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MG MGB Technical - Brake fluid

About a month ago I flushed the brake lines and put in new Dot3 fluid. Now, based upon what I've been reading here, I want to flush them again putting in Dot4 this time.

My question is how to go about this.

1. Should I do one brake at at time (standard proceedure: furthest first, then working my way closer to the master cylinder), putting new fluid in the reservoir as needed (not mixing the old and new of course)?

2. Should I try and get all fluid out of the whole system before adding any of the new fluid?

3. Should I do any kind of intermediate flushing process to completely clear the Dot3?

BH Davis
BH Davis

Drain all you can first. Then you will need to perform several flushings with DOT4. The two fluids will blend together making it very hard to get all the DOT 3 out of the system. If you have the time and want to get it all I would dissassemble the wheel cylinders and calipers. clean them in DOT 4 and reassemble. While they are disconnected flush the master cylinder and lines using DOT 4. It is hard to say how much DOT 3 can remain mixed in the system without causing you problems.
John H


Is there some sort of flushing agent I can use before putting the Dot4 in? The thought being it would clean out the Dot3 before it had a chance to mix with the Dot4.

BH Davis

You could leave all the D.O.T. 3 in if you wanted. D.O.T. 4 is better, but D.O.T. 3 won't hurt anything. It used to be said that D.O.T. 3 would destroy the seals in British brake systems, but if that was ever true it hasn't been true in decades, and if you have decades-old rubber in your brake system then you have bigger worries! ;-)

Suck all the D.O.T. 3 from the reservior, fill it with D.O.T. 4, and bleed as normal. Any D.O.T. 3 left will not hurt anything.

Rob Edwards

I agree with Rob. The master cylinder cap for 68 and later cars specify DOT 3 brake fluid. The cars must have been delivered with DOT 3 fluid. I don't think DOT 4 was avaliable until the late 70's. I made the switch from DOT 4 to DOT 3 and back to DOT 4 several years ago. I left the DOT 3 in about a year. Nothing unusual happened and I couldn't detect any difference in braking. I just changed the fluid and didn't do any flushing. For more information on brake fluid see.


Clifton Gordon

Thanks everyone. I think I will first check my resevoir cover and see if it says "Dot3". Just curious.

I'll change out the Dot3 for Dot4 though simply because I just used Dot4 in my rebuilt clutch system and would like to be consistent. Aging brain cells ya know.....keep it simple for memory sake!

BH Davis
BH Davis

DOT 3 and DOT 4 fluids are completely compatible, so you don't have to be very careful about flushing one to install the other. Their properties are similar enough that you might as well wait a couple of years and then just bleed and refill with DOT 4. Unless you are racing, DOT 3 will work fine. -G.
Glenn G

This thread was discussed between 12/06/2007 and 15/06/2007

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