You are correct except the assumption is this problem was already occurring with the original booster. Adjustable push rods are usually found on import models, but they are adjusted at the factory. They aren't supposed to need to be adjusted later. Doing so is usually the wrong fix for some other cause. Most common is a brake light switch that is adjusted down too far and is holding the brake pedal part-way applied. To identify that, pull up on the pedal with your toes when the dragging occurs. An additional hint to this is both front wheels will get hot. When just one wheel gets hot, it's either due to that constricted rubber hose I mentioned, or it's likely occurring on a front-wheel-drive car with a split-diagonal hydraulic system.
We always read warnings about not allowing a caliper to hang by the rubber flex hose as it can tear the inner liner and cause a one-way check valve, but while I suppose that's true, I've run into something else a lot more often. That is rust build-up inside the crimp on the metal bracket around the mid-point of a hose. I've solved at least a dozen of these, including three times on two of my minivans, twice on the same hose. I just spread the crimp open a little with a small pry bar or even a Channel Lock pliers to remove the restriction.
If you still suspect the push rod is too long, stop on a slight incline, place a block about a foot downhill of one tire so you don't look funny running after the vehicle, place it in neutral, then loosen the mounting nuts to let the master cylinder slide forward an eighth inch away from the booster. If that lets the brakes release, the brake light switch is misadjusted, it's the wrong master cylinder for that application or year, or someone changed the push rod's adjustment.
If loosening the master cylinder doesn't help, feel for a hot wheel, then crawl underneath and open that caliper's bleeder screw. If the hose is constricted, you'll see a little spurt of fluid, then that caliper will release.
A less common cause of this is a rusted parking brake cable that is stuck partially applied. This is most likely to occur with rear disc brakes. Drum brakes need a lot more movement of the shoes before they apply. A partially applied / stuck cable usually causes that leading shoe to grab way too hard under light brake pedal application, often to the point that wheel locks up. In its worst condition, that drum brake will release if you stop, then back up a little. It usually grabs again before you get going too fast.
When opening a front bleeder screw lets the brake release, to verify the hose is causing it to remain applied, loosen the steel lines right at the master cylinder. If that lets the brakes release, you have a much more serious problem; the brake fluid contamination I started to describe earlier. Any type of petroleum product, to include engine oil, transmission fluid, power steering fluid, penetrating oil, or axle grease will cause the rubber seals and any other rubber parts that contact the brake fluid to swell and become soft and slimy. In the master cylinder, the lip seals will grow past the fluid return ports and block them. That traps the fluid which gets hot and expands as the brake pads get hot. Being unable to flow freely back into the reservoir, the caliper gets applied harder and harder which builds more heat and makes the problem get worse very quickly. The symptoms and tests for contaminated fluid are the same as for a misadjusted brake light switch or booster push rod.
Welcome to the site, Virgil. Hope to hear more from you.
Friday, February 28th, 2025 AT 5:53 PM
