MG Car Repair: Dot 3 or Dot 4, clutch master cylinder, dot 4

Question
On a previous question you said to use only Dot 4.  You helped me with my clutch problem and when I installed the new clutch master cylinder the instruction sheet said to use Dot 3 and if I recall when the slave was installed it said to use Dot 4. What should I do?

Thanks
Wayne

Answer
The DOT 3 vs DOT 4 came from when England owned 90% of the natural rubber and used natural rubber in their seals and DOT 3 would eat into and swell up natural rubber. This would destroy the hydraulic system if used in a British car. Since then and I don't have any data as to when they claim that DOT 3 is OK now and/or the new seals from England will not be effected by DOT 3. The trouble I found was how are you suppose to know if the DOT 3 fluid is the new "no harm" fluid and/or how would you know if all the seals in your old British car is compatible with the new DOT 3. So I play it safe by always recommending DOT 4 which is a better grade of fluid anyway. It can handle much higher temperatures. Haynes service manuals even say to use DOT 3 and I contacted them about it and they could not give me a reason that they printed that.

Many times in the dealerships I worked in I seen the result of using DOT 3 in a British car but that was 30,40 years ago. But I still prefer to play it safe and recommend DOT 4 only.
Howard