Which brake fluid is best, 4 or 5.1?
Discussion
DOT 5.1 is technically "best", as it has the highest wet and dry boiling points.
However, there are a number of racing fluids that have a higher wet and dry boiling point than 5.1, but can't withstand the bottom end temperatures that 5.1 can, so are rated DOT4. Given that the sort of bottom end temperature being talked about is in the region of -40degC, it will never be a concern in this country. So while a regular 5.1 would be better than a regular 4, there could be a racing 4 that will be better than a 5.1.
However, there are a number of racing fluids that have a higher wet and dry boiling point than 5.1, but can't withstand the bottom end temperatures that 5.1 can, so are rated DOT4. Given that the sort of bottom end temperature being talked about is in the region of -40degC, it will never be a concern in this country. So while a regular 5.1 would be better than a regular 4, there could be a racing 4 that will be better than a 5.1.
Racing dot 4 is the only fluid I would put in any of my cars.
5.1 is non-hygroscopic which means it doesn't take on water. What this means is that over time you will (inevitably) end up with water in you braking system and a squidgy brake pedal.
A Racing dot 4 will take on water and still maintain a high boiling point.
5.1 fluid is one of the worst, yet most commonly used upgrades for brakes
.
Duncan
5.1 is non-hygroscopic which means it doesn't take on water. What this means is that over time you will (inevitably) end up with water in you braking system and a squidgy brake pedal.
A Racing dot 4 will take on water and still maintain a high boiling point.
5.1 fluid is one of the worst, yet most commonly used upgrades for brakes
.Duncan
DuncanM said:
Racing dot 4 is the only fluid I would put in any of my cars.
5.1 is non-hygroscopic which means it doesn't take on water. What this means is that over time you will (inevitably) end up with water in you braking system and a squidgy brake pedal.
A Racing dot 4 will take on water and still maintain a high boiling point.
5.1 fluid is one of the worst, yet most commonly used upgrades for brakes.
Duncan
You're mixing up 5.1 with 5, which is silicon based and as you say not hygroscopic. OK for use in racecars when its getting flushed every meeting, not so great for road cars.5.1 is non-hygroscopic which means it doesn't take on water. What this means is that over time you will (inevitably) end up with water in you braking system and a squidgy brake pedal.
A Racing dot 4 will take on water and still maintain a high boiling point.
5.1 fluid is one of the worst, yet most commonly used upgrades for brakes
.Duncan
IIRC its the wet boiling point requirement for DOT5.1 which means they can make a DOT4 with a higher dry boiling point despite DOT4 being a nominally lower standard.
Edited by GravelBen on Wednesday 8th June 09:03
DuncanM said:
Racing dot 4 is the only fluid I would put in any of my cars.
5.1 is non-hygroscopic which means it doesn't take on water. What this means is that over time you will (inevitably) end up with water in you braking system and a squidgy brake pedal.
A Racing dot 4 will take on water and still maintain a high boiling point.
5.1 fluid is one of the worst, yet most commonly used upgrades for brakes.
Duncan
As said, you are confusing DOT 5 for 5.1.5.1 is non-hygroscopic which means it doesn't take on water. What this means is that over time you will (inevitably) end up with water in you braking system and a squidgy brake pedal.
A Racing dot 4 will take on water and still maintain a high boiling point.
5.1 fluid is one of the worst, yet most commonly used upgrades for brakes
.Duncan
Castrol SRF and Motul RBF600 are also pretty good fluids
DuncanM said:
5.1 is non-hygroscopic which means it doesn't take on water. What this means is that over time you will (inevitably) end up with water in you braking system and a squidgy brake pedal.
Duncan
I don't get the logic here. Why, if it is non-hydroscopic will you end up with water in the braking system?Duncan
25 years ago, when I rebuilt my 1968 Spitfire, I bought a copper brake pipe set and DOT 5 silicon fluid as advertised in Practical Classics.
Never changed the fluid since then, never topped up, never leaked, always passes MOT. Car has done 140,000 miles since rebuild.
Edited to add: all my other cars (all with ABS) running regular brake fluid get it renewed every two years despite the brake fluid tester saying it's OK. Never had a brake problem on those cars either (oldest is 17).
Edited by G350 on Wednesday 8th June 09:44
I think people are missing a point here -
Good quality brake fluid (from a recognised brand, their high spec version) will more than likely be better than cheapo autolube stuff from the shelf in your motor factors.
Dot 5.1 and Dot 4 are compatible, different specifications as discussed.
Dot 5 is something different, different chemical for a differient use and not to be mixed with the others.
Good quality brake fluid (from a recognised brand, their high spec version) will more than likely be better than cheapo autolube stuff from the shelf in your motor factors.
Dot 5.1 and Dot 4 are compatible, different specifications as discussed.
Dot 5 is something different, different chemical for a differient use and not to be mixed with the others.
G350 said:
I don't get the logic here. Why, if it is non-hydroscopic will you end up with water in the braking system?
25 years ago, when I rebuilt my 1968 Spitfire, I bought a copper brake pipe set and DOT 5 silicon fluid as advertised in Practical Classics.
Never changed the fluid since then, never topped up, never leaked, always passes MOT. Car has done 140,000 miles since rebuild.
Edited to add: all my other cars (all with ABS) running regular brake fluid get it renewed every two years despite the brake fluid tester saying it's OK. Never had a brake problem on those cars either (oldest is 17).
I have to say I am surprised at your experience. I find it hard to believe that any braking system would be completely water tight and impervious to water ingress?25 years ago, when I rebuilt my 1968 Spitfire, I bought a copper brake pipe set and DOT 5 silicon fluid as advertised in Practical Classics.
Never changed the fluid since then, never topped up, never leaked, always passes MOT. Car has done 140,000 miles since rebuild.
Edited to add: all my other cars (all with ABS) running regular brake fluid get it renewed every two years despite the brake fluid tester saying it's OK. Never had a brake problem on those cars either (oldest is 17).
Edited by G350 on Wednesday 8th June 09:44
Duncan
DuncanM said:
I have to say I am surprised at your experience. I find it hard to believe that any braking system would be completely water tight and impervious to water ingress?
Duncan
Two years ago I drove it over several of Switzerland's highest passes and plenty of other high mountains, fully loaded. The brakes were never a concern. Covered 2000 miles in 6 days. The leaking gearbox on the other hand Duncan
It has always been garaged last 20 years and used 95% on dry days. (The car is run on a shoestring, mostly using s/h parts. I've never, ever changed the discs, which are well worn with rusty edges. The car is only 720 kg though. I've taken the backplates off ages ago so the discs are highly visible yet still pass a MOT).
As this topic is on brake fluid already, I have a couple of quick questions which someone should be able to answer for me:
What are the issues with having brake fluid filled to above the 'max' point - will a car fail an MOT with the fluid overfilled?;
How simple is removing some to reduce the level? Is it as simple as taking the cap off and using a syringe to take fluid out of the tank, then replacing the cap, or is it always necessary to bleed the brakes?
Sorry for being slightly off-topic, but it saves creating a new thread for simple questions.
What are the issues with having brake fluid filled to above the 'max' point - will a car fail an MOT with the fluid overfilled?;
How simple is removing some to reduce the level? Is it as simple as taking the cap off and using a syringe to take fluid out of the tank, then replacing the cap, or is it always necessary to bleed the brakes?
Sorry for being slightly off-topic, but it saves creating a new thread for simple questions.
Meoricin said:
As this topic is on brake fluid already, I have a couple of quick questions which someone should be able to answer for me:
What are the issues with having brake fluid filled to above the 'max' point - will a car fail an MOT with the fluid overfilled?;
How simple is removing some to reduce the level? Is it as simple as taking the cap off and using a syringe to take fluid out of the tank, then replacing the cap, or is it always necessary to bleed the brakes?
Sorry for being slightly off-topic, but it saves creating a new thread for simple questions.
First of all, how did the brake fluid get to be above the max?What are the issues with having brake fluid filled to above the 'max' point - will a car fail an MOT with the fluid overfilled?;
How simple is removing some to reduce the level? Is it as simple as taking the cap off and using a syringe to take fluid out of the tank, then replacing the cap, or is it always necessary to bleed the brakes?
Sorry for being slightly off-topic, but it saves creating a new thread for simple questions.
Secondly, unless it is leaking over the top of the reservoir it is unlikely to be noticed at MOT time AFAIK.
On my cars, I would simply get a clean new syringe and suck it out.
IMO, brake fluid is not a "top-up" fluid. It goes down naturally as the brake pads wear and then up again when you fit new ones. If it is going down quickly then you have a leak which needs immediate attention.
Bill said:
Isn't (and I might be making this up...) there an issue with seals in the braking system being affected by different fluid to that specified my the manufacturer?
IIRC thats if you mix DOT 4 and DOT 5.1 in a system, which is why if you change to a DOT 5.1 fluid you need to flush the system completely. I could be wrong this is just what I have heard, oh and I just stick with DOT 4 in most cases. Si_steve said:
IIRC thats if you mix DOT 4 and DOT 5.1 in a system, which is why if you change to a DOT 5.1 fluid you need to flush the system completely. I could be wrong this is just what I have heard, oh and I just stick with DOT 4 in most cases.
No!I've just said -
Dot 4, Dot 5.1, all compatible. (Dot 3 also)
Dot 5 - NOT compatible!
Gassing Station | General Gassing | Top of Page | What's New | My Stuff