Could someone confirm that a DOT 5 brake fliud is a higher spec than a DOT4 - eg higher boiling point or similar?
As with oils, is there a problem in putting, say, DOT 5 fluid in a system originaly specced for DOT 4?
Are these specs (American Dept. of Transport) on the Net?
Regards
John
|
Good grief John! If anyone else had asked this question, I would have said "Ask John Slaughter", he is the one person likely to know the answer. Personally I haven't a clue. Are you intending to decend the Alps towing a trailer?
I was in Uganda on holiday a couple of years ago. Due to unforseen circumstances I had to get a lift in a local pick-up. The driver had to frantically pump the clutch every time he changed gear. Eventually even that would not work, so he stopped and filled the (separate thank God) fluid reservoir with WATER! We did make it back to the hotel though.
|
Tristan
Thanks for the vote of confidence - much appreciated!
As for the reasons for the question, see my reply to Ian
Regards
john
|
|
|
|
Hi John,
I,m fairly sure that 5 is to a higher spec than 4.
My only info comes from my local carspares man who has been in there for about 20 years and seems to know his products.
I remember asking him this about 2 years ago when I was changing the brake fluid on my maestro.
He said you need dot 4.
I asked him what dot 5 was for and he said this was usually specified for later cars with ABS braking systems and I didn't need it for the maestro.
regards
Alvin Booth
|
Hi John
Dot 5 brake fluid is higher spec than Dot 3 and Dot 4 with a higher boiling point as follows (figures for new uncontaminated fluid):
Dot 3 boiling point 401 degrees C
Dot 4 " 446 degrees C
Dot 5 " 500 degrees C
However the big difference is that Dot 3 and 4 are glycol based fluid and Dot 5 is a silicone based fluid. Now whereas silicone based brake fluids have the advantage of not absorbing moisture it is not recommended that a braking system is drained of Dot 4 and filled with Dot 5 as the silicone can latch onto the residual sludge generated from gradual normal component deterioration and create a thicker 'murk' which is more likely to bung up metering orifices and cause sticking pistons etc.
In the classic car fraternity restorers often use Dot 5 silicone fluid as if it is spilt on paintwork it will not cause damage (unlike Dot 3/4), however the conversion to Dot 5 is only normally carried out during the restoration when a complete braking system is renewed with new everything including seals fitted. Therefore if your car has Dot 4 fluid best to stick to it and just replace it regularly.
IM
|
Ian
Ian
Many thanks. I have certainly used DOT 5 silicone fluid in my classic, folllowing a complete replacement of the braking system. I wasn't aware that DOT 5 was a silicone only spec.
I had quite some discussion with another correspondent to the Back Room, who was considering using it in an Audi. The big issue was that although we were fairly certain that DOT was a higher boiling point fluid, and that being silicone based was non-hygroscopic, we still don't know why it's not standard use - there seem to be no drawbacks, and apparently the US Military use it. It's certainly popular in the classic car world. As for the dirt it may pick up, I suspect that's a relic of the water in the original fluid. I'm told you can flush systems with meths, removing this with the new fluid.
The snag with putting it in a 'DOT 4 car' seems to be that the dealer may top up with DOT 4, negating the benefits - and if you tell them you have changed the fluid, they may well invalidate any warranty on your ABS etc. As the warranty on my Minor expired 42 years ago, I'm not too worried!
I wonder if anyone out there who works for a car maker can enlighten us?
Regards
John
|
It is my understanding that although Silicone brake fluid had some advantages (Non-hygroscopic, doesn't attack paint, higher BP) it is more compressable than glycol based fluids. This seems to me to be defeating the point of a hydraulic fluid. I have certainly heard reports of softer brake pedals using silicone fluid. I have also heard it can make systems harder to bleed.
All it's advantages can be got around with conventional fluid. Change it yearly or so, be careful when topping up and don't thrash you're braking system!
|
Jeremy
Yes, I've heard these comments too, but frankly no liquid is compressible to any degree at the pressures in a braking system. It's more likely to be expansion of the system components. There may be viscosity issues which slow bleeding, but again I've had no trouble.
I still think its basic benefits are valuable, and don't think changing the fluid annually to get round the problems of hygroscopic action is comparable to using a fluid which doesn't have the problem in the first place!
Regards
john
|
|
|
|
Ian
I've just looked at this again, and I'm sure the temperatures quoted must be degrees F, and not degrees C, don't you think?
Regards
John
|
John
Hmmmm, yes thinking about it you must be right, the figures I had were just quoted as degrees and I (wrongly) assumed they were degrees C.
I also read today that if you put dot 5 fluid in an ABS braking system designed for dot 4, because of the increased compressibility of dot 5 it can have an adverse effect on the ABS performance????
Ian
|
|
|
|
|
|
