If your rear brakes squeak when reversed, this could be an indicator that your pads have worn down or that there may be something amiss with either your rotors or brake calipers.
Chances are your brake pads are catching on an irregular lip on a worn rotor, and this issue can be solved by inspecting and cleaning them regularly.
When you press on the brakes in reverse, your car's pads may come into contact with an integral metal element - similar to tread wear indicators on tires - designed into them. This design feature serves as a warning that it is time for new pads; its purpose being to alert drivers. A squeal occurs when this spring steel pad wear indicator rubs against the brake rotor; only occurring when using reverse braking because pad and rotor orientation differ when breaking in that way.
Your brake pads could also be making noise due to contact with a worn rotor lip, such as when reverse braking. To reduce this occurrence, be sure to install or reattach anti-rattling clips that come with your pads, or remove old anti-rattling clips when replacing pads in reverse. Brake caliper grease may help by lubricating sliding components of the caliper so as to stop pads catching on it during reverse braking and stopping pads from catching onto it during forward.
Reversing the car and hearing a squeal from its brakes could be caused by something caught between the pads and disc - often something small such as a pebble or piece of metal - but could also indicate that something more serious has gone amiss, like a seized caliper or collapsed hose, needing immediate repair.
Another potential cause could be pads catching on a worn lip of a rotor. Since rotors and pads are designed to operate in one direction only, when worn-down pads create an edge lip they may catch when travelling against it and could therefore potentially be the source of this problem.
Copper slip can help lubricate caliper slider pins to help eliminate reversing squeals, but if this does not work you could try replacing pads or rotors (ensure they fit your vehicle size), although larger rotors may require you to modify the brake mounting bracket to accommodate their additional thickness.
Brake calipers work by compressing brake pads against a disc or rotor to generate friction and slow the car. They typically feature a piston that moves in and out with hydraulic pressure to apply pressure directly onto pads, often leading to clicking sounds when reversing your car or due to excessive dust between brake pads and caliper surfaces. Lubrication of this device can solve this issue by eliminating unnecessary movement between pad abutment surfaces and caliper surfaces, thus alleviating this clicking sound when reversed.
However, calipers can become stuck for various reasons such as rust. This causes them to tilt or become embedded in the brake pads, leading to pads to rattle within their carrier and make a squeaking noise that will increase over time. You can avoid this by greasing caliper guide pins with grease; additionally use brake grease at any points where contact is made between the caliper and pads/rotors as this will lessen any squeaking and increase vehicle performance.
Squeaking back brakes while in reverse is usually caused by worn brake pads that need replacing, as their wear-and-tear causes thin metal deposits at the contact points with the rotor that cause squeaking noises.
Brake lines are steel tubes which connect each wheel's master cylinder directly via flexible brake hoses to the master cylinder, and may feature flared ends to ensure leak-proof connections.
Sometimes brake lines will produce an audible squeal upon reversing due to dirt or debris in the system, and cleaning the brake lines may fix this issue. Another potential source could be that shims on brake pads are rubbing against rotors - in this instance you can free them by cleaning them.