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MG TD TF 1500 - DOT 3/4 & DOT 5

I put some Dot 3/4 in a tube and then poured some DOT 5 on top, which you will see in the first photo on the left. As you can see they don't mix. The right tube shows what happens when you shake them up, a cloudy mixture forms. In the second picture you can see their reaction between the two after just 15 minutes, the 5 pushes the 3/4 fluid to the top by creating microscopic air bubbles. I think this is a simple demonstration of what could happen in a brake system when the two are mixed if all the DOT 3/4 isn't completely removed, as in a wheel cylinder or master cylinder, the 3/4 will collect at the top when setting for a while. Not good in my estimation. PJ



P Jennings

Second photo. After setting for 15 minutes.



P Jennings

Nice illustration of what people have been saying for years about using silicone fluid. It works fine if used in a complete rebuild of the system, with all trace of the old stuff removed. Add it to a system and try to just bleed it through and you are asking for trouble.

I know that some folks think silicone is the work of the devil, but I've used in my street MGA for almost 20 years and my MG midget racecar for 10 with no issues. I did have to replace some rubber bits in the master cylinder in the MGA a few years ago, but found the cylinder bores were as smooth and shiny as new. With both cars I did a complete rebuild of the entire system before using it.
David Littlefield

What really caught my eye Dave, was the fact the DOT 5 emits air bubbles if it's agitated somewhat, which in this little demo shows by floating the 3/4 to the top. The creation of air bubbles is probably a good reason that the system should be bled a few times, (slowly), after filling. I have it in our MGB and now in the TF and have a hard pedal in both. It's been in the B since 2006. But, as I said about bleeding, I bled both cars about 4 times in about 4 days. The TF needs one more bleeding, just for self satisfaction. PJ
P Jennings

My TD has DOT 5, and the MGB I'm restoring will have it as well. The air bubbles are why DOT 5 is not acceptable with ABS brakes.

DOT 5 was originally touted as the 'Universal Brake Fluid' meaning it could be mixed with no ill effects with any fluid. While that may be true, mixing it provides none of the advantages of DOT 5. Nice illustration, PJ.

warmly,
dave
Dave Braun

As far as I've been able to determine, it's the moisture in the DOT3 fluid that causes the "goo" when mixed with DOT5. Notice in the photo below, the white layer of goo. This is a water layer reacting between the two types of fluid.

Steve S

What is the red fluid? DOT-5 is blue. PJ
P Jennings

Blue? I always considered it purple... but what do I know!

warmly,
dave
Dave Braun

It's just the color of the DOT5 reflecting into the DOT3 through the glass jar.
Steve S

I love the DOT 5 in the TD. It has been there for 15+ years and still purple as the day I put it in.

I used it in my 67 B when I restored it and still am uncomfortable with the brakes. I know I still do not have all the air out. One pump and brakes are crisp, but first push and they are less than fantastic.

I have tried every suggestion in the archives and various bleeder systems.

Because of my success with the TD, I am reluctant to drain, flush and fully replace with another, but the thought does cross my mind.
Bruce Cunha

Well, if it' helps, I have DOT5 in my '67 B as well and the brakes work beautifully. Also in the '69 C with a booster.
Steve S

...just a quick FYI.... put silicon in the TD 6 years ago after rebuilding the whole system and it has been fine, don't think I have had to top it up even...am putting new in this season 'just because'....
I had it in the Wolseley until I bought the new Brake Servo....there was a large sticker on the side 'Warranty Void if used with Dot 5/Silicon Fluid'
Had to flush out the whole system and refill with Dot 3/4... I have chatted with owners who did use silicon and have had no trouble, but brake servos seem to add a new twist to the changeover.

gblawson(gordon- TD27667)

This thread was discussed between 22/02/2011 and 02/03/2011

MG TD TF 1500 index

This thread is from the archive. The Live MG TD TF 1500 BBS is active now.