Does anyone have any input on whether hydraulic flushing is best done under positive pressure(ie. Motive Prod. 10psi) or via a suction system (ie. Mytivac). Better cleaning of the system one way or the other?
RE: Hydraulic Flush, Positive or Negative Pressure?
My experience in doing this on all kinds of cars and trucks for 25 years is right back to the old addage:
Nature abhors a vacuum
(and after years of screwing with them, so do I)
As far as a relation to better cleaning of the system goes, anything that moves fluid will accomplish about the same thing. Either way I think we're talking maybe 7 psi differential pressure, so moving fluid at about the same velocity whether using pressure or vacuum.
The only relation I have to better cleaning of the system is that the EASIER it is to do, the more fluid one is likely to flush through there. And the more fluid is flushed through, the better the cleanout of the system will be.
Bleeding/flushing hydraulics is a crummy job, especially when you're talking digging up in there next to the gearbox to get at the clutch bleeder. Anything that makes it easier is better for me. I've hasseled with any number of suction methods and they were all more difficult to do than the pressure system from MPP.
One small risk is that if there's any kind of leak in the system, you risk drawing in air when you use suction methods. This is probably more of a concern on the locker system where there are a number of pipes and hoses between the reservoir and the master cylinders, but any brake system seal that leaks a little could do teh same. Putting a suction on the system COULD POTENTIALLY draw air in, though admittedly it's a small risk, pressure flushing eliminates even that small risk.
Anyway, that's my two cents.
-Dave G.
Editing now......
YIKES!
Just noticed that you're talking new 463 with ABS. My input to folks doing these is to always let the dealer do it. The super tight tolerances inside of the ABS valving, pumps, etc make them EXTREMELY intolerant of ingress of dirt or foreign material of any kind. Those things are really expensive, not to mention the risk to human life if they malfunction. With those new systems I always recommend going by the book for recommended frequency of flush, and letting the dealer do the job.
RE: Hydraulic Flush, Positive or Negative Pressure?
Thanx for the concern, but I was thinking more about the 1980 280GE I just got which is presently on the truck enroute to its new home (in my garage.[:D])
Brakes are a big deal. When you put your foot on it, you really want to stop the damn thing.
I've been fiddling with the vacuum thingy's also with the old fashioned push and release technique. Once you've got a decent power bleeder working on over-pressure they're a thing of the past.
You simply hook it up, put her in first gear, disengage handbrake and start bleeding.
Older vehicles can do with the constant over-pressure system, newer vehicles with ESP, ABS, etc... need an electronic system which pulsates your hydraulics.
Mityvac is great for checking false air intake on small 2 stroke engines.
But brake bleeding is not it's strongest point.
__________________
Chris
Toy HDJ 80 1994
Merc GD300 1986
If its a 460 you dont need anything but what youve got other than clean fluid and tools, small plastic pipe and a jam jar, BASIC.
Syphon off all the brake fluid, fill to maximum and replace the cap. Open the bleed nipple furthest from the master attach bleeder tube and insert into jar to bottom . Use a handy tool such as a wife or girlfriend to depress the brake pedal smoothly and fully and count how many strokes equalls the volume of fluid replaced. Refill as required and watch as dirty flud is expelled, shout to hold pedal fully depressed and lock nipple.
Repeat on all nipples 1 each side at back and 3 each side at front on two pipe callipers or two nips on single pipe calipers. The contents of a litre or quart bottle more than ample for the brakes.
Lastly dispose carefully of the fluid expelled to the appropriate waste system.
Wash off the splashed areas and replace bleead caps. Refill cy7linder to mark and force test pedal for pedal creep and unexplained softening with engine running.
I dont recomend pressure as the header tank and large bore rubber hoses are not so designed, it is possible to destroy the master cylinder rubbers or to affect the brake performance with pressure bleeding as it can unset seals. and can take several hours to get things back to normal.
The secret is allways to maintain asupply of fluid and steady full strokes of the pedal and a fully immersed bleeding tube.
As always, I greatly respect Dave's experience, but is there any of you who DO flush your own brakes which have ABS?
With the G55, S420, and E320, that's alot of trips to the dealer (especially since I like to change the fluid once a year; the exessive heat and humitiy here quickly take their toll on brake fluid.) I much prefer DIY.
Point respected with info past 460 models, but would add that using the previously mentioned method to bleed all brakes had no difficulty with my Merc 124 estate after a complete repipe.
I would also note that the ignition was off, but would suggest that having the engine on and all circuits engaged the action of a working servo would blast the fluid through the system using the pump and hold method.
Do take great care of the tandem master cylinders on Mercs they are not as robust or failure free as might be anticipated and can be prone to blockage of the small breather hole from the uncompressed cylinder to the fluid tank. If this blocks it can be difficult to clear and can cause failure of the cylinder to operate as a split/reserve brake system.
If you have any doubts on component quality replace it before you have a accident.
Cleanliness and attention to detail are essential, whilst full G books are difficult to get any of the common Mec repair books will cover all that is needed as the systems are "Common family with exeption of the balence valve and the small pressure retention valve in the rear pipe line.
Suggest either a 123 or 124 book as reference to cover ABS.
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