Topping up, flushing and bleeding hydraulic oil: how to do it right

Your convertible top works on hydraulic pressure, and this requires sufficient oil. Do you already know which oil your convertible needs? Then the real work begins: how much do you need, how do you get the old oil out, and why does the top sometimes work worse after refilling than before? The latter is usually due to air in the system. Below, you can read how to prevent this.

 

Refilling is a symptom, not a solution

A hydraulic system is closed. Nothing should disappear from it. If the level is too low, the oil has gone somewhere, and that is almost always a leak. Before you remove the cap from the reservoir, you should first check this list:

• Look under the car and in the trunk: oil stains near the pump, along the cylinders, or on the floor covering directly indicate the spot.
• Check the hoses at the hinge points: there, the lines bend with every top movement. This is the spot where they tear after ten to fifteen years.
• Smell: hydraulic oil has a recognizable, sweetish odor. If the trunk smells of it without you seeing a puddle, the leak is probably higher up.

If you find a leak, refilling is only useful as a temporary emergency measure. The oil will leak out again, and a system that draws in air will eventually damage the pump. Do you find a small leak in one hose? Then you rarely need to replace the entire line. With a repair kit, you only replace the damaged part.

 

What you need

• The correct hydraulic oil for your system
• A clean funnel or a syringe with a hose
• A lint-free cloth
• Gloves and safety glasses
• A collection tray if you are flushing

Count on about half a liter for refilling. If you really want to flush the system, you will need more. One 500 ml bottle is enough for most compact convertible systems to flush twice, but check the capacity of your own system in the workshop manual.

 

Never mix two types of oil

This is the most common and most expensive mistake. Mineral oil (beige, ZH-M) and synthetic oil (green, CHF) are not compatible. If you mix them, the oil can flocculate or foam. The result: clogged channels, damaged seals, and eventually a pump that fails.

Don't know what kind of oil is in it? Look at the color of what is currently in the reservoir and compare it with what the manufacturer prescribes. If in doubt, flush the system completely instead of refilling. This costs more oil, but it is much cheaper than a new pump.

 

Step 1: park the car correctly

• Park on a level surface.
• Put the top in the position prescribed by the manufacturer for checking the oil level. For many models, this means fully closed, for some it means fully open. This varies per brand and really affects the reading.
• Engage the handbrake and turn off the ignition.
• Let the car cool down if you have just driven.

 

Step 2: find the reservoir

The reservoir is located near the pump. The pump is usually in the trunk, behind a side panel or under the floor panel. For some models, it is behind a wheel arch or under the rear seat.

Clean the area around the cap before opening it. Dirt that falls into a hydraulic system damages the pump from the inside.

 

Step 3: read the level

Many reservoirs have a MIN and MAX line. If there is no line, for most systems, the reservoir should be filled to about two-thirds. This is a rule of thumb, not a strict rule: if in doubt, consult your model's workshop data.

Never fill to the brim. Oil expands when heated, and an overfilled reservoir will force oil out past the cap.

 

Step 4: refill slowly

Pour in small quantities and let the oil settle each time. Pouring quickly draws air in, and it is precisely this air that causes the problems you wanted to solve. Check the level in between. Refill in steps of a few centiliters, not all at once.

 

Step 5: bleed the system

This is the step that most DIYers skip, and the reason why a top sometimes moves jerky or slowly after refilling. Most convertible systems self-bleed if you let the top go through its full cycle several times:

1. Let the top open completely. Wait until the movement is completely finished.
2. Let the top close completely. Wait again.
3. Repeat this three to five times.
4. After each cycle, check the oil level and top up if necessary. During bleeding, oil will disappear into the lines that previously contained air.

Does the top still move unevenly, do you hear the pump gurgle, or does the top get stuck halfway? Then there is still air in it, or the system requires a specific bleeding procedure. Some models have bleed nipples at the cylinders or a controlled procedure via the diagnostic computer. Do not force this: a pump that circulates air will be damaged.

 

Step 6: check for leaks

Leave the car for a day and then check again. If the level drops again, there is a leak that you have not yet found.

 

When to flush instead of top up?

Flushing is advisable if:

• the oil is dark, cloudy, or milky
• the wrong oil has ever been added
• the system has been open for a long time and could have attracted moisture
• you have replaced a pump or cylinder and want to ensure clean oil
• the top moves slowly and unevenly without you finding a leak

Milky oil means water in the system. This requires flushing, not topping up. When flushing, you drain the old oil, fill with fresh oil, let the top go through its cycle a few times, and drain again. For heavily contaminated systems, repeat this. Expect at least two passes.

 

When to stop

Get help if:

• the pump gurgles or squeaks and the top does not move
• the level drops again within a few days
• the oil is milky
• your model requires a bleeding procedure via diagnostic equipment
• you are not sure what kind of oil is in it

 

Safety

Hydraulic systems are under pressure. Never loosen a line or coupling while the system is under pressure, and keep your hands away from the hinge points of the top. Do not work under a top that is only held up by hydraulic pressure.

Wear gloves and safety glasses: hydraulic oil is irritating to skin and eyes. Collect drained oil and hand it in at a collection point. Hydraulic oil does not belong in the sewer.

 

Keep the top smooth

Good maintenance prevents a large part of malfunctions. Keep moving parts free of dirt and use suitable lubricants, and regularly treat the rubber seals with the correct maintenance products. A top that moves smoothly puts less strain on the hydraulic system.

 

Are you unsure about the type of oil or can't figure it out? Contact us. You can order the correct hydraulic oil directly from us.