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MG MGB Technical - Silicone back to Dot 4

I would like to replace the Silicon fluid in my car with Castrol LMA or similar. The plan is to open the filler cap and all 4 bleed screws and just let as much fluid drain as possible. Then to put the Castrol in and starting at the farthest wheel, open the bleed screw and using the slow (have a beer) gravity method let it flow until new fluid is coming out and then moving to the next wheel while at the same time not leting the fluid get low in the reservoir.
Will this be work or should I 1st use denatured alcohol and flush using the same method.
Bob R.

bob, i wish it were this easy. i initally used the silicone fuid (DOT 5) for a few years and decided to change back to DOT 3 or 4. all went well initally, but i developed a seep in the Brake M/C. research revealed silicone fluid swells the rubber? seals slightly more than the Dot 3-4 and is more than likely to leak. you can change from DOT 3-4 to silicone w/o problems, but the change back will usually be problematic. i flushed the entire brake system with rubbing alcohol (gravity flow) and then bled DOT 3 several times prior to mashing any pedals. the seep (leak) i have now can hopefully be resolved with a rebuild kit. the M/C bore is in excellent condition. 1978 "B"
your calipers, wheel cyl. and M/C may require kits.
Merry Xmas and happy holidays
kelly stevenson

Why the switch?
Steve Simmons

I would personally not make the switch, unless I were planning to race the car. I have been very satisfied with DOT 5 silicone fluid in our two MGs (nearly 30 years in the TD and over 10 years in the MGB and not a bit of corrosion in either brake system). That said, if youre are gong to do it, keep in mind there is a different absorbtion/desorbtion rate between the two different types of fluid in the rubber/neoprene seals used in the brake systems. for that reason, itis highly recommended that the system be completely flushed of all remenants of the old fluid and the changing of ALL rubber/neoprene software in the entire system before adding the new fluid. I suspect that this is what is causing the problem in Kelly's system. Cheers - Dave
David DuBois

? Why the switch: I have had Silicone in both the clutch and brake system for quite a few years. Recently I had to replace the Clutch master cylinder. Since I thought maybe the Silicone may have caused the problem, I changed fluid at that time. My brake system works fine but just thought I would change to keep both systems the same. With what you all have replied so far, I am leaning to just staying with the Silicone....
Bob R.

Yeah me too, 15 years in my GT with no problems.
Sam Sullivan

I've had DOT5 in my clutch and brake system for 24 years and 375,000 miles in my GT with no problems.
John Perkins

I don’t have any problems to report either after 5 years in my’66. I do have a question for the long term users…have you every replaced the fluid or just kept the same silicone for all these years? Thanks, -J
jjralston

JJ - I can give you a report on leaving the silicone fluid in for long period of time. In both of our MG, I have not flushed the system since installing the fluid initially. About 5 years ago we had a discussion at a meeting of our local MG T register and one of hte topics was how often should brake systems be flushed and refilled. I realized at that point I had never even checked the fluid in our TD since installing it some 20 - 25 years earlier. The next day I crawled into the foot well of the TD, removed the covers over the master cylinder, then took the cap off of the M/C and shone a flashlight down into the fill neck and was astonished to see no fluid in the reservoir - I was looking at the bottom of the reservoir and there was nothing above it. Now I had not been experiencing any braking problems so I couldn't figure out how I could be completely out of fluid and still have well functioning brakes. Finally the penny dropped and I stuck my fnger into the filler neck and there, a half inch below the neck as it should be was silicone fluid that was a crystal clear as the day I put it in. The stuff doen't draw moisture as DOT 3 or 4 does, so there is no corrosion in the system to cause the fluid to become cloudy. I quit worryin about the fluid at that point (although I do check it a bit more regularly since then). Cheers - Dave

PS. The silicone fluid in the MGB is the same way.
David DuBois

Ditto Dave's comment. I have had DOT 5 in my TD for nearly 25 years. My 67 B now has it in both the brakes and clutch. Only difference is mine is purple. Most troublefree they have ever been.
BEC Cunha

I'm leaning more toward using silicon fluid in my refurbished brake system. The MC, wheel cylinders, and calipers are rebuilt, and the lines are all new. When I installed the new rubber seals and parts, I used dot 4 for assembly. This little bit shouldn't mess me up when adding the silicon fluid, will it? Joe

Joe Ullman

I have just filled my system with Silicon. The system was all new except for the Caliper seals.
The only problem I found was when initially pumping ( when bleeding) both brake and clutch pedals I could not get rid of the mushy feeling. After leaving it overnight the pedal had hardened up completely.
Both systems have been working faultlessly.
I would NEVER use Dot 4 again mainly because I am so ham fisted that most of the paint on the calipers and around the master cylinders gets damaged.
Now I can spray it around all I like!!
Dave
D M Tetlow

Joe

Typically if you are not rebuilding the entire system, the recommendation is to drain all the old fluid and flush with alcohol then blow some air through the system to dry them.

I would not feel that little DOT 4 would affect the system.

I am like you Dave. No matter how well I try not to, I am always getting brake fluid on painted surfaces (backing plates, around the brake clutch box, etc) with the silicon no paint bubbling.
BEC Cunha

I swapped from Dot 4 to Dot 5 and was not so lucky. I flushed the entire system with alcohol, used at least three bottles. Followed it with Dot 5 and bleed the first 2 or 3 onces from each cylinder and hoped that would be good enough. A few months later, the new M/C went out.There was a small amount of black particles in the bottom of the resivour and I got lots more from the bleeds. The residue was soft like dust and would smear out on my fingertip. No idea what it was but I have to suspect it was rubber from the m/c seals. BTW, prior to the swap, everything in the brake system was new but the hardpipes, but they were flushed with air until I got nothing out but dry air.
gerry masterman

Interesting thread. I too have seriously considered changing over to a silicone based brake fluid so the paint didn't lift after and accidental spill.
Since this thread started I have checking into D0t 5.

Check this link out.

http://www.xs11.com/tips/maintenance/maint1.shtml


FWIW

Bruce
Bruce Mills

I've been a strong advocate for silicon fluid for a number of years now and have it in my GT rebuilt 1996 and in my roadster rebuilt 2000. However I have a fault in the braking system of my roadster in that as I slow down the brakes do not gradually release and so the car usually jerks to rest. The brakes then usually remain firmly on for a few further seconds. Now there are a few theories about this one the most common one being that it is the small contact piston in the servo and that a bubble of air gets trapped behind its tapered point. Putting a dod of silicon grease into the hole and refitting the valve then bleeding out of one brake will usually rectify the problem but I have found that it persists to an annoying level. The only thing that I could think of was that the lubricity of the silicon fluid was much poorer than that of DOT 4 so I duly changed the fluid back to DOT4 and bingo problem solved.
Iain MacKintosh

This thread was discussed between 03/12/2006 and 11/12/2006

MG MGB Technical index

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