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MG MGA - Master cylinder Silicone

I know, here it is again. Now, I bought my master cylinder in 2005 and it was installed and working in 2005 as I found a post in the archives where I was bragging on my progress restoring the 57 MGA. It is now 2011 and I had a clutch failure in the master. I rebuilt it. There was dissolved rubber present when I tore it down. No problem with the rebuild and all was functioning fine except I thought the brake pedal travel was a bit long say 2 inches instead of an inch and half. Not much of a bother. Took the car in to my go-to mechanic for other issues when he found cracked brake hoses which I had him replace. He could not get the system to bleed. There was no pedal. I had him pull the master to discover that the piston was hanging up and that the cylinder was pitted, scored and needing a resleeving. It was too far gone to hone. I called White Post Restorations to set up a resleeving and they told me they would not guarantee the rebuild if I was using Dot 5 silicone. He said the rubber swells and then fails. Sorry to start the argument again but why do some of you run silicone with no problems and others like me are having serious issues? Advise please. Trying to go to Reno in two weeks. Is there a rebuild kit that holds up better to Dot 5?
David Holmes

I used to be an "advocate" for silicone but no longer use it. The answer to your question may well lie in the composition of the rubber seals. Purely from my observations, the older rubber seals seem to be different from newer ones, the latter look more like plastic, very black and shiny. Perhaps these withstand silicone better, but other problems arise and I have used conventional fluid for some years now. With resleeving using stainless steel the benefits of silicone seem to no longer apply. Silicone brake fluid is no longer stocked by brake specialists here in South Australia but the last I heard was that it could be supplied if specially ordered in advance.
Barry Bahnisch

It is more trouble. Harder to bleed. More likely to leak. More time consuming to get right. BUT they ALL leak eventually. If you have paint remover in the cyls it will eat your paint. I would rather change the seals every year than EVER have to repaint the heater shelf. That said I usually get 8-10 years before it starts to leak. I have never re sleeved any of the master cyls in any of my 7 MGAs, never.
It is way more expensive and a big pain in the @$$ but it is all I will ever use in a MGA.
R J Brown

It never ceases to amaze me as to the amount of discussion this brings. Some swear by DOT-5 some swear by the glycol alternatives. For a full disclosure, I am in the latter camp. However it is difficult to dissuade the believers. I’m not sure it can be done and maybe it shouldn’t. But there must be more things at work here to get such different results.

I believe that it boils down to 3 basic conditions. The first is a driver’s sensitivity to brake pedal feel. Some like a hard pedal, some are happy with a firm pedal. It is a well known fact that air entrapment in DOT 5 is an issue that takes an extra effort to bleed out. It is fruitless to argue over perceptions and I won’t go there. 2nd is the formulation of rubber that makes up the cups in the Master and slave cylinders as well as flexible brake lines. Without going into the various accepted compounds used, it is generally an unknown when getting a rebuild kit just what it is we are getting. Even if we knew the compound we were getting, it is difficult to absolutely know the rubber compounds used in the systems with success as well as those with poor experiences. I can say is that the FMVS116, the spec used for acceptance of DOT5 fluids specifies the use of Styrene-butadiene Rubber with a specific formula. It may be that those with success have cups with this or closely matching formulae. I don’t know. 3rd is that DOT 5 is a performance specification not a chemical mix. There are 2 basic silicone fluid manufacturers. The various brake fluid companies use different additives to suit their needs. Silicone by itself will shrink the “standard” seal. Additives are provided to help swell the seals among other things. Case in point, the acceptance range per FMVSS116 is a 10 to 1 difference in swelling rates. And that is with the standard cup. Again, it is difficult to determine the different fluid formulas used in the various reports both positive and negative.

Just because it says DOT 5 doesn’t make it the same. Just because the M/C and wheel cylinder rebuild kits are for MG’s doesn’t make them the same. I suspect that the MC suppliers who will not guarantee their product are aware that many things are out of their control and will not take the risks.
Chuck Schaefer

I just looked at the Master Cyl and the pitting in the brake side is consistent with moisture and rust. I thought that was the benefit of Silicone as it did not attract water. So, how did the brake side get this way in a system that has only had silicone fluid? Now debating on making the switch back to Dot 3 in spite of the paint dissolving properties.
David Holmes

Is there no paint that will resist DOT3/4 ? As a young aeromodeller we used "fuel proofer" that resisted methanol, nitromethane etc.
Art Pearse

Silicone does not ABSORB water, so it lies in the bottom of the cylinders and eats them. Glycol fluids do attract and absorb water, such that it lowers the boiling point and you have no brakes when they get hot. But even glycol can only absorb so much before things get rusty. Barring oil contamination, dark fluid means it is loaded with water, good fluid will be near clear. If you bleed and flush glycol, it will carry the water out of the system, preserving things a while longer. Silicone just leaves the water in the bottom of the cylinders, unless the cylinders are such that they have fluid passages at the bottom, which might work for some masters, but bleed screws are generally at the top of the wheel cylinders.
If you dig through the literature enough, you will find that all manufacturers say to replace all rubber parts and fluid in hydraulic systems at about 24-30,000 miles or three years, for these reasons. Modern practice is to use expansion diaphragms to seal the MC so moisture laden air can't get in, because that is where the water comes from. Since we don't have diaphragm sealed cylinders, and nobody actually rebuilds the whole system every 3 years, the best you can do is bleed/flush the system every two years or so. People like Randy who live where it is very dry have an advantage.
I have found that bleeding a significant amount of fluid, but not a full change, from each bleeder every year preserves the systems quite well. And use DOT4, since it attracts and absorbs less water than DOT3. GTLMA is the original DOT4 - LMA means "Low Moisture Activity", and this is the difference between DOT3 and DOT4. I use nothing else, and find it even helps a lot on American and Japanese cars, as well as Britcars. Vehicles that turned DOT3 black in a year go several with moderate darkening on GTLMA.

Masterseries MCU is a moisture cured urethane, hence "MCU". It is said to be immune to brake fluid, and appears to be over reasonable time. I have not done long term tests. POR15 is also an MCU and should work, but Masterseries is better and half the price. MasterseriesCT.com, good guy, good stuff, somewhat funky website

FRM
FR Millmore

In my view, silicone is a red herring to all these MC issues. The problem is the MC itself. As I have said quite a few time recently, I am strongly of the opinion that Lockheed quality control during periodic batch production runs in the 90s and early 00s was not good.

Throw away your MC and get one from AP Caparo. Identical to the original except it is far better engineered to tighter tolerances. I used to have all the problems described above as many of you will recall during the past few years. It all disappeared at a stroke with the Caparo version. Bob West put me on them. He swears by them. He won't touch any other version.

Steve
Steve Gyles

Okay, Steve, you sold me. So where do us yanks get a Caparo MC?

--Jack
JM Morris

Having my master cylinder re-sleeved by White Post and will most likely change over to dot 4. I can't see having the MC fail in just a few short years. My 1600 (knock on wood) running dot 3 in a White Post re-sleeved master is going on year 22 with no problems.
David Holmes

Jack

This is the link to Caparo AP Braking - Classic: http://is.gd/iOotux

Bob West stocks them and he mails worldwide: http://www.bobwestclassiccars.co.uk/

Steve
Steve Gyles

Don't hate on me silicone lovers... I bow under pressure and as much as I love the idea of never having to repaint my heater shelf, I am making the conversion over to Dot 3 paint remover in the Master. All of the parts purveyors seem to make the same disclaimer that invalidates any warranty if you use Dot 5 in your car's system. My trusted mechanic got me with the "daughter" card and so, rather than risk a failure again of the hydraulic system with my 20 year old daughter at the wheel I am making the switch back to Dot 3. In my 1600 running Dot 3 since 1990 it has been problem free (knocking on wood)and so there you have it.
David Holmes

"Conventional" brake fluid is an excellent paint remover and the location of the master cylinder on MGA's is an issue. I get around this by stuffing and old bit of cloth under the m/c body (where it can't be seen) and replace the rubbers at the first sign of "weeping". In any case, it is futile to wait for a full-scale leak, the rubbers will have to be renewed anyway.
Barry Bahnisch

I agree with Steve. I think that silicon fluid is not necessarily the culprit in any of these system failures. Maybe as Chuck suggests there are some bad silicon fluids out there...

It would appear that there has certainly been some bad rubber?

Luckily I have not been plagued with either and I just checked my fluid and it is as clear as the day I filled it! I have a good firm brake pedal and shiny paint!
N McGurk

I had all my brake and clutch cylinders rebuilt (master cylinders sleeved) almost 10 years ago and have had no problems with silicon fluid. Yes, there is a spongy feel to the brake pedal that you have to get used to psychologically, but I once locked up all four wheels (with new radials) so it's not an issue of hydraulic ability, just hydraulic profile.
David Breneman

This thread was discussed between 25/05/2011 and 06/06/2011

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