DOT 5 Silicone-Based Brake Fluid

Hi there,

My Austin-Healey 3000 Mark II (1962) is going back on the road after a year of non-use (shame on me!) and I've recently had to bleed the clutch with DOT 4 hydraulic fluid.

Last evening I found a little laminated label in the glove box stating "DOT 5 Silicone-Based Brake Fluid Only." I can only assume it applies to this car and may have fallen off the reservoir and was never reattached.

a. Is there an easy way of confirming this is the fluid in my BJ8?

b. Does this mean I need to bleed the whole system of fluid and replace with silicone (clutch and/or brakes - I haven't driven it yet)?

c. Any tips on this process?

Many thanks.

conor

Re: Dot5 Silicone Based Brake Fluid

DOT 5 is silicone based and is translucent purple in color. Other fluids are translucent amber. DOT 5 fluid should not be mixed with glycol-based fluids such as DOT 4 OR 5. That can gum-up the works. DOT 5 doesn't absorb water, so for vehicles that sit, like most of our Healeys do during rainy or cold seasons, it can make sense. It is often not compatible with anti-lock brake systems that are often calibrated for DOT glycol based fluid compressibility. You can change from glycol to silicone if you are thorough about flushing the system of one before you fill with the other, but you will read a lot about seal compatibility, leaks etc. if you do this. No comment on all of the arguments pro or con from me. I run DOT 5 in my BN2, but I started with all new brake seals and hoses when I did my rebuild. I had the normal seeps at first, but once I eliminated them it has been years with no problems. In fact it is almost scary that I never have had to add fluid. Brakes are excellent as far as performance goes, and I have a bit of added security by having put in front disc brakes. Hope this helps. All this info is on the net along with opinions that run the gamut.

This can be a confusing topic, with DOT 3, DOT 4, DOT 5, DOT 5.1 all on the market. All DOT 5 also is not silicone based, as I found out recently. My '61 BT7 has silicone based fluid (label on the reservoir), so I wanted to buy some of that. It was very expensive locally, so I ordered a bottle on line that appeared to be a good deal. What arrived in the mail was DOT 5.1, which is not silicone based. Fortunately, I noticed that before topping up the reservoir. I then bought some of the correct stuff locally. Unless you have a good reason for using silicone based brake fluid, the high price itself provides a good reason not to use it.

Jon's answer is spot on. Do an internet search on 'how to tell if there is silicone in my brake fluid' or similar (you aren't the first person to ask these questions ). A couple ideas include putting the suspect fluid on a painted surface--glycol fluid will bubble or 'melt' the paint--or mix with some water (glycol fluid is miscible in water, silicone will separate out). If, as you suspect, the fluid in your hydraulic system is (mostly) silicone or 'DoT 5'--make sure the label didn't say 'DoT 5.1,' which is 'synthetic' but, IIRC, glycol-based--then I think you should drain your system and fill/bleed with DoT 5. Your fluid probably got mixed somewhat when you added the DoT 4.

If you wanted to 'make the switch' to/from Dot 3/4 to Dot 5, or vice versa, a thorough flush with denatured alcohol and lots of air may be sufficient, but total replacement of rubber components along with the flush, as Jon did, is your best bet. FWIW, I have DoT 4 in my BN2 and DoT 5 in my BJ8, and only the DoT 4 has given me problems.

Conor,
This is one of the topics that's been discussed at great length. One might want to check out the archives.

Check this one out:
http://www.britishcarforum.com/bcf/...licone-brake-fluid-conversion-existing-system

Many thanks all. Looks like I will be doing some flushing! Thankfully the system/car has not been used since adding the suspect Dot4 fluid so I believe a thorough flush of the clutch system only will suffice as the brake pedals haven't been touched. I'll double check the fluid (thank you Jon and Bob) but its hard to believe the Dot5 Silicone Fluid Only warning label is a fake. Lucky I found it by complete fluke before i drove the car.
Conor

1962 BJ8

Conor - For flushing, suggest removing all the fluid from the reservoirs first. Then all you have to deal with is the fluid in the lines.

Then when you drain the lines, you can see whether it's silicone or glycol-based in the lines without mixing from the reservoir contents.

I like to use plastic syringes for both removal from reservoir, and for pulling fluid out through the system.

Bleeding order is counter-intuitive: LR, RR, LF, RF.

Hi All,

Conor, I am sorry to say that your issue has flagged a potential issue I must address ... just in case. With the virus attacking many by surprise, I to indicate my use of Silicone brake fluid with an appropriate period label on the brake reservoir of my Healey or TR7. I am interested to know what type of label others have used?

By the way, Conor, yes Silicone (Type 5) is quite expensive but it doesn't absorb moisture that can deteriorate your brake system component and will last in good stead far longer without attention. Additionally, as others have indicated, it will not ruin your paint if, for some reason, it comes in contact. However, I have a polished brake reservoir cap because because paint would not adhere to it after scraping and ruining the finish.

Ray(64BJ8P1)

I bought this label from Moss years ago and placed it inside the hood. It stands out well against the dark background.



This last year, after ~20 years, I replaced the clutch master cylinder with one that connects to the remote reservoir. My BN4 originally came with an integral reservoir. I went to the remote reservoir because the air filter was interfering with the integral reservoirs. Since I was doing that, I replaced the rubber in the slave cylinder. The clutch hydraulics were in perfect shape after all that time because of the DOT 5 fluid.

Available in the UK...

Thanks John/Steve, I ordered 2 pairs, one for the Healey and one for the TR7. I was intending to apply them on the reservoirs but I am wondering if they will adhere as I must have spilled some DOT 5 on the sides and if paint wouldn't adhere to the cover, the sticker glue may not.

Thanks again,
Ray(64BJ8P1)