Brake Fluid Level

Brake Fluid Level

Problem: Brake drag immediately after brake job is performed

Cause: While there are many causes of brake drag there is one cause that frequently takes place immediately after having a brake job performed. If the master cylinder is overfilled it will not allow enough room for the brake fluid to expand due to heat expansion. The expanding fluid will cause the calipers to apply creating a residual drag.

The same thing can result from pushing the caliper pistons back in for a pad replacement job and never checking the fluid level after.

Solution: Only fill a master cylinder to the correct level. For more information on specific systems see below.

More Info: This issue of brake fluid level usually does not generate much interest or concern among most technicians. It is normally considered a “no-brainer”. I would agree in the majority of vehicles serviced this would be the case but I learned long ago there are exceptions to almost every rule. Achieving the proper fluid level and the process of checking the fluid level in some vehicles is much more critical than others.

Screen Shot 2016-07-19 at 9.00.15 AM

It is generally accepted that if the fluid level is low and there is no leak present it should be “topped” off. Everyone knows if there is no leak the low fluid level is being caused by disc brake pad wear. So the question then becomes do I add some or not? If you pay attention to what the car manufacturers are saying you know that many of them are taking the position that you should never add fluid. GM TSB 00-05-22-004, dated 5/2000 applies to many of its vehicles and states that once the friction is replaced the fluid level will come back to its original position. They even state it is unnecessary to remove the cap when checking fluid level on transparent plastic reservoirs.

I disagree with this position mainly for one reason. If the fluid is topped off before the friction is in need of replacement then the system’s “reserve” is at its maximum level. If there is a hydraulic failure then the driver will have the maximum available reserve if needed. Of course with some types of hydraulic failures it will make no difference in what the reserve is but I think you can see my point.

Checking the fluid level on the majority of vehicles is straightforward. There are a couple of exceptions that should be noted or one of two mistakes could be made. The system could either be left at a low level or it could be overfilled. Both of these involve vehicles that utilize integral style ABS systems. The Teves II systems used on certain Ford and Lincoln vehicles instruct you to turn the key on and let the pump run until it shuts off before checking the fluid level while late 80’s GM vehicles with the Delco III system have you pump the pedal 40 times before you check the fluid level. You can see the potential for a problem right? If I check a Teves II system after pumping the pedal down the fluid level will appear to be overfilled while if I check the Delco III system with the accumulator charged it will appear under filled. Always read the cap if you are not sure of the procedure. If you can’t read it then check your All Data or Mitchell system.

That leads us to the issue of how can someone mess up when topping off a system? It has to do with a normal process that takes place in every brake system that could be adversely affected if the fluid level is too high. The brake fluid in a hydraulic system expands after the brakes have reached operating temperature. The excess fluid caused by the expansion is allowed back into the reservoir through the vent port preventing it from causing wheel drag (See Figure 68.1). What happens if the reservoir is overfilled and there is no room for the excess fluid? It would be the same result that occurs when the vent port is either plugged or covered by the primary cup seal – brake drag.

Screen Shot 2016-07-19 at 8.53.04 AM Figure 68.1

Some reservoir types are more prone to this then others. The GM “open faced” plastic reservoir shown in Figure 68.2 is the most common example of this in my experience. The first thing that sets the stage for a problem is there really is no “Max” level mark. These masters use what is shown in Figure 68.3. If the fluid level is brought up to the top of the oval the system will be overfilled. To make matters worse the reservoir cap and gasket used on these systems will actually pressurize the fluid as the cap is put on if the fluid level is too high! I don’t know how many of these GM vehicles I have seen over the years come into the shop with over heating front brakes with the only thing being wrong is too much fluid in the system. You look under the hood and the reservoir cap is actually bowed up due to the pressure! To make matters worse many of these cases have been on brand new brake jobs. So the moral of the story is don’t always take the basics for granted because it can turn around and bite you in the behind.

Screen Shot 2016-07-19 at 8.58.36 AM Figure 68.2 Screen Shot 2016-07-19 at 8.58.53 AM Figure 68.3

To learn how to further test the effectiveness and safety of your brake fluid visit the link shown below.  Once you have arrived at the website, click on the video featuring famous car enthusiast Stacey David to learn about brake fluid contamination and the proper measurements needed to know if your brake system is safe to operate.

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