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MG MGA - Dot 4 or Silicon?

Although I am only a very occasional contributor, I am a regular and avid reader of this excellent forum. So I am somewhat concerned that it's activity seems to be reducing substantially. I assume the threads have a time limited shelf life before moving to archive, which is why the current subject list is shrinking by the day.
So in the hope of reviving this forum, I thought I would post a few old chestnuts, to see if I can get some debate flowing again?

Where best to start than advise that last year I did a brakes overhaul. I renewed the master cylinder ( twice a I used a cheap one first that was junk), replaced the pistons in the front calipers, changed the flexible hoses, etc.
So I had to decide whether to switch from dot 4 to silicon. I decided to stay with dot 4 but was this right move?

Graham V

I would say yes - silicone DOT 5 is probably less in need of regular fluid changes but I am not convinced that is a good thing - better to change the fluid and seals on a regular basis as a preventative measure.

DOT 5.1 is not silicone by the way - and is my choice for brake & clutch fluid.
Chris at Octarine Services


Worth a read of this archive article from the MMCC Safety Fast Magazine:

Steve Gyles


I guess the shortening thread list is symptomatic of the winter lay-up period for most owners. Give it a month and the queries will start coming in abundance.

The main thing with converting to silicon fluid for you is to remove absolutely every trace of the mineral fluid. As you will have read in that article I posted it involves a total system flush out and replacing EVERY rubber seal - wheel brake cylinder seals, caliper seals, MC seals and clutch seals. Obviously best done during during a total car rebuild. Most of us don't deliberately change all the fluid that often. Mine is still essentially the same from 20 years ago other than that lost during the bleeding process and MC replacement. I know others have had the same fluid a lot longer. Bit like W G Grace's original cricket bat in the Lords Pavillion. It's only had 3 new handles and 8 new blades.

Steve Gyles

Interesting article Steve. I must remember the army's emergency procedure for loss of hydraulic fluid when in combat (look out for me on the M25 ).
I stayed with dot 4 for the simple reason that it appeared to be a lot of work to switch and wasn't sure the benefit would be worth all the work. And I was keen to get the car back on the road asap!

Interestingly my eezibleed sprung a nasty leak in the MC cap and I quickly washed the wing down with water and all seems well.
Graham V

This always deteriorates into a pissing contest or a religious war. Truth is, any commercial brake fluid will work well in these vintage cars. Natural rubber seals are long gone, so don't worry about the specification for the Girling crimson red fluid.

Do not mix different types of fluid. If you change fluid type, then you need to flush the system clean and dry, and change out all of the rubber parts.

Otherwise DOT-5 is more expensive, but it doesn't eat paint when you get a leak (the best selling point), and it should extend life of the rubber parts. In any case you should still flush fresh fluid through the system every couple of years to eliminate moisture that accumulates from condensation.

Lots of opinions out there on this subject, but sticking to the hard facts makes the decision easier. Simply put, silicon fluid is trickier to bleed, costs a bit more and is harder to find. BUT when (not if) your master cylinder leaks, it won't destroy every trace of paint and rust the shelf like glycol will. So in my opinion it's more of a individual judgement call. There is no wrong fluid.

My personal preference on the MGA is for silicon, which I've been running now for several years without issue. I made the switch because of what the glycol fluid did to my shelf when the MC leaked.

If we're going beyond the hard facts and into the grey area, you could argue that glycol has slightly better lubricating properties, and that different brands of silicon seem to act a little differently from one another. Someone in that indusrty told me that standards are looser on silicon so that's why it happens (diferent anti-swelling agents, etc). Personally I've tried several brands of silicon over the years and while it's hard to tell sometimes if it's just in my head or one is actually better than the last, the Dynolite fluid Moss and others are currently selling is one of the best I've used. Last system rebuild from bare lines (on an MGB last month), it bled very easily to a nice firm pedal on both clutch and brake.
Steve Simmons

I believe Lockheed are far from keen on Silicone. Although silicone, unlike mineral, is not hygroscopic it does tend to hold air which makes it more difficult, if not impossible, to get a hard pedal. I use 5.1.
Allan Reeling

The important thing with silicon fluid is not to reuse expelled fluid until it has sat for a week or two to let condensed water, rubber particles and micro bubbles settle out.

I have been using it for thirty years with no problems at all (bleeding is easy with an eezibleed)

Dominic Clancy

Dominic, do you re-use brake fluid? Anything that comes out of my bleeders has always been disposed of. I would never under any circumstances re-use DOT3/4 but hadn't considered that silicon might be ok to use again since it doesn't absorb moisture. Interesting idea.
Steve Simmons

I changed to Silicon fluid mainly because it does not absorb water which with regular fluid will cause pitting corrosion of the master cylinder and wheel cylinders particularly if they sit over winter. Flushed the system with alcohol and have had no problems since. The added advantage of course is that the silicon fluid won't lift paint if spilled. I doubt if it is worth changing out all the rubber seals if you decide to convert. Mine seem to function fine with the conversion - I would be interested hear if Barney has a different view.
Tom Heath

Some interesting reading on the subject of brake fluids, from Paul Hunt's MGB site

You may need to copy and paste the link
Dave O'Neill 2

I reuse my 'bled' silicon fluid after filtering through very fine cloth and leaving to stand, usually for months, before another bleed etc. I keep the filtered stuff in a separate bottle. Like Dominic I use the Eezibleed and have never had any issues with bleeding. Just give any newly poured fluid plenty of time to settle before going through the process.

Steve Gyles

Re Graham's post I have been occupied with a 3 year restoration that is thankfully almost finished and can now give more time to my MGA coupe that is in need of TLC. I run on DOT 5 but I know many owners that are very happy with DOT4. All I can say is that I've managed to have spillages whist bleeding using both DOT 4 and DOT5 using the ezibleed system and in the case of DOT 5 they heve been spectacular. I put the spillages down to too high air pressure in the system and have gone back to the 'foot method. I accept that ezibleed does work but I have a brake servo that lockheed say can give problems to fully bleed and I think the foot system is more able to clear out any entrapped air.

J H Cole

I've had problems with eezibleed leaking, so now only use the bottle for pressurising, and not as a reservoir. It does mean you have to keep stopping to refill the m/c, but it avoids spraying fluid all over the front wing.
Dave O'Neill 2

My eezibleed leaked at the lid of the master cylinder and squirted every where. I'm not going to risk that again - I am back to old fashioned pumping on the pedal
Graham V

With the comments on glycol brake fluid being hygroscopic we need to look at what is happening.
Every time you apply the brakes the fluid level in the master cylinder drops and returns to the original level on release.
When the fluid level drops it draws in air from outside containing water vapour. This is absorbed by the brake fluid and the air, less some of the water vapour, is expelled from the master cylinder when the brakes are released.
This is repeated thousands of times and slowly builds up the amount of water in the brake fluid.
On modern cars a bladder is fitted inside the master cylinder and connected to the hole in the cap.
In this way the air entering and leaving the master cylinder never comes into contact with the brake fluid.
This is a simple modification for an MGA.

M F Anderson

Mick have you done this mod? If so could we have details and a photo?

Barry Gannon


I did it years ago when I had a 1500 pushrod car.
However I sold that car when I bought my Twin Cam.
I never bothered fitting it to the Twin Cam's two master cylinder setup but just changed the fluid every couple of years.
It is simple. Just solder or weld a steel tube to the inside of the cap so it covers the air hole in the cap. It only needs to be about a 1/4 inch long. I then fitted a small party balloon to the tube and secured it with a cable tie. It does not need to be very secure as the air inside the balloon is always at atmospheric pressure whether extended or collapsed.
You must test the balloon for some time in a container of brake fluid to check that the balloon material is compatible with the brake fluid.
Another alternative is to find a compatible cap from a modern car which will fit your master cylinder. Most Japanese cars sold in Australia have this arrangement as standard.

M F Anderson

Yes Mick, I think many of us would like to do this mod, especially if it is not too difficult. I will go along with Barry, details and a picture will be of great help.

F. Camilleri

I don't reuse the normal stuff which can have water absorbed into it.

When I bleed a new system with silicon I end up with a lot of new fluid that has micro-bubbles but is otherwise brand new. l leave it to sit in a closed container for a few weeks and throw away the bottom 5mm or so. the remaining fluid is perfectly reusable.

Dominic Clancy

"Every time you apply the brakes the fluid level in the master cylinder drops and returns to the original level on release.
When the fluid level drops it draws in air from outside containing water vapour. This is absorbed by the brake fluid and the air, less some of the water vapour, is expelled from the master cylinder when the brakes are released"

I don't think so - the shuttle moves forward with a brake press and the volume between the shuttle seals stays the same so the fluid level doesn't drop & rise.

The fluid level slowly goes down over time as the pads / shoes wear and fluid passes over the front shuttle seal to fill the larger volume.

The amount of air drawn in is minuscule.
Chris at Octarine Services

Sorry Chris I cannot agree that the fluid level in the master cylinder does not rise and fall.
When you apply the brakes fluid must move into the slave or caliper cylinders.
This fluid must come from somewhere it cannot be just magically appear from nowhere. It comes from the master cylinder.
To put it more clearly as the master cylinder piston moves fluid forward this must be replaced by a drop in the master cylinder level.

M F Anderson

As long as the cylinder is pushed forward,the connection between the cyl. and it's reservoir is shut off.The level will not change IMO.
When the pedal is pumped,there is extra fluid added and the level will move.
Dirk Van Ussel

This is difficult to understand.
If fluid is moved forward to the slave cylinders it must come from somewhere.
I understand that the piston as it moves forward it closes off from the reservoir and pushes the fluid in front of it, but what happens behind the piston? It must draw fluid from the reservoir and therefore change the reservoir level. It cannot just leave a vacuum behind it.
The fluid can be considered as a "closed system". At one end we have the master cylinder fluid level and at the other end the fluid against the slave pistons. If one moves forward the othen must move the same amount (volume).
As the piston retracts it uncovers the return port where the volume returned by the slaves is returned to the master cylinder.

It seems simple physics (hydraulics) to me I guess it just my explanation that is lacking.

M F Anderson

When the master cylinder piston is pushed, fluid moves out the pipe end, while air moves in the pushrod end in equal volume. Keep in mind that the rubber cup seal you can see on the outside is a dust excluder around the pushrod, not a fluid seal, and not air tight. That visible "seal" breathes air every time the pushrod moves, but that air does not enter the reservoir.

If brake shoes are out of adjustment so you have a low pedal condition, and you pump the pedal a second time, then the anti-return valve holds fluid in the line so more fluid is drawn from the reservoir on the first return stroke (and the reservoir fluid level drops).

Even when the shoes are adjusted properly, there is the normal movement of fluid on the first push stroke. On the return stroke high pressure downstream will upset the pressure foot just long enough to relieve the high pressure, pushing some fluid back along with part of the MC piston return. Then the non-return valve does its thing to retain fluid downstream momentarily during the rest of the return stroke, which is when some more fluid is drawn into the MC piston cavity, and the reservoir fluid level drops a bit. If you wait long enough for the residual pressure downstream to bleed off, then more fluid returns from downstream, ultimately going back into the reservoir.

So yes, the reservoir fluid level does change a tiny bit with each stroke, and it changes more if the brakes are out of adjustment. It does not change as much as the full displacement of the master piston.

Fluid level in the reservoir changes with thermal expansion and contraction of the fluid. Air mass in the reservoir also changes with thermal expansion and contraction. The regular small changes of fluid level and air mass make the air breath in and our of the reservoir, bringing moisture along with it.

Temperature changes ultimately condense moisture from the air so it very slowly "rains" inside the master cylinder reservoir.

With silicon fluid (DOT-5) a tiny amount of moisture can be absorbed into the fluid, while excess moisture will ultimately puddle as droplets of water at low points in the system. The free water can boil at lower temperature than the silicone fluid, and it can cause corrosion, so it is a good idea to avoid free water.

With glycol fluid (DOT-3/4/5.1) a lot more moisture will be absorbed into the fluid, lowering the boiling temperature of the fluid. Moisture can also be absorbed into the fluid from air without temperature changes. When the fluid is completely saturated (pretty serious moisture condition) excess moisture can puddle as free water. Even without free water, moisture in the glycol fluid can cause corrosion in the system.

In any case, it is a good idea to bleed fresh fluid through the system every couple of years to get rid of accumulated water. Some modern cars do have a rubber bladder in the master cylinder cover to seal the breathing air away from the fluid.

This thread was discussed between 11/02/2018 and 16/02/2018

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