Bleed brakes "correctly" FuGedaboudit!
Bleed brakes "correctly" FuGedaboudit!
12-12-2006, 09:45 AM
#3
Three Wheelin'
Originally Posted by Flying Finn
I've never done depressurization either, nor have problems. I'm little lost here, how do you actually depressurize it?
Haven't had any brake problems whatsoever and, yes, I do have ABS/ABD.
I used the Motiv Power Bleeder and ATE super blue. The poifect single-guy-DIY combo IMHO.
Gerry
12-12-2006, 09:59 AM
#5
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Originally Posted by Martin S.
Santa Claus brought me an early present, Porsche 993, The Essential Companion by Adrian Streather. Very informative 640 page book, full of 993 arcana that every 993 geek may need access to, sometime.
On page 116, Adrian writes about bleeding the brakes for “993s fitted with hydraulic boost, ABS and ABD.” Here he writes, “Depressurize booster circuit before changing the brake fluid. A pressure of up to 180 bar (2,610 psi) is present in the system.”
Yes, I did a search and discovered feedback from Adrian, "You do not mention if they bled the hydraulic accumulator and brake booster. The hydraulic pump has nothing to do with the master cylinder. There is no direct pressure/fluid link between the MC and the brake booster.
The hydraulic pump pressurises the accumulator which in turn provides pressurised fluid to the brake booster onto which the MC is mounted. There is a mechanical link between the booster and the MC to provide the power assist.
Using the hydraulic pump refills the accumulator, fills the brake booster and raises the pedal, but does not remove the air.
If the boost cct is not bled properly the pedal will be low and feel soft and using the pump only provides a temporary fix.
A lot of people believe that if you bleed the PDAS cct you also bleed the accumulator/ brake booster cct, this is not true. There is a bleed nipple on the accumulator.
My first recommendation would be to ensure the accumulator and brake booster are bled again and all air removed. You must ensure the fluid level is maintained throughout the process otherwise you are wasting your time. You must ensure that the far right chamber on the brake fluid reservoir remains full at all times during this process. If the fluid is sucked down and it's emptied, air follows really quickly. The accumulator holds around 300cc of fluid.
Ciao,
Adrian.
There is a very for me what seems to be an elaborate protocol to follow to accomplish this depressurization. I must confess I believe my car has hydraulic boost, but know it has ABS and ABD. I have never done this depressurization…I have bleed and have had my brakes bleed on many occasions. I have never seen this done. Adrian implies it is a critical step in brake bleeding. Hmmmmmmmmmmm. What about you?
On page 116, Adrian writes about bleeding the brakes for “993s fitted with hydraulic boost, ABS and ABD.” Here he writes, “Depressurize booster circuit before changing the brake fluid. A pressure of up to 180 bar (2,610 psi) is present in the system.”
Yes, I did a search and discovered feedback from Adrian, "You do not mention if they bled the hydraulic accumulator and brake booster. The hydraulic pump has nothing to do with the master cylinder. There is no direct pressure/fluid link between the MC and the brake booster.
The hydraulic pump pressurises the accumulator which in turn provides pressurised fluid to the brake booster onto which the MC is mounted. There is a mechanical link between the booster and the MC to provide the power assist.
Using the hydraulic pump refills the accumulator, fills the brake booster and raises the pedal, but does not remove the air.
If the boost cct is not bled properly the pedal will be low and feel soft and using the pump only provides a temporary fix.
A lot of people believe that if you bleed the PDAS cct you also bleed the accumulator/ brake booster cct, this is not true. There is a bleed nipple on the accumulator.
My first recommendation would be to ensure the accumulator and brake booster are bled again and all air removed. You must ensure the fluid level is maintained throughout the process otherwise you are wasting your time. You must ensure that the far right chamber on the brake fluid reservoir remains full at all times during this process. If the fluid is sucked down and it's emptied, air follows really quickly. The accumulator holds around 300cc of fluid.
Ciao,
Adrian.
There is a very for me what seems to be an elaborate protocol to follow to accomplish this depressurization. I must confess I believe my car has hydraulic boost, but know it has ABS and ABD. I have never done this depressurization…I have bleed and have had my brakes bleed on many occasions. I have never seen this done. Adrian implies it is a critical step in brake bleeding. Hmmmmmmmmmmm. What about you?
Martin,
You are mixing up 993 information and 964 information. The paragraph you have posted in blue has NOTHING to do with the 993. You lifted it from the 964 forum and the 964 hydraulic brake boost system is far more comkplicated than the 993.
If you have a 993 with a hydraulic brake boost circuit (as opposed to the vacuum boost), before you bleed the brakes you make sure the boost circuit is depressurised by pumping the brake pedal until it goes hard.
There is NOTHING complicated about this at all and is just a precaution really. Something you should do to make sure nobody can depress the pedal when you have the brake calliper nipples open.
Ony the 993 Carrera 4, Turbos and RS have the hydraulic brake boost system.
ABD is available as an option with the vacuum boosted 993 Carrera.
Just repeating that all the 993 Carreras including the Carrera S uses vacuum boost.
ABS is standard on all 993s.
Ciao,
Adrian.
PS: I think it might be a good idea to delete this thread Martin. People will read what you have posted long before they get to this reply and they will be very confused. Mixing one model's system with another can cause great uneccesary concern.
Last edited by Adrian; 12-12-2006 at 10:16 AM.
12-12-2006, 07:14 PM
#6
Rennlist Member
I did this recently on my RS. The pressure accumulator is a tad difficult to get at with the 92 L fuel tank. I did depressurize it as the workshop manual suggested, although there was a section in the manual about depressurising with the brake pedal. What is the difference between the 2 procedures?
Harry
Harry
12-12-2006, 07:20 PM
#7
Weathergirl
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No, don't delete, since it has this nugget of gold from Adrian, which clarifies a topic in his book nicely:
Good to know.
There is NOTHING complicated about this at all and is just a precaution really. Something you should do to make sure nobody can depress the pedal when you have the brake calliper nipples open.
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12-12-2006, 08:17 PM
#10
Rennlist Member
By the way, if you have the hydraulic brake booster, I think it may be a good idea to depressurize the accumulator as per the manual insruction by turning the key to position 1 and waiting til the whirring of the pump stops. Then, remove the electrical connection to the pump and then bleed the accumulator. The reason why I say this is if the brake pumping method to depressurise makes depressing the brake pedal difficult during the bleeding procedure you may not be able to bleed the brakes by depressing the brake pedal that easily (?) - I've never tried the brake pedal depressurising method so perhaps Adrian can educate me. Despite the power bleeder, one of my calipers did not bleed so I used my hand to partially depress the brakes (disconnect the bleeder 1st) and the brake fluid came out quite easily. Be careful not to fully depress the brake pedal during the process....
Harry
Harry
