RV Auto Hold Parking Brake: Complete Guide to Operation and Troubleshooting Basics

The Rundown

Your 1999 Damon Daybreak 3270's ineffective auto hold parking brake is a common issue on these GM 6.5 diesel chassis, and you're right to address it - it's a safety concern when parked on any incline. The parking brake system on your Daybreak (note: automatic engagement when shifting to park was not standard on 1999 GM chassis - verify your specific setup), but after 25 years, the vacuum lines, diaphragm, and adjustment mechanism often fail or become severely out of adjustment. Since you're in the UK, you'll want to focus on parts that can be sourced locally or shipped internationally without excessive cost.

The good news is that most of the components for your GM chassis parking brake system are standard GM parts that should be available through European GM dealers or online suppliers. Your Daybreak uses the same parking brake setup as many GM trucks from that era, so don't let anyone tell you it's an exotic RV-only part. The system consists of a vacuum actuator connected to the parking brake mechanism via a cable and lever system.

Why It Happens

The vacuum-operated parking brake on your 6.5 diesel relies on engine vacuum to actuate, but diesel engines produce much less vacuum than gasoline engines. Over time, the rubber diaphragm in the vacuum actuator deteriorates, vacuum lines crack or become disconnected, and the cable mechanism stretches or seizes. Additionally, the parking brake mechanism may need adjustment or service depending on your specific brake configuration.

Your specific symptoms of the brake "doing virtually nothing" commonly indicate either a complete loss of vacuum to the actuator, a failed diaphragm inside the vacuum unit, or severely worn brake shoes that are no longer making proper contact. The parking brake shoes are separate from your regular service brakes and are located inside brake drums on the rear axle. Many owners don't realize these shoes need periodic adjustment and eventual replacement, just like regular brake shoes.

Another common culprit is the vacuum storage tank and check valve system. Your Daybreak should have a vacuum reservoir tank that stores vacuum when the engine is running, allowing the parking brake to function even after the engine is shut off. If this tank has a leak or the check valve fails, you'll lose vacuum pressure and the brake won't engage properly.

Gather These Items

For the vacuum system diagnosis, you'll need a vacuum gauge (available from most auto parts stores in the UK), basic hand tools including wrenches and screwdrivers, and possibly vacuum line (correct diameter for your model - verify with dealer). The vacuum actuator itself (correct part for your model - verify with dealer) might need to be sourced from a GM dealer or online - prices vary but expect £150-250 estimated cost for a new unit if needed.

For the brake adjustment or service, you'll need brake shoe adjustment tools or a long flathead screwdriver, and potentially new parking brake shoes if yours are worn beyond adjustment. The shoes are typically available through GSF Car Parts, Euro Car Parts, or similar UK suppliers as they're standard GM truck parts.

You'll also want brake cleaner, a good flashlight or work light, and jack stands rated for your RV's weight. Consider getting a basic vacuum pump tool (around £30-50) which will help you test the vacuum actuator and can be useful for other RV maintenance tasks. Don't forget safety gear - safety glasses and gloves are essential when working with brake components.

The Fix

WARNING: Before lifting your RV, ensure you're on level, solid ground. Use properly rated jack stands - never work under the RV supported only by a jack. The weight of your RV can cause serious injury or death if it falls.

Start by checking the vacuum system before assuming you need new parts. With the engine off, locate the vacuum line going to the parking brake actuator. Use a vacuum gauge to test vacuum at this line when the engine is running - if not, trace the vacuum routing back toward the engine to find breaks or disconnections. The vacuum source typically comes from the intake manifold through a series of lines and a vacuum reservoir tank.

If you have good vacuum at the line, reconnect it and test the actuator by having someone shift into park while you watch the actuator lever move. If the lever barely moves or doesn't move at all, the internal diaphragm has likely failed and you'll need a replacement actuator. However, if the actuator moves but the brake still doesn't hold, your issue is likely with the brake shoes or cable adjustment.

To adjust the parking brake shoes, you'll need to access the brake drum adjustment holes, typically located on the backing plate of each rear wheel. Look for an adjustment wheel or star wheel mechanism - turn it to expand the shoes until they make firm contact with the drum surface. This is a bit tricky on your first attempt, so take photos as you disassemble to help with reassembly. Some mechanics recommend marking the current position of adjustment screws before making changes.

If adjustment doesn't solve the problem, the shoes themselves may be worn down to the rivets or severely glazed. Replacement involves removing the old shoes and springs, cleaning the drum surface, and installing new components. This job requires patience and attention to detail, as the spring arrangement must be correct for proper operation. Consider having this part done professionally if you're not comfortable with brake work, as improper installation could be dangerous.

Know Your Limits

While vacuum line replacement and basic adjustments are within most DIY capabilities, working inside the rear differential requires experience and proper tools. If you've never done differential work before, consider having a local truck service center handle the brake shoe replacement - the cost savings of DIY aren't worth the safety risk if done incorrectly. Many UK truck service centers are familiar with GM chassis and can source the correct parts.

Be aware that parking brake work on a heavy RV like your Daybreak requires proper support and safety precautions. Never work under the RV supported only by the factory scissor jack - use proper jack stands rated for the weight. Additionally, if you discover that multiple vacuum components need replacement or if there are signs of brake fluid contamination in the differential, it's time to call in professional help.

Given your UK location, establish a relationship with a local commercial vehicle workshop that handles medium-duty trucks. They'll have access to GM parts through European channels and can often source components faster and cheaper than RV-specific dealers. Many commercial workshops are more familiar with the actual running gear on your Daybreak than RV service centers, since they work on GM trucks regularly.