Driving down the road and had no brake

CONCETTA A DALRYMPLE

MEMBER
1996 TOYOTA 4RUNNER
4WD
AUTOMATIC
165,000 MILES

I was driving down the road and had no brake after pumping them they came up slightly, this happened a few times same day and I parked the car. Checked the tires an no leaks checked fluid and it was full. My nephew said it was the master cylinder so my son took it off and replaced it. We cannot seem to get the brakes bled or any pedal pressure. The cylinder it self was bench bled, then each wheel bled from the furthest to the closest. Petal felt tight until tuning the vehicle on and then it just goes to the floor. Each side was bled and re-bled three times and still the same thing. Took off the master cylinder and replaced again and now the same thing. What the hay? Was it never the cylinder and now what?

1 Reply

CARADIODOC

MECHANIC
33,744 POSTS

Your description is exactly what a master cylinder will cause, but I have never had to bleed at the wheels. Doing so most likely put air in the lines that hasn't bled out yet.

The way to do this is to loosen the line nuts slightly at the master cylinder, then unbolt it from the power booster. Pull it forward off the mounting studs, then use it as a handle to bend the steel lines upward a little. That will stop the fluid from running out of the lines. Finish unscrewing the line nuts and remove the master cylinder.

Screw the lines to the new, bench-bled master cylinder. Make them just snug with a wrench so the double flares do not deform from bending. Push the master cylinder down to bend the lines back the way they were, then slide it onto the mounting studs and bolt it tight. Loosen one of the line nuts about 1/4 turn, then have a helper push the brake pedal very slowly. You will see air bubbles coming from around the loose nut. The helper must holler when they reach halfway to the floor, and they must hold the pedal there until you holler that the nut is tight. At that point the helper should release the pedal quickly. If they release it before you have the nut tight, air will get sucked in again and you will have not made any progress.

Loosen the nut again and have the helper push the pedal again. Often this only has to be done once. When there's no more bubbles coming out, tighten that nut and repeat the process for the other line.

By pushing the pedal slowly, air in the lines and ports will have a chance to float back up as the brake fluid is going down. By releasing the pedal quickly, the fluid rushing back will wash any air into the reservoir.

The next issue is if the vehicle has anti-lock brakes, you may need to have the system bled by a mechanic with a scanner that can activate the valves. If there were a few air bubbles in the lines, those would have ended up in some of the chambers and passages. That air will not make it through to the wheels until the valves are opened.