MGB Register Forum

Hi

What brands are the best when renewing or topping up brake and cutch master cilinders of an MGB.
Thanks a lot.

Hi Pierre
I don't think the choice is so much between brands as types of Brake Fluid.
The traditional brake fluid is hydroscopic so absorbs water, and at high temperatures this boils off and because a gas is compressible the pedal goes spongy. The modern standard for this is DOT 4. The other type is silicon based (DOT5) which doesn't absorb water but there are two problems. Firstly the pedal 'feel' is a bit different (but you do get used to it!) and secondly it is incompatible with DOT 4 brake fluid, and even a small amount left in the system can give very strange results. So if you want to change to DOT 5 you need to replace all the brake or clutch hydraulic system so there is no DOT 4 Fluid left in the system.

I mixed DOT 5 and DOT 4 in a Mini Cooper I had and it cost me new calipers, servo and wheel cylinders plus new brake lines and hoses to get rid of the result.

Thanks a lot Pete.

Confusingly there is also DOT 5.1 specification! This is glycol based and so is compatible with DOT 4 and other glycol based fluids.
DOT 5 is the odd one out and does not mix with any others.
DOT 5.1 has a higher boiling point than DOT 4 so is often preferred by racers as it gives a little margin for higher temperature in the braking system. I believe it is recommended to change DOT 5.1 at more frequent intervals, but whether this is due to it being more hygroscopic I'm not sure?

Ian F