Tech Topics

The New England M.G. ‘T’ Register has a long history of providing its members with advice and technical expertise on the restoration and maintenance of our special cars. We present some of our favorite technical articles and tips here for members and visitors alike.

Taking Care Of Your Brakes

What is the difference between a "brake job" and brake maintenance?

The majority of us who drive our M.G.s and our regular cars figure we'll need a brake job about every two years, depending on our style of driving and the distance we travel. Do you ever think about brake maintenance between brake jobs?

A properly operating brake system with an extra margin of effectiveness for safety is the most important safety system on your car. I am a firm believer in regular flush and refill of fluid in a brake system as a part of brake system maintenance. All brake systems accumulate sludge over a period of time.
Unscrew the filler cap from your master cylinder and remove a sample with an eyedropper. If your fluid has been in your car's system for any length of time, the fluid will be dark and tiny black particles will be suspended in the liquid. Flushing the system can remove this sludge; but, once you have disturbed it, you want to be sure to force it all out of the system. Bleed the system until the outgoing fluid is as clear as the new fluid.Removing built up sludge isn't the only reason for a regular change of brake fluid. The fluid itself can lose its effectiveness over a period of time.

The specifications of all automotive brake fluids are defined by SAE (Society of Automotive Engineers) standard J 1703 and Federal Motor Vehicle Safety Standard (FMVSS) I 16. Fluids classified according to FMVSS 1 16 are assigned Dept. of Transportation (DOT) numbers - DOT 3,4, and 5. The higher the DOT number, the more stringent the specifications for the fluid. The most apparent difference among the grades is the minimum boiling point. I use DOT 4 fluid in my M.G. TC and most European manufacturers call for DOT 4 in their cars although DOT 3 is still used in all our domestic cars and can be safely used in your M.G. DOT 3 and DOT 4 fluids sold on the shelf today are polyalkylene-glycol-ether mixtures, called "polyglycol" for short. DOT 5 fluids are all silicone-based because only silicone fluid can meet the DOT 5 specs. No vehicle manufacturer recommends DOT 5 for use in its brake systems and White Post Restorations, who re-sleeves a lot of our T-Type hydraulic cylinders, also does not recommend its use.Polyglycol fluids are hygroscopic, which means they absorb water. Brake systems aren't completely sealed and some exposure to air and thus water vapor is inevitable so all of our systems have a small amount of water in the lines. An SAE field evaluation program number R11 showed that the average year-old car has about 2% water in its brake fluid. Even this small amount of water is enough to lower the boiling point of DOT 3 fluid from 401 degrees F to less than 320 degrees F. For those of you with some extra cash a brake fluid tester is now on the market that actually boils a small amount of fluid and displays the boiling point on an LCD in less than a minute. Shark Industries makes this tester and recommends a brake fluid flush if the fluid is beyond 2% water contamination.

Silicone DOT 5 fluid does not absorb water, but it has some other characteristics that some folks find undesirable. Silicone fluid compresses slightly under pressure, which can cause a slightly spongy brake pedal feel. Silicone fluid also attracts air more than a polyglycol fluid does, which makes bleeding all that much harder. I have had several first hand experiences installing a DOT 5 system in a customer's car. On a difficult system to bleed, such as on a Jaguar E-Type, it can be a time consuming and frustrating experience to gain a respectable pedal.

Silicone fluid also has a lower specific gravity than a polyglycol fluid. If the two types are mixed, they will not blend; the silicone fluid separates and floats on top of the other fluid. That means if you are intent on having DOT 5 silicone in your car, all the DOT 3 or DOT 4 must be flushed from the system. The best time to convert to DOT 5 is when all your components are renewed.

How often should you flush your system? If you have never done it and have owned your car over a year and have no records from the previous owner, now is as good a time as any. I inspect my fluid annually and flush it every two years in all my vehicles. It may be a bit of overkill but it gives me peace of mind and an excuse to get out in the shop with the TC! If you don't already know, I recommend checking to make sure of the type of fluid that is in your system. Put an eyedropper full in a small amount of DOT 4 and see if the fluids stay together or separate. If they separate, you have silicone DOT 5 in your system. Remember to start your bleeding with the wheel cylinder farthest from the master cylinder. Left hand drive cars - start in the right rear. Right hand drive cars - start in the left rear. Drive Safe!

by Don Lawson #5687



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Disclaimer: All Tech Tips presented here are copyright of The New England M.G. ‘T’ Register. Be advised that every effort was made to verify the validity of each tip. But as in any free advice, it’s up to the user to judge the usefulness of each tip presented here.


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