General Automatic or Manual?

[Goudrons]

My son has a 2017 Kia Ceed 1.6 diesel estate with a 7-speed DCT. It pulls well, changes gear really smoothly and returns high 50s mpg on a good run. VAG DCTs had loads of faults reported on Honest John a few years back, so are probably best avoided like the Dualogic.

I seem to think that will be the same Getrag DCT as the Clio, it's also in some Merc A Classes and I think the latest auto Mini, Puma and Fiesta (replaced the troublesome 6 sp Powershit.

Different versions, 7dct300, 400, 500 handle more or less torque, but the overall design and operation are pretty similar.
These are all wet clutch rather than the 6dct's dry clutch that caused some many problems.

 

[mj2k]

Brake and clutch fluid is hydroscopic

Brake and clutch fluid is hydroscopic, meaning it attracts water and moisture from the air.

If you leave it too long all the moisture in the fluid starts rusting the internals of the master and slave cylinders and then bits of rus and pitted pistons start ripping at the seals within which is why they start to leak and fail. This is why you should replace the brake and clutch fluid every 3 or 4 years (see the service schedule) and only use new, clean fluid from a sealed container (not any old fluid that's been sat on the shelf in a garage for years attacting moisture).

It's one of the main reasons masters and slaves fail.
If you pull the piston out of a failed master or slave, 9 times out of 10 it's full of rust like this.

Agreed, corrosion in the cylinders / pipes is the main reason for brake / clutch failure, aside from seal failure, which is often caused by the seals catching on corrosion in the cylinders...

Though I did come across one failure caused by a particularly fine piece of 'maintenance' by the previous (numpty) owner - topping up the brake / clutch master cylinders with engine oil. Not much left of the original systems by the time I'd sorted that out

DOT 5 silicone fluid is supposed to be non-hygroscopic, but it's got more detractors than fans because it readily traps air which makes the pedals feel spongy, and generally it isn't recommended for ABS systems since the trapped air will turn the fluid into foam if rapidly agitated, which is not ideal.

 

[vexorg]

Brake and clutch fluid is hydroscopic

Brake and clutch fluid is hydroscopic, meaning it attracts water and moisture from the air.

I know the theory, but always think it's dubious. Brakes are supposed to "boil" the water under heavy braking. I've had second hand cars with very oil brake fluid when I've changed it, never shown up as a problem.

The clutch pressure is contant, or wont get more over time, therefore the water will stay in the fluid.
And the water is suspended in the fluid, so wont rust metal without air and some means of getting the water out. More likely the rust in those photos is happened after the seal has started to go, but I dont believe the moisture in the fluid would cause that degree of rust (it never has in any car I've had and even the brake lines are all steel, they dont rust!)

 

[vexorg]

Agreed, corrosion in the cylinders / pipes is the main reason for brake / clutch failure, aside from seal failure, which is often caused by the seals catching on corrosion in the cylinders...

Where's the 'dislike' button

 

[mj2k]

Where's the 'dislike' button

Really? You've never come across corroded brake lines or failed seals in a master cylinder / brake balancer, with particles of seal floating in a black sludge of rust and rubber?

You should buy more French cars

https://shop.advanceautoparts.com/r...ake-pedal-so-hard-to-push-and-how-do-i-fix-it

 

[vexorg]

Nope, all brake lines I've replaced have corroded from the outside. The inside are normally still clean. I've owned a lot of older cars and do hate steel brake and fuel lines.

I also have a citroen, and the slave cylinder did leak, but it's all plastic, must be magic rust that corrodes the plastic. The fluid that came out the pipe when bleeding (8+ years old) looked very clean too. I flushed the brake lines when I got it, and that would have been 6 year old fluid, it was only a little cloudy comapred to the clean fluid. The reservoir does have a rubber seal for the cap.

I dont believe anyone changes the clutch fluid, definiately not since most cars now have CSCs fitted.

 

[mj2k]

Nope, all brake lines I've replaced have corroded from the outside. The inside are normally still clean. I've owned a lot of older cars and do hate steel brake and fuel lines.

I also have a citroen, and the slave cylinder did leak, but it's all plastic, must be magic rust that corrodes the plastic. The fluid that came out the pipe when bleeding (8+ years old) looked very clean too. I flushed the brake lines when I got it, and that would have been 6 year old fluid, it was only a little cloudy comapred to the clean fluid. The reservoir does have a rubber seal for the cap.

I dont believe anyone changes the clutch fluid, definiately not since most cars now have CSCs fitted.

Most likely the reservoir was plastic, with the master cylinder itself being cast alloy or steel, machined to provide a smooth bore for the piston / seals to run in. Plastic wouldn't last terribly long as a master cylinder in a braking system

 

[chris3234]

I know the theory, but always think it's dubious. Brakes are supposed to "boil" the water under heavy braking. I've had second hand cars with very oil brake fluid when I've changed it, never shown up as a problem.

The clutch pressure is contant, or wont get more over time, therefore the water will stay in the fluid.
And the water is suspended in the fluid, so wont rust metal without air and some means of getting the water out. More likely the rust in those photos is happened after the seal has started to go, but I dont believe the moisture in the fluid would cause that degree of rust (it never has in any car I've had and even the brake lines are all steel, they dont rust!)

Definitely very unlikely especially as the water and fluid mix so will still have boiling point well over 100C and the fluid itself dosnt really get that hot on a road car

 

[mj2k]

Definitely very unlikely especially as the water and fluid mix so will still have boiling point well over 100C and the fluid itself dosnt really get that hot on a road car

It's more the relative boiling points of the water vs the brake fluid which are a concern - ethylene glycol has a boiling point of 230 degrees vs water at 100 degrees, and certainly the localized temperature in the brake pads / shoes can get pretty high under prolonged braking. This is why performance cars are fitted with larger discs, often vented, than smaller cars - the size of the disc and ventilation allows the heat to be transferred away from the braking surface more efficiently than on the smaller disc (or drum).

The friction of the brake pad / shoe against the disc / drum generates a considerable amount of heat under heavy braking (up to 600 degrees), and though the brake pads insulate the brake pistons from a lot of that heat and most of it is transferred to the discs / drums instead (which can reach about 400 degrees) some of that heat is transferred to the brake pistons. Under prolonged braking it's easy to imagine the temperatures around the brake pistons getting to well over 100 degrees, though. Pressure affects the boiling point however, so the boiling wouldn't occur during the heavy braking, but immediately afterwards when you take your foot off the brake, leading to brake binding (generating more heat) and then sponginess the next time the brakes are applied since the vapor would need to be compressed back into liquid form (which in itself releases heat energy) before the brakes can actually start functioning.

Edit: 'course you'd have to be doing something seriously wrong to boil your clutch fluid in that way, but if a hard clutch pipe passes too close to an exhaust component, once again temperatures could get above 100 degrees given excessive amounts of heat soak, though that's far less likely and considerably less dangerous than boiling your brake fluid. Well, unless the pipe ruptures and the brake fluid catches fire, in which case I suspect a high water content would be rather a good thing...

 

[vexorg]

Most likely the reservoir was plastic, with the master cylinder itself being cast alloy or steel

Clutch "slave" cylinder, body was all plastic, and actuator was plastic coming out the end. I dont have the old one to cut open, but I'd image any metal in there would just be another potential failure point for leaking as they'd need to seal that to the plastic body.

 

[mj2k]

Clutch "slave" cylinder, body was all plastic, and actuator was plastic coming out the end. I dont have the old one to cut open, but I'd image any metal in there would just be another potential failure point for leaking as they'd need to seal that to the plastic body.

Ah, clutch slave cylinder, yeah, that seems sensible - cheaper to produce than a metal one and won't have to stand up to high fluid pressure (or temperature). But more prone to wear than a metal one, no doubt.