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MECHANICALS With a 30 year-old automobile, there will always be discussion here - maintenance, modifications and mechanicals.

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  #1  
Old 24th September 2008, 05:43 AM
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Styria Styria is offline
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Default Changing Brake Fluid

It is recommended that the brake system should be flushed at least once a year. Apparently, the fluid attracts moisture, and therefore affects the efficiency of the calipers and various other components in the system.

To be honest, I had never really followed the recommended procedure as I felt that 'okay, I can see the brake fluid, I haven't lost any and therefore everything must be alright'.

Well, that's the theory. It isn't until, for instance, you open up calipers and when you see the gel-like substance and the crud around the caliper piston dust seal, that you appreciate the importance of changing and flushing out the old fluid. Mind you though, I do seriously doubt that flushing the system is going to get rid of the gel in the caliper housing. Regards Styria
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Old 24th September 2008, 05:53 AM
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Default Re: Changing Brake Fluid

Quote:
Originally Posted by Styria View Post
Mind you though, I do seriously doubt that flushing the system is going to get rid of the gel in the caliper housing. Regards Styria
What is the option then?
Take the calipers apart?
Or pressure clean them on the car?
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Old 24th September 2008, 06:05 AM
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Default Re: Changing Brake Fluid

Hi Michel, the way I see it, only complete dismantling off the car will clean them out. If you look at it realistically, there is a helluva lot of dirt and crud that will call calipers their home, and just because they're out of sight, they tend to become neglected.

Best thing to do is to check the caliper seals every time the pads are changed which, in my experience, occurs at about 30k. kilometer intervals with my kind of driving. It is also a good practice to blow out the calipers from time to time to get rid of accumulated brake dust. Regards Styria
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Old 24th September 2008, 07:31 AM
BenzBoy BenzBoy is offline
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Default Re: Changing Brake Fluid

I read your post with interest Styria -as a matter of course I change my brake fluids on everything at least every two years which is the Benz and R-R recommendation. As you say, this probably does not remove all the gunk but the way I see it, if the fluid is regularly chagned, the amount of gunk build up should be less in the first place. Of course the Phantom never needs a change at all. Just adjust the rods and the servo and cross fingers no-one cuts too close in front.....
Regards,
BenzBoy
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Old 24th September 2008, 08:42 AM
John S John S is offline
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Default Re: Changing Brake Fluid

Itís the water absorption and gunk in the calipers that causes the problems, so a regular bleed before the problems build up is the way to go. However by changing over from the Dot type hydroscopic brake fluids to silicone brake fluid a lot of problems can be eliminated.
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Old 24th September 2008, 10:57 AM
Lukas Lukas is offline
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Default Re: Changing Brake Fluid

I wasn't aware of the existance of silicone-based fluids, so thanks for that. Are there any compatability issues between those and braking systems of older cars
(say a w116 perhaps)?
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Old 24th September 2008, 01:35 PM
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Default Re: Changing Brake Fluid

2 years is the M-B recommendation and if you do that regularly with M-B Dot 4+ there should be little crud present and all the water which will lower the boiling point is also removed.

Silicon Fluid was used by the US Military and was thought to be the answer for systems held in storage and cars which cover low mileages and was all the rage about 25 years ago but it has three problems.

1) It is incompatible with most of the Dot 3 and 4 fluids and it is not advisable to use it in a system unless it is new.

2) It does not absorb water so any water that accumulates in the system is at liberty to corrode whatever it comes in contact with.

3) It has a small amount of compressibility and is not an incompressible fluid like Dot 3or 4 so will produce a soft feeling pedal.

I can produce a reference that discusses these points if needed but Silicone Fluid is best avoided as far as I am aware.

Bill
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Old 24th September 2008, 02:06 PM
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Default Re: Changing Brake Fluid

I believe the silicone based stuff is probably the best but its only usable on a "new" brake system, one in which all brake fluid has been flushed, dried out, etc. this includes all the rubber components in the calipers and master cyl, hoses, etc. Those that use it on race cars that I know are very pleased with it.

As for the need to change brake fluid regularly, as already stated, its hygroscopic (absorbs water) which over time lowers its boiling point... so after 5 or 10 years the boiling point might be so low that hard braking generates enough heat to boil the fluid even at moderate speeds (admittedly this is an extreme example).

Also the absorbtion of moisture tends to - over a period of years - cause localised pitting in the surface of the pistons.

Best to just change the fluid every 2 years. I couldn't trust who i bought whitey the race car from and while the brakes felt OK, I just cleaned out and rebuilt the calipers (they don't need to be taken apart) and let all the old fluid leak out, put in new fluid and now its perfect.

Ian.
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Old 24th September 2008, 04:07 PM
John S John S is offline
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Default Re: Changing Brake Fluid

If the old Dot grade fluid is flushed out properly with the silicone fluid there is no compatibility problems. Silicone fluid does not absorb water so there is much less likelihood of moisture entering the braking system to cause problems than when using Dot graded fluids. Another minor advantage is that silicone fluid does not damage paintwork.
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Old 24th September 2008, 05:53 PM
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Default Re: Changing Brake Fluid

Ok, most of the above is correct, but to clarify, regular brake fluid (not silicone) is hydroscopic, which does indeed mean it absorbs water.
How did water get in there?
It absorbs the moisture in the atmosphere, because the cap on the reservoir is vented - usually a little hole in the top - and that is all it takes.
Brake fluid in a well maintained and leak-free system will still get dirty over time, and if you have ever flushed a car out that has not been serviced regularly - I have seen brake fluid almost black - you will see a change in colour.
Again this dirt is absorbed with the moisture.
If there is 'crud' INSIDE the calipers, you have a major problem, and they will need a complete strip and rebuild with new seals.
I suspect you were refering to the stuff on the OUTSIDE of the caliper body, the dust and rubbish that collects behind the brake pads.
This is normal build up and can easily be washed down with brake cleaner.
It is worth checking there is no sign of fluid around the seals at this point, and certainly it is worth cleaning all the surfaces and the locating pins, as even a little build up can cause sticking pads or a spongy pedal as the pressure is having to compress other material before the brake pad moves, and this can cause longer travel.
Got those alloy wheel brake dust protection discs?
Your choice, and they certainly keep the wheels cleaner, but they also restrict airflow, and your brakes will run hotter, so, particularly if you are in hilly country, or tow a lot, get rid of them.
Another important tip is, despite what well meaning car maintainence books and NRMA advice guides may say, you should never 'top up' brake fluid!
The difference between the MIN and MAX marks on the reservoir are tha amount lost down the system due to pad wear - and hence the pistons are further along the caliper bores to compensate.
If fluid is low, it is for two reasons only;
1 - your pads are worn and need changing, and with new pads fitted the fluid would come back to the MAX mark again - except - you are changing the fluid at the same time anyway, aren't you?
2 - you have a leak - investigate urgently!
One other point, I have found a lot of garages and servicing outlets are now using the vacuum type bleeders, which suck fluid down from the calipers.
If it works for you, fine.
I have always used, and still prefer the pressure bleeder which attaches to, and pressurises the reservoir, for the simple reason that the vacuum type will only (with a good vacuum) one BAR, that is 14 PSI, which is often not enough to pull any trapped air bubbles, that will sit at the highest point in any bends in the hoses and pipes.
Apressure bleeder can often shift these at 25 or 30 PSI.
There are potential hazards with pressure bleeders though, so be careful;
reservoirs have been known to pop off master cylinders, due to weak locating grommets, and you have to be careful to disconnect the pressure supply BEFORE opening the reservoir cap, and then you have to syphon fluid out down to the MAX line.
The risk of spillage is I guess why the trade have gone over to the vacuum type.
And finally, I lost the pedal completely on my 280E coming down the MacQuarie Pass a few years back due to old (and subsequently boiling) brake fluid. I had only had the car a couple of weeks, but that is no excuse from somone who used to be employed by Midas (in th UK) twenty-five years ago specifically to train staff in brake servicing!
That will do for now,
Chris M.

Last edited by GreaseMonkey; 24th September 2008 at 05:56 PM.
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Old 24th September 2008, 06:07 PM
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Default Re: Changing Brake Fluid

Thanks Chris for a good description. I too prefer the pressure bleed arrangement. I made up adapters to rig up to my air compressor. It allows one-man bleeding of a complete system in about 30 minutes, and the results are perfect, every time.
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'76 W116 6.9 euro - trusty rusty
'78 W116 6.9 euro - the AMG
'74 W116 350SE euro - the stand-in ride
'82 W126 500SEL euro - full hydraulics - the limo
'83 FORD XR3 - the beast

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Old 24th September 2008, 10:31 PM
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Default Re: Changing Brake Fluid

Quote:
Originally Posted by s class View Post
Thanks Chris for a good description. I too prefer the pressure bleed arrangement. I made up adapters to rig up to my air compressor. It allows one-man bleeding of a complete system in about 30 minutes, and the results are perfect, every time.
Indeed.....
You make me wanna flush all my cars..... (the brakes I mean)
One by one...
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Old 25th September 2008, 11:32 AM
Lukas Lukas is offline
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Default Re: Changing Brake Fluid

Indeed, thanks to all, sounds like DOT fluid is the safe bet for novices like me then.

Re the pressurised bleeding - is that an adaptor that sits over the hole in the master cylinder where you ordinarily add the fluid (and applies compressed air through that point), or does it attach somewhere else?

Last edited by Lukas; 26th September 2008 at 10:37 AM. Reason: Typos
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Old 25th September 2008, 03:19 PM
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Default Re: Changing Brake Fluid

Lukas, you got it. Its really as simple as that. I fill up the reservoir with fluid to the brim, connect the pressure, and slowly crank it up to about 2.5 bar. THen I crack open the bleeder screws one at a time. Just keepan eye on the reservoir level, cause it goes down quickly. WHen you need to top up, just lower the pressure back to zero before disconnecting, else you get a brake fluid shower..

I've found that above 2.5 bar, there is serious danger of the reservoir blasting itself of the M/C.
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'80 W116 280SE euro - the ride
'76 W116 6.9 euro - trusty rusty
'78 W116 6.9 euro - the AMG
'74 W116 350SE euro - the stand-in ride
'82 W126 500SEL euro - full hydraulics - the limo
'83 FORD XR3 - the beast

0 benzes on stands, 5 RUNNING
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  #15  
Old 25th September 2008, 05:48 PM
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Default Re: Changing Brake Fluid

Or get yourself a Gunson EeziBleed kit.
Google it for suppliers, or email me as I may still have supplier's details.
Cost about $70, but worth it once you've used it a few times.
It consists a half litre plastic bottle that holds brake fluid, a set of different m/c reservoir caps to suit most cars, including one that fits Mercedes, and an air pressure hose that clips on to the valve on your spare tyre, which presumably is inflated to around 30PSI.
All instructions included, easy to use, check for leaks when you first use it, as you don't want brake fluid spraying all over the place.
You will also need a piece of clear plastic tubing to fit the bleed nipple, and a glass jar for the old fluid, and I recommend one of those rubber bulb on a tube type syphon things (someone will post here and tell me what it's called) to remove brake fluid from the top of the reservoir to get the level back down to the MAX mark when you've finished.
Good luck,
Chris
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