BACKYARD MECHANIC: Don’t mix brake fluids

Hey there, time traveller!
This article was published 21/01/2011 (5423 days ago), so information in it may no longer be current.

Question: I just did some brake work on my 2000 Astro van. I wanted to top up the master cylinder, but when I went to purchase brake fluid, the only fluid there was synthetic. I asked if synthetic was compatible with regular brake fluid presently in the system. The answer was yes. Before I went any further, I asked another source and the reply was no.

I checked the Internet and I arrived with the answer as being yes it was OK to mix the synthetic with regular fluid. I am asking if you can enlighten me on this subject, as I still get conflicting answers from dealers, etc. on my query.

— Whitney

Answer: Most brake fluids are either glycol ether- or borate ester-based fluids. Then there is silicone-based brake fluid. They are classed by the Department of Transport based on their chemical makeup and boiling points. Almost all vehicles use either DOT 3, DOT 4 or DOT 5.1 brake fluid and it is amber in colour. I am not aware of any vehicles using the silicone-based DOT 5.0 brake fluid from the factory. This fluid is purple in colour.

The DOT 5.0 silicone fluid is compatible with all brake-system components, just like the regular brake fluids, but it is not recommended by most auto manufacturers because it has a different viscosity and characteristics that could affect the efficiency of ABS and traction control systems. One of these characteristics is that it tends to foam when rapidly pulsed, as would happen when the ABS or traction control systems are active.

Although both regular and silicone brake fluids are compatible with brake systems, they should NOT be mixed. This information comes right from the brake fluid manufacturers. There are advantages and disadvantages of using silicone brake fluid, including the fact that it doesn’t damage paint if spilled on a fender and it doesn’t absorb moisture from the air. For this reason, it is often used in older special-interest vehicles without ABS when they are being restored with rebuilt brake parts. However, if there is moisture in the brake system already, the silicone fluid will let it just sit on the bottom of the chamber and it can rust parts even quicker than regular brake fluids.

Since you have combined both types of brake fluid, you should flush the system completely to replace the old fluid with new fluid. You can do this by bleeding the brake system just as if you were trying to get air out of the system. It may take two to three litres of brake fluid to completely flush all the contaminated fluid and this is a lot of pedal pumping, so you may want to take it to a repair shop that has a pressure bleeder. This tool can make the task much easier.

Q. I have a 2006 Chev HHR with a 2.2L engine, and 59,000 km. The “service engine” light is intermittently coming on. A scan tool shows the code as P0300, with no specific cylinder indicated. When this happens, there is no other indication such as poor driveability. The car runs perfectly. The problem first appeared about a year ago, then went away for almost a year and then has returned recently. Can you suggest what I should do, and in what order, to try to fix this problem?

— Ray

A. The P300 code is a general engine misfire code. It often occurs when the engine is first started and is warming up. It can be caused by a poor spark, contaminated fuel injectors that create a lean fuel mixture, or even a vacuum leak. The most common cause is a spark plug that misfires intermittently. You may not sense the spark plug misfiring but the engine computer does. It monitors crankshaft acceleration and deceleration during every power pulse to determine if a cylinder has fired.

I would start by installing new spark plugs. The next step is to install new plug wires, although in your situation, the vehicle doesn’t have many miles on it. A technician can monitor misfires with a scan tool and that can help locate cylinders that are misfiring, but I would start the repair by replacing spark plugs.

 

Jim Kerr is an experienced mechanic, instructor and member of the Automobile Journalists’ Association of Canada.

kerr.jim@sasktel.net