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MG MGB Technical - Silicone Switchover

I recently replaced the master cylinder in my 72 B. The system had been filled with silicone by the Previous Owner, and when I removed the old cylinder I had opened each of the 4 bleeders, and pumped the pedal a few times, emptying most of the system. I am now at decision time, since I haven't refilled the system. I replaced all hoses last year, as well as one of the rear wheel cylinders. I had a good pedal before I replaced the master, just had an annoying leak that dripped down on the pedal causing me to refill the master every month or so. If I decide to switch over to DOT 4 (Castrol LMA)can I just pour it in the new master and bleed out the system. I don't want't to flush with solvents, fearing this would do more harm than just refilling with silicone. I really don't have a problem with silicone, but in talking with various master cylinder rebuilders (3) they all recommended DOT 4 fluid over Silicone. I am at the point of decision, if I can easily refill with DOT 4 at this point, I might do that, otherwise if I have to go through an elaborate purge of the system, I might as well pour in silicone fluid, which costs 3 times as much but won't have to be changed out as often.
Joe

Joe;
From everything I've read, you cannot mix the silcone with DOT 3 or 4 fluids, even a little bit.
I would stay with the silicone. I have had it in my big Healey for 5 years now without any signs of problems, and did a TR6 3 years ago, still without problems. To change to DOT4 would require a flush, however I am not sure what you would use for a flushing agent.
Pete Haburt

Joe and Pete,

There's a wide range of opinions on this. Some folks say you can't mix even a little bit, and others say you can freely mix and match. Still others say you can mix and match, but you won't get as good a result as either one by itself.

I'm no expert by any means, but I can tell you my own experience. When I replaced a leaky MC several years ago, not knowing any better, I decided to swap from DOT4 to silicon so I wouldn't have a repeat of the paint damage if the MC leaked again. All I did was pump silicon through the system until it showed no visible traces of the old fluid, and then drove it for 8 years with no problems at all. I didn't flush anything through the system to clean it before putting in the silicon.

After 8 years, the only part of the system that was replaced when I did the swap - the MC - began to leak, so I rebuilt it. None of the rubber parts, seals, etc, in the rest of the system gave any trouble, even though most folks will tell you that they are not compatible with silcon.

I'm of the opinion that if you flush it with the new fluid until all visible traces of the old are gone, you should have no trouble.

I agree with Pete, though. Stick with the silicon. In spite of what you may hear to the contrary, I had no problem at all with a "soft" pedal using silicon, and, evidently, neither have you two. I wouldn't use anything else.

YMMV.
Dan Masters

Hi Dan,

I can't add much more, other than say that I share Dan's experience with the silicon fluid. I have run it for over 10 years without problems in my MGA. Just a word of caution though, you definitely cannot and should not mix the fluids. If you want to convince yourself, take a bit of the silicone fluid and mix some regular in it. You'll see a bunch of greay sludge globs forming and they do not dissolve at all...it's clear why: silicone does not bind with water, the other does. So it's just like mixing oil and vinegar.
Having said that, I too flushed the system the same way Dan described it and it still works.

Conclusion? Silicone does not attack my car and it works great. Ah there is one more thing: I had several (2 i think) break stop light switches give in, I don't know if it's related, but I remember a while back folks talked about that as a silicone draw-back

Good luck.
corneliu

I’m another who would recommend staying with silicon(e).

I have used it in my B GT V8 for over fifteen years, and for lesser periods of time in my MGA & MG RV8.

In every case I have been delighted, and have had no problems.

I have only once changed the fluid in the B GT V8 about six years ago when I fitted four pot calipers.

Silicon(e) - is this a UK / US spelling thing? <G>

Nigel
Nigel Steward

Not everyone has had a good experience with Silicone fluid, and I choose to use Valvoline Syntec DOT 4 Brake fluid in my 68 GT as it simply works for me. I had a bad experience with GE's silicone fluid back in the late 80's (jillions of tiny bubbles that NEVER went away!). However, my recently acquired 73 B roadster has silicone fluid in it, and frankly there is no perceptible difference in performance between these fluids.

Also, I have since seen enough other cars with MODERN, CURRENT formulations of silicone fluid in them that have hard pedals and no bubbles, and which work every bit as well as their glycol counterparts. My opinion at this point is that like many other things we use, silicone brake fluids have evolved from what they might have been originally, into a much better product.

Many of the comments and concerns about using silicone fluids we hear today may well be the persistence of old wives tales, poor experiences from the early years of silicone fluid use, and just plain ignorance of how to use ANY brake fluid (like how to properly bleed systems, etc). I can't say much, I certainly wrote some negative comments from my experiences until I started to rethink this topic in the light of fresh experiences of my own and from others. FWIW
Bob Muenchausen

Bob

Somehat off topic I admit, but I always read your sage posts with interest.

It is so nice to hear that others who rethink from time to time in the light of fresh experiences of their own and from others.

I find the information on this Board invaluable. Thank-you everyone.

Seasonal Greetings

Nigel
Nigel Steward

I have always thought of silicone fluid as having many advantages, but have never been game to make the change. There is always some disadvantage to making the change every time I research the issue.

This thread is enlightening, I didn't realise the silicone products had been improved. Perhaps you could answer 2 questions for me please.

Does the pedal feel the same? I heard that it has a slightly course (rough) feel when applying the brakes.

Have the lubricating qualities been improved. Perhaps this is related to the above question but I understood there was potential for greater wear in cylinders.

cheers
Ian G Buckley

Ian,
my experience is that if bled properly, they do indeed seem to feel the same. This is with currently available silicone and glycol DOT 4 fluids. Lubricity seems to have been improved as there is none of the "miniscule chattering" felt back through the pedal as some folks have reported in the past. What exactly has been done to silicone fluid, I have no idea, but it certainly seems much more like the glycol fluids in all important aspects than it did just 13 yrs ago.
Bob Muenchausen

Dan,

You mentioned that you pump the silicone through the system until no trace of the old fluid is seen. Can you elaborate a bit on the process. I am considering switching to silicone, but am confused as to whether this can simply be done by adding the silicone to the master cyclinder resevoir as you bleed the system. To me that would mean the two types would mix, at least to start with. I was under the impression the brake system would have to be purged somehow before adding the silicone.

Thanks

Mike
Mike

Mike,

All I did was put the silicone in the MC, as you suggested, and bled the brakes at all four corners until I could see no more of the DOT 4 fluid.

Some people claim that you have to purge the system of all the old fluid, flushing with alcohol before putting in the silcone, but I don't believe it's necessary at all. The two fluids and incompatible in the same way as gas and oil are incompatible, that is, if left to stand the two fluids will separate. In my opinion, if each fluid works alone, and they separate in the system, then the two should work together as well. The more of the DOT4 you leave in the system, the more DOT4 type problems you'll have, such as moisture retention, paint removal, etc, but if you purge the system until you have no visible signs of the DOT4, you'll have so little left of it that it shouldn't matter.

Of course, I am NOT an expert on brakes by any stretch of the imagination, so take all that I say with a large grain of salt. All I do know for sure is that I had no trouble of any kind doing it the way I did.

I have some of each type in the garage, so I'm going out in a few minutes and mix a little of each and see what happens. I'll let you know if anything worth reporting on occurs.
Dan Masters

Thanks BOB M

This chattering was the main reason I was put off making the change when I rebuilt my brakes about 2 years ago.

I also understand that silicone is more 'searching'than glycol based products and more likely to weep if cylinders and seals are even slightly imperfect.

However I am being swayed!!

cheers
Ian G Buckley

Dan & Corneliu;
Thanks for sharing your experience. You're the first that I am personally aware of that have actually done the switch without flushing or rebuilding everything, and have a history of success. I have wanted to change my B over for some time, just didn't want to go thru rebuild/flush to do it. As I will be rebuilding the M/C for brakes this winter, maybe I'll take the plunge.
Pete Haburt

I have extensive fleet and personal experience with silocone (DOT 5) brake fluids and changovers . I have an old V.W. that I changed over 15 years or so ago and it is going strong , never needs topping up and the fluid is as clean as the day I installed it during a major hydraulic overhaul .

We used to change over the new police cars by flushing them , about one gallon of DOT 5 per vehicle , this worked well and we did carefull checks every 20,000 miles and _never_ found any noticeable wear/seepage with the silicone , untill the shop mechanics became lazy and only bled the brakes untill they saw the (red) silicone fluid coming out - almost immediately we had numerous crashes caused by catastrophic brake failure due to residual DOT 4 fluid contaminating the silicone fluid , what happens is : it turns to _gell_ and attacks the rubber seals .

You are supposed to flush the old fluid out with alcohol but I find just flushing really really well with new silicone works O.K. .

I also have some customers in Canada and Minnesota who discovered the water that silicone fluid doesn't absorb settled in one place and _froze_ whilst driving in the interstate in winter causing brake failure .

Silicone also tends to get more tiny hard to bleed bubbles and when you get frustrated
and/or don't know how to correctly bleed the system and kick the pedal , they stay behind causing the much mentioned soft pedal . in reality silicone fluid has the same firm pedal feeling when properly bled . many folks bleed @ sea level then drive to Colorado and discover no brakes ! this is due to the tiny bubbles expanding .
re-bleed whilst up there and no further problems .

It is good stuff but must be carefully installed or problems will result .

I'd just re-fil with silicone as that is what's in ther now , simpler that way .

BTW : you are supposed to paint the fluid resivoir _red_ to indicate the car uses DOT 5 silicone fluid .

-Nate
Nate

Thanks to everyone for their advice. I feel a lot better about refilling with silicone fluid. I purposely didn't refill the system last weekend in order to seek the wise counsel of this thread. Wasn't in any rush anyway, the highs around here have been in the 20s (F.) for the past few days, and the B is nice and snug. Good thing the majority went along with refilling with silicone, because I had already bought the stuff, but also have a bottle of LMA for the clutch.
Joe

Any suggestions as to which brand of silicone fluid to use? Is one better than the other? Nate mentioned silicone fluid being red. Years ago when I rebuilt my MG, the fluid was clear.
Mike D

Thanks Nate for the first hand and extensive experience with this stuff. Most of us only have our own experience and our own research to go by, and even with the internet, it is sometimes inadequete. To me, first hand experience is a better guide than what is supposed to be as read in technical articles. Thanks again.
Bob Muenchausen

Not scientific by any means, but here's what I found:

Last week, I replaced a wheel cylinder in my pickup truck, but I was too lazy to empty the jar from the bleeding. It's been sitting on my workbench, and a nasty sludge has formed in the DOT4 fluid in the jar. If I shake it well, the sludge dissipates into the fluid, but then settles back out after it sits awhile. The fluid had been replaced about two years ago, DOT4 for DOT4, when I had the MC replaced.

Today, I mixed a little fresh DOT4 with a little fresh DOT5 in a jar and shook it real well. Now, several hours later, the two fluids have separated, but each layer is perfectly clear.

There's a lesson here, but I'm not sure what it is. Perhaps mixing 4 & 5 is not a problem, but mixing 4, 5, & the sludge is the problem? Maybe the sludge isn't a problem with DOT4, but becomes a problem when the DOT5 is introduced? I guess I'll dump the old DOT4 with the sludge into the 4/5 mix and see what happens. Does anyone have a system that has never had anything but DOT5 in it? Wonder what that looks like after a few years of use. What the heck is the sludge and how did it get there?
Dan Masters

Is there a reason, other than the usual objections to silicone fluid, that it shouldn't be used in the clutch system?
Joe Ullman

Dan,
I've been running DOT5 in my '69 B for about 5 years now. It has never had DOT4 in it since the system was completely overhauled. I replace the fluid every few years or so (it's due right now, in fact).

I've found that new, fresh DOT5 I use is dyed purple and over time the dye fades out to clear. Additionally, the fluid seems to darken, tho not as much as DOT4.

Interestingly, I recently opened an old bottle of DOT5 to top off my system, and the dye had faded out whilst on the shelf...

FWIW, I use DOT5 in the brakes and the clutch. I've never had a hydraulic problem with this car. Not sure if I should credit the DOT5 for that.... I've never seen any elephants around the car either, so maybe the DOT5 is keeping the elephants away as well...
Rob Edwards

I began using silicone fluid in the brakes 22 years ago. At that time it was because I was given about 3/4 gal of some industrial fluid, don't know the name but it was blue. It never changed color and never deteriorated, and in fact I still have a little of it in a bleed jar. That's not to say the brake/clutch components did not deteriorate, as they did wear and eventually begin leaking. Having recently installed a HTOB and being alerted to the wear issue I did a quick "feel" comparison of the glycol fluid and a modern silicone fluid. The glycol based fluid felt noticeably slipperier. Based on this test I went with the glycol based fluid in the HTOB system. BTW, the purple fluid, such as NAPA sells does indeed deteriorate and is in my opinion an inferior product. It evidently breaks down, and the sludge that results appears to attack the metal parts. Also, the sludge that Dan mentioned in the Dot4 fluid is apparently caused when the glycol, which is hygroscopic and actively seeks water molecules, combines with water.

I cannot provide a source for high quality silicone fluid such as the blue fluid at this time, since the label fell off the bottle many years ago, but it performed well, except for the wear issue. I suspect component wear was a good bit faster than with glycol but as I did not know this was an issue, I have no baseline for comparison. If anyone has a new system on both sides and uses silicone in the brakes and glycol in the clutch, please track any wear issues and get back to us on that.

Jim
Jim Blackwood

May I humbly direct your attention to www.buckeyetrimphs.org. Nelson Riedel has an excellent article on silicone brake fluid. Go to the technical section.
Berry

Hi Joe,
Regarding a reason to switch away from silcone fluid, I did when I bled out my clutch this summer. I was having problems getting the air out because A) its an MGB clutch and B) the bubble problems w/ silicone and yes, in case anyones wondering, I tried Paul Hunt's method. Anyway I did eventually end up getting a good clutch pedal but if I'd done all that bleeding with silicone, I'd have been out a hell of a lot of money! Silicone costs about 5x what valvoline syntec did and its much easier to find. So especially if your flushing out old or a different type of fluid, think of the money you'll be spending on fluid.
Wade Keene

I have been running silicone in one my cars for 20+ years and never had a problem with it although it was harder to bleed, but as mentioned before the fluids have improved. The real problem is rusty brake lines from years of PO neglect. When I installed all new lines in my car I never had a problem. The fluid is just as clean as the day I put it in. And it doesn't lift paint!
datsooonmike

This thread was discussed between 09/12/2002 and 13/12/2002

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