One. DOT5 and DOT3 (Glycol) brake fluids do not mix physically or chemically. They can be used in the same system without any damage to the system or chemical degradation of the fluids themselves. So, if you put a little of one in the other, no immediate problem but you do introduce water and therefore rust into the system..
Two. DOT5 boils at 500 degrees F. Fresh, uncorrupted DOT3 boils at 400 degrees F. Water boils at 212 Degrees F. DOT3 absorbs water leading to: A. Lower boiling point and B. Corrosion in the system. This rust is what scores the cylinders and wears the rubber cups leading to failure. Lower boiling point leads to easier brake fade and failure due to the heat of heavy braking. DOT5 prevents all that.
Three. One drawback to DOT5: IF water gets into the system - usually by submerging the vehicle such as the military does driving through rivers - the water pools at the low points in the system. This is corrected by bleeding the system again. If you don’t drive in rivers, it won’t happen. If you do, bleed it soon after. To get water in the system, the water level would have to be over the master cylinder cover.
Four. Using DOT5 in an old system will precipitate the sludge in the lines down to the wheel cylinders causing failure. See tech tips below.
Five. “Silicone brake fluid is completely compatable with elastomer materials used in conventional brake systems.” Translation: It wont hurt the rubber bits.
Six. The silicone will not eat your paint job if you spill it.
Flush the system with DOT3 before changing to DOT5. Flushing with alcohol is NOT indicated.
Blowing the lines out with air creates a highly flammable vapor!
DOT5 aerates easily. Don’t shake the bottle and pour slowly.
To change over to DOT5, flush out your old system with DOT3 then rebuild or replace the wheel cylinders and the master cylinder. This is a good time to replace all the flex lines in any vehicle over 20 years old (Any light line truck or Scout). Fill the system with DOT5 and bleed. Drive it a few days. If the pedal feel is soft or spongy, bleed again because silicone releases air bubbles slowly. Sometime a third bleeding is indicated.
The DOT5 will gradually move all the sludge in the steel lines down to the wheel cylinders. At six months to a year after your system rebuild, bleed thoroughly again to remove this sludge. Otherwise, the sludge will wear the cups and the cylinders will leak.
I neglected this last step on Old Red, the 65 Travelall . The system was rebuilt in May, 2000 and a cylinder leaked in November, 2003. On inspection, all the cylinders had yuck in them. The cylinder walls were good with no sign of corrosion at all. A kit in each cylinder solved the problem.
Some of you desert rats will remember the 68 desert tan 4X4 Travelall I drove all over the west for 20 years and 200,000 miles. I rebuilt that system using DOT5 when I got it in 1978. We finally stripped and junked it after five years of rotting behind the barn. The brake system still worked! I pulled all the cylinders apart and found no sludge or dirt at all.
If you are rebuilding your brake system and are going to keep the truck, DOT5 silicone is indicated.