — or all new .
Generally i do all new with cupro nickel as often it involves dual master . . If a conversion l would change fluid to be sure not contaminated by old stuff — after a year or two . Or stay with glycol and drain for sure every three years , as MB directs .
I have never converted old stuff , to best of my recall , without all new rubber and wheel cylinder seals / inspection , or all new parts .
Ultimately up to you , on all this . But hopefully we understand it , not opinion
jg
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