RV Air Bags Dropping But Tanks Hold Air: Complete Diagnosis Guide

Quick Take

Your air system has a classic leak pattern that points to a faulty check valve or unloader valve in your air compressor system. The fact that your air bags drop uniformly and your tanks lose pressure only after driving (but stay full when you air up while parked) indicates the compressor is backfeeding air through a bad check valve when hot, but seals properly when cool. This is a common issue on motorhomes after the compressor has been working hard during travel and the internal components expand with heat.

The uniform drop across all four bags tells us the leak isn't in individual bag lines but somewhere upstream in the main air supply system. Since you've tested this multiple times with consistent results, we can rule out random leaks and focus on temperature-related valve failures in the compressor assembly.

Common Causes

The most likely culprit is a failing discharge check valve in your air compressor's discharge line. When these valves get worn or warped from heat cycles, they don't seat properly when the compressor is hot, allowing compressed air to leak back through the intake system. This explains why the problem only occurs after driving when everything is hot.

Another possibility is a faulty unloader valve that's supposed to dump air from the discharge line when the compressor shuts off. If this valve is sticking partially open due to heat expansion, it can create a slow leak path that becomes more pronounced as components cool and contract differently.

Heat-related expansion of O-rings and seals in the pressure switch or governor can also cause this symptom. These components work fine when cool but develop small leak paths when hot from engine compartment heat during travel.

Less common but possible causes include a cracked air line near the compressor discharge that only opens under thermal stress, or a failing pressure protection valve that's designed to prevent over-pressurization but starts leaking when hot.

Preparation

Start by locating your air compressor - typically mounted on the engine or chassis near the engine bay. You'll need a soap solution (dish soap and water in a spray bottle), a multimeter, basic hand tools, and safety glasses. Always completely depressurize the air system before disconnecting lines or removing components. Make sure you have the compressor model number handy as check valve kits are model-specific.

Gather compressed air system schematics for your RV if available, as air line routing varies significantly between manufacturers. You'll also want to pick up a check valve repair kit (usually $15-30) and possibly an unloader valve ($25-50) before starting, as these are the most likely repair parts needed.

Plan to do this work when the RV has been sitting overnight so all components are at ambient temperature. You'll need access to shop air or a portable compressor to test your repairs, and allow 3-4 hours for the complete diagnosis and repair process.

Have a notepad ready to document pressure readings at various time intervals, as this data will help pinpoint exactly which component is failing and confirm your repair was successful.

Repair Steps

Begin by reproducing the problem - drive the RV for at least 30 minutes to heat everything up, then park and immediately check your tank pressure and bag height. Record these baseline numbers and note the time. Monitor the pressure drop over the next hour, checking every 15 minutes to establish the leak rate pattern.

While the system is still warm, spray soapy water around all fittings at the compressor discharge, the pressure switch connections, and the unloader valve (usually a small valve with a single air line). Look for bubbles indicating external leaks. Pay special attention to where the discharge line connects to the compressor head.

If no external leaks are found, the problem is likely internal to the compressor. Remove the discharge line from the compressor head and completely depressurize the system. With proper safety precautions and the system isolated, you can test the discharge port for leaks using soapy water. Bubbles here confirm a bad check valve inside the compressor head.

To replace the check valve, first completely depressurize the air system, then disconnect the discharge line fitting where the check valve is typically located. The check valve is usually a spring-loaded valve or ball valve assembly. Clean all surfaces thoroughly and install the new valve according to the kit instructions, ensuring proper orientation and torque specifications per manufacturer guidelines.

If the check valve appears good, test the unloader valve by first depressurizing the system, disconnecting its air line, then re-pressurizing to check for improper air flow. This valve should only allow air through when the compressor is running. Replace if it's passing air when it shouldn't.

After repairs, test the system by heating it up through normal driving, then monitoring pressure retention. A properly repaired system should hold full tank pressure for 24+ hours and bags should only drop slightly due to normal permeation through rubber components.

When to Get Help

Call a professional if you're uncomfortable working with pressurized air systems or don't have the tools to safely remove the compressor head. Air system repairs require careful attention to torque specifications and proper component orientation - mistakes can lead to catastrophic failure.

Seek help if your testing reveals multiple leaks or if the compressor itself shows signs of internal damage like metal particles in the oil or unusual noises. These symptoms suggest more extensive repairs that may require compressor rebuild or replacement, typically costing $300-800 depending on your RV's system.

If your RV is still under warranty, document the problem thoroughly with photos and pressure readings before attempting repairs, as air system work might affect warranty coverage. Some manufacturers require dealer service for air system repairs to maintain warranty protection.

Consider professional service if you've replaced the obvious components but the problem persists, as this could indicate issues with the air dryer, pressure protection valves, or other complex system components that require specialized diagnostic equipment to properly test.