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How should I flush a brake systems if I don't know type of Brake fluid was previously used?

I have no way of telling what they used and if it was Dot 5 isn't it really hard to flush out?

I want to use Dot 3....

Buy your DOT 3. Smell it. If the old fluid smells the same it's Dot 3 and if it smells different it's Dot 5. Or take some old fluid and pour it on a painted object. If it lifts and wrinkles the paint it's Dot 3 and if it does nothing to the paint it is Dot 5.

One method I read in this forum or on G503 was to use rubbing alcohol to flush brake lines then blow them clear with compressed air. I'm planning on doing this on my M38 project to see if the brake lines have rust or contamination in them. If they are OK I'll hook them up to the new master and wheel cylinders, if not, I'll replace the lines and hoses. I've also read that a few drops of one type of brake fluid in a system probably won't do much harm if you do switch from one type to another. Just try to get as much of the old fluid out as possible by thoroughly draining and bleeding the system. Any other thoughts out there? This is a timely post for me as well.

I think there are two types of rubbing alcohol available,
70% (30% water) and 90% (10% water), at Wal Mart. I'm
not sure what the water content would do to the brake lines
and the other parts.

Jim in Darkest Arkansas

You use Isopropyl Alcohol to clean out Dot 3 & 4 plumbing and then flush the alcohol with Dot 3 or 4.

Dot 5 is a real animal to deal with when cleaning. It also does not leave the sytem completely. It always leaves behind slicon residue. The only safe way to switch a system from Dot 5 to Dot 3 or 4 is to replace the entire system.

Cleanliness is akin to Godliness in a brake system.

Draining and flushing are two entirely different procedures. Draining utilizes gravity and flushing utilizes both gravity and some external source of pressure.

Here's a good read for you:
http://importnut.net/brakefluid.htm
http://www.popularmechanics.com/how_to_ ... 72446.html
http://www.type2.com/library/brakes/brakef.htm

Thanks Wes!