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Which type Brake Fluid to use?


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For many years I was using standard brake fluid in my 3A unfortunately the master cylinder decided to spring a small leak from the brake pipe union which has nicely ruined the paint.

 

Once repaired I decided to refil the system with silicone fluid. Now I have had two brake switch failures and have seen on ebay a note with recon calipers that you must not use silicone.

 

Has anyone else out there had similar experiences or thoughts on this.

 

Regards

 

Dave

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Dave,

 

I use silicone brake fluid in my 3a and will do the same with my 4, once I get to the point of filling with various fuids. I have never had any problems and don't want to run the risk of ruined paintwork due to leaks.

 

There are various threads on the forum discussing the pros and cons of silicon brake fluid, in particular the issues of filling the system, replacing old mineral fluid and the problems of air in the fluid when filling the system.

 

David

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Hi Dave

 

Another thing to bear in mind is that Silicon fluid DOES have a shelf life (and an in service life) - in my experience it is not a fit and forget consumable - so protection of paint and non absorbtion of water are the primary advantages. It definately starts to go off after 2 years or so in my experience.

 

Also silicon fluids do tend to have a lower temperature rating, so if you run a car with big brakes or have a tendency to use them to stop quickly you will boil them up - I have done this on a fairly standard TR4a and now use a non silicon racing fluid (Castrol SRF) in my TR4. This is corrosive to paint, but does not boil.

 

I also believe that there are issues with moving from silicon back to normal fluid (not the other way round) in that the composition of the 2 damages rubber ....... perhaps this is what you have suffered.

 

On recon calipers there should be no problems if they have been done properly and by a reputable / skilled supplier. If there are buy from someone else ! there are lots of good suppliers for our marque.

Regards

 

Tony

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Firstly I am no expert on brake fluids. However, I have a TR4 which has been on the road now over 8 years, and I put silicone fluid in it when rebuilt and apart from topping it up twice it is the same fluid, and it has not changed colour. I was of the understanding that as it wasn't hydroscopic it didn't need changing very often, but Tony has now got me thinking/ worried, although I have noticed no deterioration in braking performance. I however have only done about 20K miles in the car since the rebuild. The 2 reasons for topping up was due to a failed brake light switch after about the first 8 months. The replacement has been fine since. I have heard of several people having to replace this switch both with conventional and silicone fluid, and I believe the fault is most likely with a poor quality switch. Two years ago (while abroad of course) the brake master cylinder developed a leak. Seals subsequently replaced and OK since. I have no idea therefore whether these 2 failures are coincidental or due to the fluid. However, personally I am sticking with silicone. I am restoring a TR2 at present and just fitted professionally rebuilt calipers (rather than drums) and the instructions with them made no mention of not using silicone. I intend to use silicone on the TR2 as well.

 

Trevor.

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I agree with you Trevor, I've used SBF for the last 20-25+ years (albeit, mainly in Land Rovers) and never had any problems to date - even without renewing all seals, just thoroughly flushing through, catching the SBF and allow to stand/separate. A minor thing does need to be noted (I'm not trying to be pedantic Trevor) mineral BF is - hygroscopic - it absorbs moisture from the air, the second the seal is broken, this means the Boiling Point gets progressively lower and closer to 100ºC. The BP of Automec SBF is quoted at >260º C or 500ºF, I don't know the standard or racing mineral fluid BP without looking on a bottle - anyone got it handy?

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I use silicon brake fluid on my TR6 with no problems and have used the same fluid on my TR3 rebuild. All I can say is thank goodness as with all the minor leaks I would have had to completely repaint the car!

I have done a fair amount of research and the only negative comment I have heard from members over here (RSA) is that the brake pedal is not as "hard" as before and certainly it is harder to bleed, particularly the clutch on a 3!

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I am at the stage now where I am thinking about filling my newly rebuilt clutch and brake systems, (recon calipers, new rear slave cylinders, new master cylinders and servo + kunifer pipes throughout) and have decided to plump for silicone fluid. Just have a few minor reservations about it's availability. Where do you all get your silicone fluid from?

 

Seems like the ball park figures are around £20-25 for a litre, is this enough to fill the system on a TR6?

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I have used SBF in my TR3A for 16 years - 89,000 miles. I never had a problem but you have to bleed the systems every 5 years.

 

I once needed to top up the reservoir and I was able to find it at a Harley-Davidson shop. If all the bikers use it where you live, it may cost you a bit more than from a TR supplier - that's why I always take some with me now.

 

Don Elliott, Original Owner, 1958 TR3A, Montreal

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  • 2 weeks later...

When I rebuilt my 3A in 1984 I filled the system with SBF using all new cupronickel pipes, recond/new cylinders. I changed it in 2004 just because .. the stuff that came out was slightly discoloured but otherwise no problems with it brake wise. One thing I have noticed though is that sometimes the clutch seals 'stick' a bit ... not sure if it's the master or the slave ... it doesn't have the strength to hold the clutch out but makes for a few inches of nil resistance at the pedal. A pump or two and it's fine. On this lubricity issue, when rebuilding the calipers in 2004 I found it impossible to get the pistons it when lubricating the seals with SBF. I struggled for hours!! I was all ready to take the seal kit & pistons back to the supplier when I thought I'd just try a bit of Girling red brake grease & glycol fluid ... pistons went in a treat!!

Here's an interesting link:-

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

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I no longer use Silicone. I found it causes the brakes to hang on.

I know lots of people who use silicone and have not suffered this problem - I'm just throwing it in the mix for your consideration so that if it happens to you you can't say you weren't warned!!

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  • 3 weeks later...

I have replaced the normal DOT3 break fluid by DOT5 (silicon) on several of my oldtimers, in order to avoid rust in the cylinders due to moisture in the DOT3.

The only problem I had (on two very different cars) was blocking of the little hole in the master cylinder: when the piston is fully back the rubber cup on the piston must free this hole, so that the liquid can circulate between the cylinder and the fluid reservoir above it.

With the silicon fluid the (old) rubber cup swelled slightly, so that the hole stayed covered even in the fully back position of the piston. After a few miles of driving the liquid expanded because of heat, but could not flow back to the reservoir! It was captured in the system! Pressure was building up and the brakes stayed on and brought the car to an undesired halt. The only cure I had on the road was to open one of the bleed screws to depressurize the system, so I could continue the road.

Replacing the cup by a new one helped.

I had brake switch failures on two systems with normal DOT3 liquid and beleave this has nothing to do with silicon fluid, but rather with long periods of non-use.

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The only problem I had (on two very different cars) was blocking of the little hole in the master cylinder: when the piston is fully back the rubber cup on the piston must free this hole, so that the liquid can circulate between the cylinder and the fluid reservoir above it.

With the silicon fluid the (old) rubber cup swelled slightly, so that the hole stayed covered even in the fully back position of the piston.

I wonder if this problem is caused by swelling of the rubber cup by SBF : if SBF would cause this amount of swelling of rubber parts, it would not be used in brake systems I suppose.

A second reason : freeing of the passage to the master cylinder reservoir is provided by a circular ondulated spring washer : this is pushed flat when the pedal is actuated and thus the passage (a little hole or bore) is shut off.

Some brake master cylinders (TR3 i.e.) have a bolt with nut to stop the return of the pushrod : if the bolt is screwed too far, the pushrod cannot return far enough to free the passage to the reservoir when the pedal is released. Or this spring washer can be missing or mounted in a wrong position.

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I believe that the MG Owners Club do not recommend silicon fluid on the MGB for that very reason- swelling of the rubber seal and blocking the little return hole.

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I believe that the MG Owners Club do not recommend silicon fluid on the MGB for that very reason- swelling of the rubber seal and blocking the little return hole.

Rhodri, I was imagining (falsly then) that the swelling of the rubber cup was causing a blocking of the hole by elongation of this cup. If it is true that the swelling is blocking the hole, than only because the cup gets stuck in his bore and doesn't move enough anymore.

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Marvmul you have a point, if the fluid does not lubricate so well then , perhaps, the seal fails to return past the hole, rather than it swelling. I have never used silicone fluid, therefore I have not personally sufferred from the problem.

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Silicone fluid is a great lubricant and the pistons in my TR3A front brake calipers are the original ones with over 171,000 miles on them and they never get stuck, even after 5 months of winter storage.

 

I've never had anything get stuck in the last 16 years and 91,000 miles with silicone fluid.

 

Don Elliott, Original Owner, 1958 TR3A, Montreal, Canada

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