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**dmjlambert** · 07-30-2023, 01:08 AM

DOT 5.1 is glycol based and can be mixed with DOT 3 and DOT 4, although there is no need to mix any of these fluids with each other or think about mixing the fluids. It is easy and cheap to just mostly empty the reservoir in the master cylinder and put fresh of whatever fluid you want to use, and bleed about a quart of it through the system thoroughly to complete replace and refresh. So a newly-acquired truck where you don't know what fluid is in there should just be flushed and put the good ol' DOT 3 (labeled synthetic or not) in it and be done. If you ever have a leak or need to replace a part such as a cylinder or caliper or tubing, or even brake pads or shoes, it's time to repeat the full flush and change on the whole system, no reason not to do it. On a 67-72 truck there is no need to consider using anything other than DOT 3. If it's kept fresh, it's not going to boil or break down or give any problems. Just my opinion.

07-30-2023, 01:08 AM	#20
dmjlambert Senior Member Join Date: Apr 2016 Location: Cypress, TX Posts: 3,615	Re: Brake fluid DOT 5.1 is glycol based and can be mixed with DOT 3 and DOT 4, although there is no need to mix any of these fluids with each other or think about mixing the fluids. It is easy and cheap to just mostly empty the reservoir in the master cylinder and put fresh of whatever fluid you want to use, and bleed about a quart of it through the system thoroughly to complete replace and refresh. So a newly-acquired truck where you don't know what fluid is in there should just be flushed and put the good ol' DOT 3 (labeled synthetic or not) in it and be done. If you ever have a leak or need to replace a part such as a cylinder or caliper or tubing, or even brake pads or shoes, it's time to repeat the full flush and change on the whole system, no reason not to do it. On a 67-72 truck there is no need to consider using anything other than DOT 3. If it's kept fresh, it's not going to boil or break down or give any problems. Just my opinion.