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What Are Brake Wheel Cylinders And Why Are Mine Leaking?

Photo of backing plate from a set of drum brakes with brake fluid leaking from the lower edge onto the wheel and tyre

A squishy or soft brake pedal is something that no one wants to experience. Especially if it has been good up until now and it fails all of a sudden as you're approaching the stationary back end of a flatbed truck at highway speeds because someone ahead has stopped across traffic on the Bruce Highway to help an elderly echidna across the road.

In this situation, you've only got a split second before things get really bad for you, the truck driver and the echidna.

First things first, if you're in this shit-uation, pump the brake pedal. A loss of brake pressure can sometimes be made up for by stomping that pedal over and over like it owes you money in the hopes that it will get hard again.

Second to that, try not to hit the back of the tow truck. The cross-section of a flatbed looks awfully like a knife and you are as soft as a 3 minute egg compared to the 4.5 tonne kitchen implement approaching you at 60km/h.

Thirdly, when all of the panic is over, if you're still alive you'll probably be wondering what went wrong with your car (you'll probably use a few more expletives).

In this blog post, we're not going to cover all of the components that might cause this but, we are going to look at one specifically, drum brake wheel cylinders.

Why is this coming up now? Well, it's quite simple. In the last week, we saw a certain Nissan Navara suffering from this problem and it got us thinking.

Strangely enough, the only reason this person realised there was a problem was because someone else drove their car and had a big panic the first time they had to stop (though it wasn't as dramatic as described above).

So, what is a wheel cylinder and why do they leak?

Brakes, on the whole, are actually a really interesting system. They utilise the theory that certain fluids that are completely incompressible can transfer and multiply braking force from the itty bitty 45 psi you put on the pedal to a stonking great 2000 psi of line pressure.

They also use a really interesting combination of fluids that have extremely high boiling points to prevent them from turning into gas in the lines, a phenomenon that would make an incompressible fluid a compressible gas. This is all a bit sciency for today so more on this another time.

In this system there has to be a hydraulically actuated piston that applies force to the brake pad or brake shoes and (in drum brakes at least) this is the wheel cylinder. The wheel cylinder (our candidate for the day) sits inside the brake drum and applies the (up to) 2000 psi of pressure outwards forcing the brake shoes into the brake drum and creating enough friction to slow you down.

All really simple right? But how do wheel cylinders leak then?

Like everything that has to move, there has to be a minute gap left between the piston and the cylinder wall. If there wasn't, the friction between both components would be too high to overcome. To allow for this little tiny gap, a set of (usually) rubber seals is used.

With age and time, the heat and forces applied to this seal cause them to degrade and this is where cracks and damage can form and where leakage can start.

Now, here is the real question. How can you fix leaking wheel cylinders?

In the olden days (a time we like to talk about quite regularly), the go-to method was repair. A task that included taking the wheel cylinder apart, honing the inside to a fine finish and replacing all of the dodgy seals.

Nowadays, a brand new wheel cylinder is close to the cost of a repair kit and (seeing as there is a lot of added labour) it's a lot cheaper to replace.

The unfortunate part of all of this is the conversation around the brake drums/shoes. While it might be a common thought that you can just replace the wheel cylinders and get back on the road, the fact that the brake shoes are an organic compound means that the leaking brake fluid is soaked up like a beer after a hard day in the sun.

Due to the flammable nature of brake fluid, soaking it into a component that regularly sees high levels of friction and temperatures in the multiple hundreds of degrees can lead to yet another shit-uation.

To cut a long story short, wheel cylinders leak because of aging or damaged seals, brake drums (and more importantly) shoes need to be replaced at the same time, and no one likes a soft pedal.


If you have any questions about this blog post, would like a second opinion from a mechanic or would like to find an honest mechanic in Brisbane, check out Kashy here.

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