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MG Midget and Sprite Technical - The brake fluid debate

I apologise in advance as I know this is one of those controversial topics. But I need to decide ahead of testing the brakes in the near future.

Frogeye, front discs, later 3/4" rear cylinders, new 3/4" tandem m/c. New brake pipes throughout so unblemished. Car currently on wheeled frame so brakes easier to bleed and test. Will be road use only. So, mineral (dot 4) or synthetic (dot 5)? I simply can't decide.
Bill Bretherton

I hate handling Dot4 near the car, and a new brake system may have leaks which isn’t good for new paint. Every time you do anything on the brakes or clutch you risk paint damage.

I haven’t tried Dot 5 but just the fact it doesn’t damage paint makes it appealing. The next time I start with a new braking system I will try it..
Chris Madge

Bill, that's exactly the question I am at.

All new empty system so new seals throughout. So which one to go for? Dot 4 or DOT 5? I am leaning towards synthetic for what will be a low mileage car and therefore I won't really want to be draining and replacing the fluid every couple of years. That is factoring in my advancing years as well!
GuyW

For the reason Chris says I used synthetic on a newly painted shell (and it has been spilt, ahem, but no damage) and it doesn't absorb water so can have a longer life.
Some say as it doesn't absorb water then water will form in 'puddles' and may cause localised rusting but for me, the advantages outweigh the additional cost.
If you have to bleed anything like oh, I don't know, say the clutch for example, chances are fluid will escape and I didn't want the fresh paint ruined.
The brake pipes, hoses and cylinders were all new so there was no chance of any contamination.
Jeremy MkIII

Im beginning to wish Id gone for DOT5, as I had a leak from the master cylinder which took paint off the bulkhead and pedal box. Its a lot more effort to remove the pedal box on an MGB than a midget.

I think I saw something on the MASC website, where Mark Boldry was saying that DOT3 is more suitable for the type of seals in our systems than DOT4.
Dave O'Neill 2

I've had DOT5 brake fluid in my BV8 for over ten years now. It has never been changed and the brakes are fine. The fluid also restore plastic parts that have gone grey, back to black. Made a big improvement on the dash of my Elan.
Mike Howlett

I understand that silicone fluid (DOT 5) is more compressible than DOT 3 or 4 and may therefore lead to a more spongy feel to the pedal.

For that reason alone I stay clear of it, I have used DOT 3 & 4 for 55 years and never had paint damage.
Chris at Octarine Services

I started using silicon brake fluid in the early - mid 1990's
I have a Frogeye on drums and a Sebring Rep on disc / drum set up.
My 'modern' daily driver is a 1990 Ford Escort XR3i
I won't use anything else.
At one stage every car in the household was on silicone fluid.
I often wonder if those that hold forth against it have ever actually used it.
Alan
Alan Anstead

Dot5 may not be the paint stripper that dot4 is but it can still mark fresh paint. When I painted the engine bay on our Frog 8 or so years ago, a short time later I bled the brakes and clutch. I used an eezi-bleed and over filled the bottle. I placed it on top of the fresh paint on the inner wing while I went to get some rags to mop it up. I cleaned up but didn't notice anything at the time. A week or so later I noticed a ring in the paint. I thought it might be a tea cup ring but it was smaller. It took a while to realise it was the same size as the eezi-bleed bottle. It's still there now. It appears as a change in texture of the paint, where the ring is, is dull in an otherwise shinny painted wing.
Rob
MG Moneypit

I'm leaning towards synthetic/ silicon (DOT5). The fact that you don't need to keep changing it appeals, especially into the future, as Guy says. I've read it doesn't lubricate as well as mineral and isn't as good for the clutch perhaps but, although expensive initially, seems the preferred option.
Bill Bretherton

Circa 210000 on the XR3i I did have a change of master cylinder as the silicone fluid began to leak.
Alan Anstead

I've used DOT 5 for the past 15 years without any problems.
Philip Sellen

I've also used synthetic dot5 for the last couple of years and very happy with it.
G Lazarus

I put DOT5 in my Midget when I restored it in 1990. Paintwork around the cylinders is still perfect.

Any car that I do a complete system rebuild get DOT5 - did a 1936 SA last year - perfect brake pedal.

You need to take great care not to get any air in the fluid as it takes an eternity to "bubble out", but done right it's simply brilliant.

The Midget has done a bit of motorsport too over 30 years, with no brake dramas at all.

Silicone is standard equipment on all new Harley Davidson's sold around the world, so easy to buy at your local HD parts outlet.

Cheers

Tony Slattery
The Classic Workshop
Black Mountain
Australia



A L SLATTERY

I was at the same point, new everything including newly painted shell. Within a week I was glad I’d gone for silicon, a leak from the brake light switch saw dripping all down the chassis. Just wiped off, no damage at all.

The brakes don’t feel great though, not sure whether it needs more bleeding, the cheap pads/shoes I used or possibly the dual master cyl cars feel different to the later type I’m used to. Something I’ll be investigating over the next few weeks.
John Payne

I first used Silicone (Automec) on my then mgb restoration in about 94.
My Midget rebuild was completed in 2008 and its been in there unchanged since.
I have not noticed any pedal issues and have a good firm brake pedal. Also used for the clutch systems.

I would not use anything else in a car that may see long periods of inactivity. It doesnt absorb water and is paint friendly.

There may be performance issues when used for track/racing etc but Ive not had any issues with road use.

R.
richard b

For those who have used silicone for any years and DoT3 and DoT4 before, have you found any difference with the longevity and serviceability of rubber seals and flexi pipes? I ask as we are always worrying about quality of replacement parts (not that I can remember anyone moaning about seal kits and flexible brake pipes...)

Always good to hear of experience, and Alan I have not tried silicone fluid, but have an open mind.

Not switched yet from DoT4 but next time I am replacing lots of seals and flexi pipes might do.

I know it is seen as a fit and forget item, but what would count as a cautious service interval for fluid flushing silicone? I.e.beyond 2 years for DOT3 or 4 (due to its hydroscopic nature)

Cheers
Mike
M Wood

I was n early adopter and have used silicon fluid in my MGA since its restoration in 1988. No problems apart from seals just wearing out after 25 years, at which point I replaced all the cylinders as I decided they were cheap enough and as safety critical items I would rather fit new than recondition.

You absolutely DO need to bleed silicon fluid periodically, as water from air humidity condenses in the MC and beads at the lowest point of the bore and corrodes / pits the cylinder wall. This can happen on wheel cylinders too, so bleed when you service just like DOT4 and DOT3.

As to making the brakes spongy or soft, most other MGA drivers comment that my brakes are really good, and when we go to investigate the cause of their brake issues, it is always corrosion and leaks..... old hoses (once the originals!!!!) are often cracked and weeping, yet my 25 year old cylinders and callipers looked like new inside and outside apart from the worn seals.

So for me silicon fluid has a long history without complaints or problems.
Dominic Clancy

Some interesting posts - the consensus is clearly in favour of silicon (Dot5).
Bill Bretherton

Not to be left behind...

I started using silicon brake fluid (DOT 5) in about 1990 when I was living overseas and the Midget was stored. Storage is hard on brakes, even in the desert so DOT 5 solved the wheel cylinder rusting problem. The moisture that accumulated in the DOT 3 or 4, would always migrate to the lowest point in the system.

The brakes are a bit squishier with DOT 5 but certainly are effective, even in panic mode.

I run DOT 4 in the B since it is driven almost daily. The 1970 B didn't come with a booster so it needs all of the firm braking it can get anyway.

If you car is stored through the winter or not driven constantly, I vote for DOT 5. One thing that is critical is replacement of caliper seals, wheel cylinders, the master cylinder and the pressure differential switch when changing from DOT 3 or 4 to DOT 5. Residual DOT 4 will get trapped under the DOT 5 in calipers or wheel cylinders and create a serious rust condition.
Glenn Mallory

I've never used silicone fluid and other online discussions have put me off. But I am sick to death of the paint damage. The master cylinder is in such a difficult position that it's virtually impossible to fill it without spillage, and fluid gets under the cylinder and strips the paint off the pedal box (as well as the cylinder if that's painted). I had the pedal box powder coated but the fluid damaged that as well. Bleeding the system also creates some spillage, especially the clutch. The idiotically tiny bleed nipples on the later rear drums mean the tube falls off easily. So I think I am convinced. It will be worth replacing all the seals.

Les
L B Rose

Looks like there are clear benefits using DOT 5. When I rebuilt the car in 2009 I didn't feel so sure. I'll stick with DOT 4 for now, its working well and re-sealing the entire braking system seems a bit unnecessary. To be honest I have no problem replacing the fluid every 2 years as its an opportunity to check all is in order.
Bob Beaumont

That makes sense Bob. If I already had Dot4 in I'd stick wth it as changing is a major hassle it seems.
Bill Bretherton

This thread was discussed between 19/05/2020 and 23/05/2020

MG Midget and Sprite Technical index

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