When looking at DOT 5 Manufacturers websites, I have found that none of them recommend mixing dot5 with dot3. They actually warn you NOT to.
[FONT=Arial,Helvetica]DOT5[/FONT]
[FONT=Arial,Helvetica][SIZE=-1] DOT5 brake fluid is also known as "silicone" brake fluid.[/SIZE][/FONT]
[FONT=Arial,Helvetica][SIZE=-1] Advantages:[/SIZE][/FONT]
[FONT=Arial,Helvetica][SIZE=-1] DOT5 doesn't eat paint.[/SIZE][/FONT]
[FONT=Arial,Helvetica][SIZE=-1] DOT5 does not absorb water and may be useful where water absorption is a problem.[/SIZE][/FONT]
[FONT=Arial,Helvetica][SIZE=-1] DOT5 is compatible with all rubber formulations. (See more on this under disadvantages, below.)[/SIZE][/FONT]
[FONT=Arial,Helvetica][SIZE=-1] Disadvantages:[/SIZE][/FONT] [FONT=Arial,Helvetica][SIZE=-1] DOT5 does NOT mix with DOT3, DOT4 or DOT5.1. Most reported problems with DOT5 are probably due to some[/SIZE][/FONT]
[FONT=Arial,Helvetica][SIZE=-1] degree of mixing with other fluid types. The best way to convert to DOT5 is to totally rebuild the hydraulic[/SIZE][/FONT]
[FONT=Arial,Helvetica][SIZE=-1] system.[/SIZE][/FONT]
[FONT=Arial,Helvetica][SIZE=-1] Reports of DOT5 causing premature failure of rubber brake parts were more common with early DOT5[/SIZE][/FONT]
[FONT=Arial,Helvetica][SIZE=-1] formulations. This is thought to be due to improper addition of swelling agents and has been fixed in recent[/SIZE][/FONT]
[FONT=Arial,Helvetica][SIZE=-1] formulations.[/SIZE][/FONT]
[FONT=Arial,Helvetica][SIZE=-1] Since DOT5 does not absorb water, any moisture in the hydraulic system will "puddle" in one place. This can[/SIZE][/FONT]
[FONT=Arial,Helvetica][SIZE=-1] cause localized corrosion in the hydraulics.[/SIZE][/FONT]
[FONT=Arial,Helvetica][SIZE=-1] Careful bleeding is required to get all of the air out of the system. Small bubbles can form in the fluid that will form[/SIZE][/FONT]
[FONT=Arial,Helvetica][SIZE=-1] large bubbles over time. It may be necessary to do a series of bleeds.[/SIZE][/FONT]
[FONT=Arial,Helvetica][SIZE=-1] DOT5 is slightly compressible (giving a very slightly soft pedal), and has a lower boiling point than DOT4.[/SIZE][/FONT]
[FONT=Arial,Helvetica][SIZE=-1] DOT5 is about twice as expensive as DOT4 fluid. It is also difficult to find, generally only available at selected[/SIZE][/FONT]
[FONT=Arial,Helvetica][SIZE=-1] auto parts stores.[/SIZE][/FONT]