Right Rear Drum Brake Not Holding and Brake Pedal Going to the Floor After Wheel Cylinder and Hose Replacement: Causes and Diagnosis

Introduction

A right rear drum brake that will not hold, combined with a brake pedal that sinks all the way to the floor, usually points to an active hydraulic problem or a mechanical fault inside that corner of the brake system. When only one wheel behaves this way, the issue is often assumed to be isolated to the wheel cylinder or flexible hose. In real repair work, that is only part of the picture.

Replacing the wheel cylinder and rubber hose does eliminate two common failure points, but it does not rule out air trapped in the system, a leaking hard line, a misadjusted drum brake, damaged brake shoes, or an internal problem elsewhere in the hydraulic circuit. A symptom like this can be misleading because a rear drum brake can appear to be the source of the complaint even when the real cause is upstream or hidden inside the brake hardware.

How the System Works

A rear drum brake depends on hydraulic pressure, friction material, and mechanical adjustment working together. When the brake pedal is pressed, brake fluid pressure travels through the master cylinder, brake lines, and hose to the wheel cylinder. The wheel cylinder pushes the brake shoes outward against the drum. If everything is sealed and adjusted correctly, the shoes move only a short distance before creating braking force.

That short movement matters. Drum brakes are designed with very little extra travel. If the shoes sit too far from the drum, the wheel cylinder has to extend farther than normal before the shoes contact the drum surface. That extra movement increases pedal travel. If the system has air in it, fluid compresses instead of transferring pressure efficiently, and the pedal can drop low or go to the floor.

A pedal that goes all the way down is not always caused by the same fault that makes one wheel weak. It can happen when one corner is allowing too much movement, but it can also happen when the master cylinder cannot build or hold pressure, or when a leak is still present somewhere in the circuit.

What Usually Causes This in Real Life

When only the right rear is involved, the most common causes are usually mechanical or hydraulic in that corner, but not always the wheel cylinder itself.

A very common cause is improper drum brake adjustment. If the shoes are too far from the drum, the wheel cylinder piston has to travel too far. On a drum brake, that can create a low pedal and a feeling that the brake on that side is not really applying. A self-adjuster that is stuck, assembled incorrectly, or simply not working can create the same complaint.

Another common issue is trapped air in the right rear circuit. If the wheel cylinder and hose were replaced, air likely entered that side of the system. Drum brakes can be difficult to bleed completely if the shoes are not adjusted close enough to the drum or if the bleeder position is not allowing the air to move out cleanly. Air in one rear corner can make the pedal feel soft and let it drop too far.

A damaged or partially collapsed hard line to that wheel is another realistic possibility. The rubber hose may have been replaced, but the steel line feeding it could still be restricted, rusted internally, or cracked near a bracket. A line that leaks only under pedal pressure may not leave an obvious puddle, especially if the fluid is being thrown by the wheel or caught in the backing plate area.

There is also the possibility of contamination or mechanical failure inside the drum assembly. Brake shoes that are soaked with fluid, worn unevenly, installed incorrectly, or contaminated with grease will not grip the drum properly. If one shoe is not seated correctly against the anchor or adjuster, the wheel cylinder can move more than it should and the drum will not hold well.

Less commonly, the problem can still involve the master cylinder or another part of the hydraulic system. A master cylinder with internal bypassing usually affects more than one wheel, but it can show up as a pedal that sinks to the floor even after individual wheel parts have been replaced. If the pedal drops without any visible external leak, internal bypass in the master cylinder becomes a real diagnostic possibility.

How Professionals Approach This

Experienced technicians usually separate a complaint like this into two questions: is the wheel failing to apply, and is the pedal losing hydraulic support?

The first question is answered by checking drum brake adjustment and hardware condition. The drum should not be treated as a sealed mystery part. It needs to be removed and inspected for shoe wear, spring condition, adjuster operation, anchor seating, and signs of fluid contamination. If the shoes are too far from the drum, the system may never feel right no matter how many hydraulic parts are replaced.

The second question is answered by pressure and leak logic. A pedal that goes to the floor after the wheel cylinder and hose have already been replaced means the system still needs a careful hydraulic inspection. That includes looking at the steel line, the bleeder screw, the junctions, and the master cylinder behavior. If the pedal improves when the rear shoes are manually adjusted closer to the drum, that strongly points toward excessive shoe-to-drum clearance rather than a major hydraulic failure.

A proper diagnosis also considers whether the brake pedal is sinking slowly or immediately. A slow sink under steady pressure suggests internal bypass in the master cylinder or a leak that only opens under pressure. A very low pedal that improves after pumping often points toward air in the system or excessive shoe clearance. Those two behaviors may feel similar to the driver, but they mean different things in the shop.

If the vehicle has rear drum brakes on both sides, the opposite rear wheel should not be ignored just because it is not showing the same symptom. Brake systems work as a balance, and one weak corner can be caused or amplified by problems elsewhere in the rear axle braking setup.

Common Mistakes and Misinterpretations

One of the biggest mistakes is assuming that replacing the wheel cylinder and hose automatically fixes a low pedal complaint. Those parts fail often enough, but they are not the only parts that affect pedal height.

Another common mistake is overlooking brake adjustment. Drum brakes are not like disc brakes, where pad clearance is mostly self-managed. If the adjuster is not working or the shoes are sitting too far away from the drum, the pedal will travel farther than normal. That can look like a hydraulic failure even when the real issue is mechanical clearance.

It is also easy to miss a small leak at the bleeder screw, line fitting, or hard line rust point. A leak at the right rear may not drip enough to leave a noticeable spot on the ground. Sometimes the fluid is pushed onto the backing plate or inside the drum area where it is harder to see.

Another misinterpretation is blaming the master cylinder too quickly. A master cylinder can cause a floor pedal, but if only one wheel is behaving oddly, the rear shoe adjustment and local hydraulic condition should be checked first. Replacing the master cylinder without confirming the rear brake assembly is correct can lead to unnecessary work and the same symptom afterward.

Finally, brake shoe installation errors are more common than many people expect. A shoe that is on the wrong side, a spring that is in the wrong location, or an adjuster assembled backward can all create excessive pedal travel and poor holding power on one corner.

Tools, Parts, or Product Categories Involved

A proper diagnosis usually involves brake inspection tools, brake adjustment tools, a brake bleeder, hand tools for drum brake hardware, brake fluid, replacement brake shoes if contamination or wear is found, wheel cylinders, flexible brake hoses, steel brake lines, and sometimes a master cylinder or brake hydraulic test equipment.

In some cases, inspection of the backing plate, adjuster mechanism, return springs, self-adjuster hardware, and drum surface condition is just as important as the hydraulic parts.

Practical Conclusion

A right rear brake that will not hold, especially after the wheel cylinder and rubber hose have already been replaced, usually means the problem is not finished yet rather than solved. The most likely next steps are checking shoe adjustment, inspecting the drum hardware, confirming the right rear line and bleeder are dry and sound, and making sure all air has been removed from that corner of the system.

This symptom does not automatically mean the master cylinder is bad, and it does not automatically mean the new wheel cylinder failed. More often, it means the brake on that corner still has too much travel, trapped air, or an overlooked leak or assembly issue. The logical repair path is to verify the mechanical setup first, then confirm the hydraulic integrity of the entire circuit.