m8kwr
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| posted on 6/1/09 at 08:55 AM |
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High viscosity brake fluid
Does anyone know what brake fluid has a high viscosity index?
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Mr Whippy
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| posted on 6/1/09 at 09:02 AM |
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why would you want this? surely it would make the brakes heavier and slower to react, there are various different hydraulic oils available but
I'd personally only consider using approved fluid like Dot 4 etc. Anything not approved may invalidate your insurance in the event of an
accident and they find you have oil meant for a digger in your brakes 
[Edited on 6/1/09 by Mr Whippy]
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m8kwr
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| posted on 6/1/09 at 09:12 AM |
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My mate has a V12 cobra, with jag running gear, and we are having some braking problems.
On the second press of the pedal they work.
We thought the problem was the brake reservoir, where it had 2 pipes exiting from the bottom, and one going out the side (this is for the clutch), so
we assumed that air was getting into the system, causing the problems.
We have got a reservoir, that has 3 exit points at the bottom, and still the same issues.
My mate was talking to someone yesterday who had the same problem on their cobra, which lasted 4 years, with changing reservoirs etc.
And the solution that worked was getting brake fluid which had a high viscosity... but it had been some years ago and he could not remember what dot
the fluid was...
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Mr Whippy
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| posted on 6/1/09 at 09:43 AM |
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For a start I’d fit one separate reservoir for the clutch and keep the brakes on their own system, its much safer that way and the bottles cost a
couple of quid. It sounds far more likely there is still air in the system or the rear brakes are not adjusted correctly. It doesn’t matter if the
fluid is like water, if it has no air in it and it not leaking past the seals then it will work. Forget about the reservoir that is just a wild goose
chase and nothing to do with the problem. Go round each wheel, check the mechanism is free, that the handbrake is working correctly (some cars will
not bleed correctly until there is no excess play there), check the pedal has the correct free play and is actually returning to the released
position, it must have some free play to reset. Then bleed the brakes again and check the operation. Look round the piston side of the cylinders for
any brake fluid which would indicate the seals need replaced. Also if you stamp on the pedal and hold it here it should not slowly sink down, if it
does then again the seals are worn.
[Edited on 6/1/09 by Mr Whippy]
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MikeCapon
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| posted on 6/1/09 at 09:49 AM |
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Do you mean a high viscosity or a high viscosity index? These are not the same thing. Viscosity is the fluidity or 'thickness' of a liquid
while the viscosity index is a measure of the rate of change of viscosity against temperature.
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bilbo
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| posted on 6/1/09 at 09:49 AM |
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Another thing is most calipers need to be bled while in a certain orientation (normally with the bleed nipple at the top). This orientation is
sometimes not how they are mounted on the car (particually with kit cars), so you may need to take the calipers off and hold them in the correct
position to bleed them (don't forget to fit a block of wood or something inside to stop the piston poping out )
---------------------------------------
Build Diary: http://bills-locost.blogspot.com/
Web Site: http://locost.atspace.com
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m8kwr
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| posted on 6/1/09 at 09:52 AM |
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Mr Whippy - thanks for your input, i will check seal etc and do what you said.
As for the Viscosity - i would assume it would be the index, as temperature was mentioned - apparently the brake system gets very hot.
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Mr Whippy
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| posted on 6/1/09 at 10:12 AM |
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quote:
Originally posted by m8kwr
As for the Viscosity - i would assume it would be the index, as temperature was mentioned - apparently the brake system gets very hot.
The upper working temperature won’t be relevant unless the car is thrashed or used for racing, only then might fancy fluids be required. Since the car
is fitted with jag brakes I doubt they are particularly struggling anyway so just use the Dot stuff, note too that some oils and silicone fluids
destroy rubber seals.
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bartonp
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| posted on 6/1/09 at 12:06 PM |
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Some seals don't suit 'silicone' fluid (whatever DOT that may be) - it causes skipping of the seal intermittently and loss of
fluid-tightness, usually on the first push but can be anytime. Jag XJS cylinders are one of those (ask me how I know!)
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britishtrident
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| posted on 6/1/09 at 01:45 PM |
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quote:
Originally posted by m8kwr
Does anyone know what brake fluid has a high viscosity index?
Really won't make any difference.
A few things could cause it.
If the car has non standard calipers or caliper brackets fitted then it is more than likely that the calipers are not mounted 100%
"square" to he disc surface.
The other main possibility is seized piston in one of the calipers -- check each piston in the caliper is moving freely.
The master cylinder can easily be tested by discconecting the brake pipes from the cylinder and screwing bleed nipples in -- give a quick bleed, if
after bleeding the pedal is rock solid even when pressed very slowly and gently the master cyilinder is OK.
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britishtrident
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| posted on 6/1/09 at 08:40 PM |
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quote:
Originally posted by bartonp
Some seals don't suit 'silicone' fluid (whatever DOT that may be) - it causes skipping of the seal intermittently and loss of
fluid-tightness, usually on the first push but can be anytime. Jag XJS cylinders are one of those (ask me how I know!)
Good old recuperating valve seal failure quite common on Lucas-Girling master cylinders of the 1980s & 1990s. Test is to slowly put very gently
pressure on the pedal if the pedal sinks very slowly then the recuperating valve seal is banjaxed. It Won't show up if you press the pedal hard
or quickly.
really a wear & tear problem it usually occurs over 80,000 miles especially after work has been done on the brakes such as changing the pads.
Many older cars have the fault but the vast majority of drivers don't notice it and most MOT tester don't know about it.
[Edited on 6/1/09 by britishtrident]
[I] “ What use our work, Bennet, if we cannot care for those we love? .”
― From BBC TV/Amazon's Ripper Street.
[/I]
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bartonp
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| posted on 7/1/09 at 04:40 PM |
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quote:
Originally posted by britishtrident
quote:
Originally posted by bartonp
Some seals don't suit 'silicone' fluid (whatever DOT that may be) - it causes skipping of the seal intermittently and loss of
fluid-tightness, usually on the first push but can be anytime. Jag XJS cylinders are one of those (ask me how I know!)
Good old recuperating valve seal failure quite common on Lucas-Girling master cylinders of the 1980s & 1990s. Test is to slowly put very gently
pressure on the pedal if the pedal sinks very slowly then the recuperating valve seal is banjaxed. It Won't show up if you press the pedal hard
or quickly.
really a wear & tear problem it usually occurs over 80,000 miles especially after work has been done on the brakes such as changing the pads.
Many older cars have the fault but the vast majority of drivers don't notice it and most MOT tester don't know about it.
[Edited on 6/1/09 by britishtrident]
....on a NEW master cyl?
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