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MG Midget and Sprite Technical - Autumn planned work!

Hi Guys,
Planning to replace calipers, disks etc on my midget as my main autumn job this year.
I'm considering swapping over from old school Brake fluid to a Silicone fluid. I understand to bleed off & discard all the old liquid, but how do I flush the brake system thoroughly ready for the Silicone?
Cheers colin
colin frowen

It is best to replace all cylinders (master and wheel) and the calipers. You will find that residual DOT 3 - 4 will remain and you will end up replacing everything anyway. I would also replace hoses. Why bleed it twice?

For the most part, I have been happy with the conversion. The pedal is slightly squishy but I have not touched in during the last 15 - 20 years.
Glenn Mallory

I would be far less concerned about this than Glenn. ( sorry to disagree, Glenn)
First, the bottle of DOT5 I have just bought says "compatible with DOT4 fluids". So I would drain our the old and fill with fresh and not worry about it.
Secondly I would use the fresh DOT5 to flush through anyway, bleeding through each bleed nipple until a good stream of clean fluid had come through. The old fluid will be discoloured and so it will be very apparent when you start getting a good stream of uncontaminated fluid coming through. You could start by emptying the reservoir using a syringe fitted with a short length of windscreen washer pipe. Then fill with new fluid and start pumping at the furthest rear corner. That way there is little or any place in the system where any significant amount of old fluid could lurk.

I have read that flushing the system through with meths will clean it but that just seems to introduce the possibility of getting residual meths trapped somewhere which would be more of a problem than any small amount of old fluid!

I generally don't hold with the idea of automatically replacing old parts with new. There are so many reports of poor quality in newly manufactured items. Certainly replace all the flexible hoses and inspect all lines and parts, but I would keep the original components if at all possible if they appear in good condition. Just inspect regularly after you have made the change over.

And at last, I am not convinced that DOT5 gives a softer pedal than DOT4. A myth in my view. At STP fluids are normally considered incompressible or so nearly so that there is no way you could tell the difference between two oils both designed because of their hydraulic characteristics. Any difference would be in very small fractions of a percent and impossible to feel in practice.
GuyW

Hi Guys,
Many thanks for the advice.
Cheers colin
colin frowen

Colin - and anyone else.
THIS IS IMPORTANT!

It has just been brought to my attention a detail that I had skipped, and not been aware of. I wasn't clear that there was a distinction between DOT 5 and DOT 5.1. My comments earlier were about DOT 5.1 which is a Glycol based fluid, similar to DOT 4 and DOT 3. Which explains why the bottle says it is compatible and can be mixed.

It is different from DOT 5 which is the proper Silicone based fluid, and the one you, Colin, were asking about. DOT 5 is NOT compatible with earlier DOT 3 or DOT 4 fluids. Sorry for my mix up (literally!) And my apologies also to Glenn who clearly saw the distinction more clearly than I!

I am now rather surprised that no-one else corrected me on this.
Guy
GuyW

Yes, fancy calling a non-silicon fluid DOT 5.1, surely DOT6 would have stopped all the confusion.
As to spongyness with Silicon, I noticed this when I went Silicon on our Frogeye about 8 years ago. I know liquids are not compressible but there is for some reason a spongy (or should I say less rigid) pedal feel.
Rob
MG Moneypit

I'm with Glen on this, as the Midget has been DOT5 synthetic since rebuild/replacing the entire braking system 13 years ago. The pedal seems fine but I do have a servo...
Jeremy MkIII

This thread was discussed between 02/07/2020 and 05/07/2020

MG Midget and Sprite Technical index

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