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MG TD TF 1500 - Silicone brake fluid

I am planning to change from DOT 4 to silicone brake fluid in my 52 TD.

I have a mighty-vac and wonder if just sucking as much of the old fluid out through the bleed valves is sufficient prior to adding the new fluid.

I have seen restoration recommendations about using compressed air to blow the lines the cleand and then run solvent through the system, but I don't have access to that.

Does the silicone fluid mix well with the DOT 4?

Has anyone experienced any other problems with the switch? I have heard some stories about leakage problems.

Is the silicone fluid skinner than DOT 4?

Any input would be appreciated. Thanks.

David Balfour

David,

Sorry to say, the mighty-vac method will not work very well for you. If you look at the bleaders on each wheel, you will see that they bleed lines only. Any fluid (or air)in the cylinders remain there, so you would not get all of the DOT 4 out of the system. Since the main advantage of silicon fluid is that it does not absorbe moisture and as a result there is no, or little corrosion in the system (I have had silicone in my TD for 20 years and it is as clear as the day I put it in), you don't want any of the DOT 4 fluid to remain in the system to absorbe any moisture. This means that you need to open up all of the cylinders and get all the old fluid out, plus flushing the system with alcohol or Brake-Klean, then follow up with compressed air to dry everything out. My recommendation is always to make the switch at a time that you are going to have your system apart anyway because it is such a long and involved procedure. In the mean time, continue with the DOT 4 and flush the system every other year.

To answer your other questions. 1) Silicon (DOT 5) and DOT 4 fluid are compatiable but they do not mix - they just coexist in different strata. 2) Some people have experienced various problems with silicone fluid. It is thinner than DOT 4 fluid and like synthetic oils, will escape through the smallest hole, so one has to have the brake system in tip top shape. Silicon fluid is somewhat more compressable than DOT 3 or 4 fluids so some people complaine of a spongy feel to the pedal (I have not been bothered by this). Silicon fluid will trap air bubbles, so bleeding the air is more difficult than with DOT 4, just remember to doo things sloowly to avoid arification of the fluid while bleeding.

There was a rather spirited discussion of different brake fluids on the MGB BBS a while back. I will e-mail you the findings of two people involved (one of the me) and some web sites that you can go to for what the Federal and ASE specifications say about various brake fluids, silicon (DOT 5) included.

Good luck,
Dave
David DuBois

David,
When I rebuilt our 52 TD in '84, I went to silicon brake fluid... But I was cautioned not to mix any part of the system that has been in touch with anything other that silicone,,,, ie. any rubber seals etc. must be replaced with new. because of possible contamination... I guess this might have been in reference to your first paragraph about the DOT 4 or any part exposed to DOT 4 keeping the moisture ... Your comments and knowledge are always appreciated

Keep Your Stick On The Ice,
SPW
Steve

Hi David,
I have to ask the question, why bother?
I have never experienced any problems with ordinary brake fluid. It needs to be replaced occasionally, but the wheel cylinder cups need to be replaced about every 5 years or so anyway. I just do it at that stage.

I can't see that it would improve braking at all. TD brakes are fine if in good condition.

Just my opinion,

Cheers,

Paul.
Paul van Gool

5/6 years ago we changed to silicone fluid on the TF after a rebuild and then we had problems. Major problems. These resulted in draining all the fluid and changing all the seals which swelled up. As a previous correspondent said; why change? The standard fluid whilst hygroscopic (i.e absorbs water) is perfectly satisfactory adn providing it is changed every 2 years there shpould be no problem.
However if you are in the habit of driving falt out all the time and braking extremely hard; driving down Pikes Peak heavily loaded then I suggest that your brake fluid is very unlikely to boil.
Bob Marshall

Gentelmen - I did forget to include the cavet to change all the seals when changing the fluid to silicon. Bob, I believe that it was in the Record from the Octagon MG club over there in England, that Wim Jenten wrote an article about silicon fluid. He stated in that article that the seals, when soaked in one type of fluid, will swell when the other type of fluid is used. The contamination of silicon fluid by DOT 3 or 4 fluid is a non issue as long as the seals have not soaked in one type of fluid or the other for a long period, but then why leave any of one type in when changing to the other? They will coexist fine with each other, they just don't mix. I believe that it is one of the SAE specs that talks to testing the fluids when mixed, but I may be getting that mixed up with another article that I read recently.

Why change? That is a good question and one that I would pose if I had not already done it. At the time it seemed like a good idea and I have had excellent success with silicone fluid in both my TD and MGB. The silicone fluid was developed for the military for use in vehicles that spent a great deal of time out of service (Wim Jenten's article) and that kind of describes a great number of the T series cars (although I was just telling Bob that we have been forced to drive our TD daily the past two weeks because our MGB is being repaired - I want all of you to feel sorry for me).

Paul, as I said in my first posting, I have had silicone in our TD for 20 years and I finally had to replace the seal in the master cylinder about 2 years ago. Like I said, I have had excellent success with the stuff, but I do know several people who have had serious problems with it. Some of that was growing pains for the fluid suppliers themselves, some was a result of arification of the fluid and I am sure that some was a result of putting it in a system where the seals were soaked with DOT 3 or 4 fluid. I have heard of less and less problems in recent years, so maybe the bugs have been worked out. Regardless, I believe that a brake system will work fine with any of the modern fluids used in it (This is the statement that touched off a rip roaring debate on the MGB BBS recently, I certainly hope that it doesn't here)
David DuBois

I switched to silicon fluid in my TD 10-12 or so years ago. I cannot recall a cylinder rusting/sticking since, and am now out several years on wheel and master cyls. At that brake job, White Post sleeved the front cyls, ( they advise against sil. fluid). Three were fine, and one leaked- never could tell if between the sleeve/cyl or the cup- but it does totally instantly ruin the linings. Brakes were perfect even when the motor was out of the car over a year. This is a very humid area, and I do end up driving in the rain on occasion- with the regular fluid things just rusted and froze up in a couple years. My TD has never had a problem with bleeding, and pedal feel fine. As an aside, I tried sil. in my 280z (with brake job), bled it a dozen times, etc., and the pedal felt like a lot of air was still in there somewhere- so out it went and back to the regular in the Datsun!
George Butz

I have been running Silicon brake fluid for about 20 years also. Changed to it after having my brake cylinders freeze up for the second time after winter storage in a dry garage. Never had any problems, leaks or brake fading. Oh one other nice point. I have never had to repaint my rims after getting silicon brake fluid on the paint.

I took the whole system apart and did a total rubber rebuild prior to adding the silicon, have not had to replace anything yet.

Bruce Cunha

George - The probable reason that you had a problem with the silicon fluid in your 280Z is that silicon fluid has a nasty habit of retaining tiny air bubles in suspension. When it passes through very small orfices under pressure, it tends to cavitate, producing the bubbles that are then retained in suspension. Thus the problems getting things to bleed properly. Our T series cars are not plaged with small orfices in the braking system, so there is not the problem with them. Cheers - Dave
David DuBois

The biggest reason for me to convert to silicone was that it does not remove paint. Normal brake fluids will take paint off faster than stripper. Since the cylinders leak from time to time unless you constantly maintain them, you are bound to drip fluid on your backing plates and/or wheels. Then a repaint is necessary. Not with silicone.

Chris
Chris Couper

This thread was discussed between 11/08/2002 and 13/08/2002

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