Why are car brake fluid/coolant reservoirs always made of thick white plastic?

[pinzaboy]

Hi 

I was checking the brake fluid/coolant reservoirs on the car and got to wondering why they are so difficult to see through to establish the levels.

 

They are always made from thick plastic; you might imagine an element of the reservoir would be see-through for a quick visual check.

 

Perhaps others find the current reservoirs easy to check. But I don't. 

Thanks

Tim

 

[spamcan61]

I suspect they're made of thick plastic because they need to be robust in case of an accident, modern brake fluid is highly inflammable so you don't want it spilling over a hot engine.

 

Why they couldn't use a thick but clear housing I don't know, I agree it can be difficult to see the level. OK most cars have a low level sensor but that only triggers when it's getting really low I suspect.

[didcot]

I did a test at work using a lubricating mechanism, the sort that is used on machine tools to lubricate sideways etc with brake fluid to see if we could use it in a radioactive environment. We are limited on materials we can use. So I filled the lubricator with brake fluid in a normal environment to check compatibility. After 3 weeks the clear plastic sight gauge made of a perspex type material had turned milky and the plastics inside had turned brittle. It wouldn't pump either. So the brake fluid actually did what radiation would have done in a similar period of time.

Brake fluid is hygroscopic, it absorbs moisture. Some plastics such as nylon do have a percentage of water in them. Makes them rubbish for cryogenic work as they expand and crack. So I suspect the plastic is thick for several reasons. Strength, heat from the engine and coolant can soften plastics, effects of brake fluid and possibly they need to be strong enough to allow the use of pressurised brake bleeding kits.

Coolant reservoirs will be pressurised so some of the above applies. I had a Vauxhall Fronteria that went through reservoirs. Constantly cracking. I used to drive it with the cap slightly cracked open to prevent it pressuring.

[meil]

16 minutes ago, didcot said:

 

Brake fluid is hygroscopic, it absorbs moisture.

I thought DOT 5 wasn't hydroscopic?

[Titan]

16 minutes ago, didcot said:

I did a test at work using a lubricating mechanism, the sort that is used on machine tools to lubricate sideways etc

 

Took me a while to realise you meant slideways. makes sense now!

[didcot]

Yes, my mistake. Slideways, auto corrupt strikes again.

Depending on which grade, brake fluid is or isn't hygroscopic. Both my wife and I had our 2014 Vauxhall's recalled due to hygroscopic brake fluid.

[tigerburnie]

Cars used to have metal reservoirs a long time ago, plastics would no doubt have been brought in to save money for the manufacturer. There's a lot on a car that makes no sense , the factory I worked in had a problem with mixture strengths of window wash fluid, too strong and it was eating the plastics on light clusters in Scandinavia(where they asked for higher concentrations of the anti freeze). I spent weeks mucking around re programing PLCs trying to get a happy mixture.

[tigerburnie]

27 minutes ago, didcot said:

Coolant reservoirs will be pressurised so some of the above applies. I had a Vauxhall Fronteria that went through reservoirs. Constantly cracking. I used to drive it with the cap slightly cracked open to prevent it pressuring.

To be fair quoting a Vauxhall, especially a Frontera in a debate about car quality is a bit like using Genghis Khan as a reference for a job in diplomacy...……………………………….

[Kickstart]

2 hours ago, meil said:

I thought DOT 5 wasn't hydroscopic?

DOT 5 is very rarely used. It is silicone based. Harley Davidson used it for ages but they have switched to conventional fluids now.

 

Big problem with DOT 5 is that it doesn't absorb water. So any water that gets into the system is just about impossible to remove. DOT 5 can also give sprongier brakes.

 

DOT 5.1 is a conventional fluid

 

All the best

 

Katy

[Nick C]

I think it all comes down to cost. Plastic is cheaper than metal for something like that.

 

I find many of them you can get an idea of the level by shining a torch at one side, then you can often see the shadow of the level on the other.

[Yardman]

11 hours ago, Kickstart said:

DOT 5 is very rarely used. It is silicone based. Harley Davidson used it for ages but they have switched to conventional fluids now.

 

Big problem with DOT 5 is that it doesn't absorb water. So any water that gets into the system is just about impossible to remove. DOT 5 can also give sprongier brakes.

 

DOT 5.1 is a conventional fluid

 

All the best

 

Katy

Dot 5 isn't compatible with ABS systems apparently. Slightly different viscosity maybe? A pity, I still use it in some of my older vehicles.

[Kickstart]

48 minutes ago, Yardman said:

Dot 5 isn't compatible with ABS systems apparently. Slightly different viscosity maybe? A pity, I still use it in some of my older vehicles.

That is why Harley switched. Not sure on viscosity (DOT 5.1 seems far less viscous that DOT 3 or 4). Possibly down to compatibility with seals or lubricating qualities, but could just be with abs pumps there are more places for water to accumulated if not absorbed.

 

All the best

 

Katy

[big jim]

15 hours ago, didcot said:

Coolant reservoirs will be pressurised so some of the above applies. I had a Vauxhall Fronteria that went through reservoirs. Constantly cracking. I used to drive it with the cap slightly cracked open to prevent it pressuring.

BMW mini are notorious for it too, if it isn’t the reservoir It the pressure release cap that goes faulty 

 

my mini one has a normal coolant reservoir with a non pressurised lid, but my cooper S has a pressurised system In it, the reservoirs on them fail by splitting along the centre seam 

 

 lots of companies do aftermarket metal ‘racing’ reservoirs but they want over £100 for a decent one for the cooper 

[The Stationmaster]

I remember the first time I had a car with a plastic brake fluid reservoir and I concluded that it was no more that saving money on using metal   My view has not altered since then.

[tomparryharry]

The reservoir on a braking system is normally made from polypropylene. Once moulded, it's quite tough, and gave effective protection of the contents to meet the then motor quality requirements. Also, it's fairly impervious to solvents. The reservoir  was moulded in two halves, top & bottom, then, they were heated up at the joint face, and stuck together. After that, they were pressure tested to ensure the weld seam was 'sound'. Lucas Girling made thousands & thousands of them.