A vacuum bleeder sucks fluid out of the system through the bleeder valve at each caliper/wheel cylinder. It’s powered by shop air and uses a flow eductor to create a vacuum on the fluid collector bottle (see sidebar, next page). The only real trick to using this tool is to check the fluid reservoir often. If all the fluid runs out, air is sucked into the system and you have to start all over again. A hand-operated vacuum pump with a fluid collection bottle can also be used to bleed or flush a brake system. Those tools are inexpensive and easy to use, but the job takes more time.
One company offers a unique tool that fills the brake system from the bottom up. Phoenix Systems makes several versions of a hand-operated pump that “injects” brake fluid into the caliper or wheel cylinder through the bleeder. This offers several options. Instead of forcing fluid up to the master cylinder, you can open the other brake bleeder on that circuit and force new fluid through just that part of the system. Repeat on the other circuit, then empty the reservoir and inject new fluid through any bleeder. The company says this will force any air in the system to go where it would naturally go anyway: up. They also emphasize that this is not the way to flush a brake system because it doesn’t flow enough fluid to wash away contaminants in the system. Still, on brake systems that are difficult to bleed, this technique may be the answer.
Whenever there’s a hydraulic problem, especially a seized caliper, it’s important to check hydraulic pressure at each wheel. There are two different ways to do this: with pressure gauges threaded directly into the hydraulic system or with caliper pressure gauges.
Hydraulic gauges with adapters can be threaded directly into the brake bleeder ports. To get an accurate reading, the gauges must be bled too. Caliper pressure gauges have a pressure sensor that fits into the caliper in place of the brake pads. These save a lot of time and brake bleeding, but if the rear axle has drum brakes, you’ll still need hydraulic gauges to check those pressures.
Rear brake pressure will always be lower than the front, but pressures should be equal side-to-side on each axle. Pressures at each side should also rise and fall together. If there’s a difference between left and right brake pressures, swap the gauges to check their calibration. Vehicle height sensors or height-sensing pressure limiting valves can also cause different left/right brake pressures.
On the Mazda described earlier, the collapsed brake hose caused pressure on the left caliper to rise and fall slower than on the right. If the master cylinder was clogged, left-side pressure probably wouldn’t release when the pedal was released. One way to diagnose this is to remove the master cylinder as suggested. But tools that measure brake pressures and test the condition of the fluid not only save diagnostic time, they also produce solid information that will help determine the proper repair and help sell it to the customer.
Techs can prevent brake comebacks with the right tools and procedures.