Vacuum Line(s) question for Rear Brakes
10-31-2002, 03:16 PM
#1
Nordschleife Master
Thread Starter
Vacuum Line(s) question for Rear Brakes
84 S2 Auto 4 spd -
I replaced the grommets on the Master Cyclinder and bled everything accordingly (MC,RR,LR,RF,LF)but my Rear Brakes are not grabbing as they should; so I think I may have a vacuum line issue. Currently there are two vacuum lines, entering the engine compartment, that are below the Brake Master Cyclinder. One line is yellow and the other line is black. The Yellow line connects to another vacuum line which goes into the firewall and the other vacuum line connects to the secondary inlet for the Brake Booster. Does anyone know if this configuration is correct?. Also, when I first start my shark, the brake warning lamp comes on but immediatley goes out when I depress the brake pedal. (does not come on until I start the car again) Fluid level checks out and my pads are good to go. One last thing, I have bled the system twice and still the same issue.
Any advice would be appreciated! -
Thanks everyone - Happy Halloween
I replaced the grommets on the Master Cyclinder and bled everything accordingly (MC,RR,LR,RF,LF)but my Rear Brakes are not grabbing as they should; so I think I may have a vacuum line issue. Currently there are two vacuum lines, entering the engine compartment, that are below the Brake Master Cyclinder. One line is yellow and the other line is black. The Yellow line connects to another vacuum line which goes into the firewall and the other vacuum line connects to the secondary inlet for the Brake Booster. Does anyone know if this configuration is correct?. Also, when I first start my shark, the brake warning lamp comes on but immediatley goes out when I depress the brake pedal. (does not come on until I start the car again) Fluid level checks out and my pads are good to go. One last thing, I have bled the system twice and still the same issue.
Any advice would be appreciated! -
Thanks everyone - Happy Halloween
11-05-2002, 06:24 AM
#2
Pro
My car always have the brake warning lamp lighting when I start it until I press the brake. I always considered this an extra check of the brake warning lights built into the system?
Ian
Ian
11-05-2002, 08:15 AM
#3
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How are you judging that the rear brakes are not doing their share? This can be quite difficult to do without a rolling road to check braking effort of each wheel/axle. I doubt it is a vacuum issue - if vacuum was not getting to the booster, you need Schwarzenegger legs to stop it. Also it doesnt take much braking to stop you from turning a lifted wheel by hand, so thats not much of a test. If you knew which end locked up first under heavy braking before the work, and now its changed, thats a fair test, but....
jp
jp
11-05-2002, 08:27 AM
#4
Burning Brakes
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Drew,
I sounds like you have air in the braking system for the rear wheels - It has nothing to do with the vacuum hoses you mentioned, they are for the vacuum actuators for climate control and for the cruse control - Bleed the rear brakes again.
The brake light when you start-up is normal, at least if you mean the text up-left, that's for check of the stop light
I sounds like you have air in the braking system for the rear wheels - It has nothing to do with the vacuum hoses you mentioned, they are for the vacuum actuators for climate control and for the cruse control - Bleed the rear brakes again.
The brake light when you start-up is normal, at least if you mean the text up-left, that's for check of the stop light
11-05-2002, 01:20 PM
#5
Nordschleife Master
Thread Starter
Hey everyone -
I thought the rear brakes weren't working properly because of the noticeable surface rust. I bled the system again and there definately is no air in the system.
What Ive noticed when braking is this....
Upon gradual stops, it seems the front grab 85% and the rear grab 15%. Upon hard stops, it levels out 50/50. I say 50/50 because when I hard brake, (the first couple of times) the pads/rotors smoked like an expensive cigar. (no more surface rust).
I know the front stop 3/4 of the vehicles mass/velocity, so would it be safe to say that the rear won't engage equally to the front unless used in a hard braking situation or should all four grab equally?. Other than the vacuum question, I think she's good to go but I'm still a little concerned about the brake warning light which will not go out until I depress the pedal. Thanks again everyone for the feedback.
I thought the rear brakes weren't working properly because of the noticeable surface rust. I bled the system again and there definately is no air in the system.
What Ive noticed when braking is this....
Upon gradual stops, it seems the front grab 85% and the rear grab 15%. Upon hard stops, it levels out 50/50. I say 50/50 because when I hard brake, (the first couple of times) the pads/rotors smoked like an expensive cigar. (no more surface rust).
I know the front stop 3/4 of the vehicles mass/velocity, so would it be safe to say that the rear won't engage equally to the front unless used in a hard braking situation or should all four grab equally?. Other than the vacuum question, I think she's good to go but I'm still a little concerned about the brake warning light which will not go out until I depress the pedal. Thanks again everyone for the feedback.
11-05-2002, 01:35 PM
#6
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There are four brake warning lights on most 928s.
Parking Brake - on when the brake handle is up.
Brake Light - Comes on with the ignition, goes off the first time the brake pedal is pushed - if the brake lights are OK.
Brake Pressure - Comes on if the pressure is different in the two brake hydraulic circuits (earlier cars only).
Brake Pads - Comes on if one of the pad sensors is worn enough to break the wire loop.
Which one is your problem?
Parking Brake - on when the brake handle is up.
Brake Light - Comes on with the ignition, goes off the first time the brake pedal is pushed - if the brake lights are OK.
Brake Pressure - Comes on if the pressure is different in the two brake hydraulic circuits (earlier cars only).
Brake Pads - Comes on if one of the pad sensors is worn enough to break the wire loop.
Which one is your problem?
11-07-2002, 12:04 AM
#7
Nordschleife Master
Thread Starter
Wally - You the MAN and you deserve a fat Raise!!!
The warning lamp which illuminates and stays on until I depress the brake is ...drum roll please......
BREMSLICHT.
From your reply, I gather this is F.A.D. (Functioning as Designed)
Thank You Wally!
<img src="graemlins/beerchug.gif" border="0" alt="[cheers]" />
The warning lamp which illuminates and stays on until I depress the brake is ...drum roll please......
BREMSLICHT.
From your reply, I gather this is F.A.D. (Functioning as Designed)
Thank You Wally!
<img src="graemlins/beerchug.gif" border="0" alt="[cheers]" />
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11-11-2002, 12:48 PM
#8
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So, are your front/rear brakes sharing the load properly Drew?
Anyone know a simple test to determine that the rears are doing their share?
Over the years I'm going through approx. 3 sets of front pads to each set of rears. Seems like the rears should be doing more work than they currently are.
Anyone know a simple test to determine that the rears are doing their share?
Over the years I'm going through approx. 3 sets of front pads to each set of rears. Seems like the rears should be doing more work than they currently are.
11-11-2002, 12:53 PM
#9
Nordschleife Master
Thread Starter
I would say - no.
It seems the fronts handle more than the rears. In my research I haven't been able to determine the load balance, so I assume that this is how it is. 3/4 front 1/4 rear (just like a motorcycle)
It seems the fronts handle more than the rears. In my research I haven't been able to determine the load balance, so I assume that this is how it is. 3/4 front 1/4 rear (just like a motorcycle)
11-11-2002, 02:55 PM
#10
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[quote]Originally posted by Randy V:
<strong><...>
Anyone know a simple test to determine that the rears are doing their share?
Over the years I'm going through approx. 3 sets of front pads to each set of rears. Seems like the rears should be doing more work than they currently are.</strong><hr></blockquote>
Nothing like a wet or icy offramp at speed, stand on the wide pedal and see which end of the car smacks the guardrail first...
Looks like the differences between fronts and rears are mostly limited to piston size. Looks like the pads are different size too, but not by much.
Based on the piston diameters (looking at my S4 specs...) the fronts have about 65% of the total available piston area. The fronts have 59% of the total available pad friction area. The difference in effective rotor diameter is negligible. There's more to the calculation than these factors, of course.
In addition to that basic stuff, the factory includes a pressure limiting valve on the rear brakes. When brake system pressure exceeds the threshold of the valve, no addition rear braking is available. Several list threads have been devoted to the relative merits of swapping in larger valves to give additional rear brake support. My opinion is that you should not even consider a bigger valve for a street car unless you have working ABS. The factory valve is 18bar, and there are 33bar and I think 55bar available readily from the usual suspects. Unless you are driving at the limit on dry pavement, you might never know the difference. I'm sure there are more opinions. (see guard rail bias test in the first paragraph before responding...
)
My front and rear pads are wearing pretty evenly, by the way. The original pads lasted less than 30k, of which 22k was the original owner. In contrast, the metal-master replacements have the same miles on them now, and are about half gone. Differences in driving style I'm sure. PO: "I can't keep my foot out of it!" dr bob: "Tach seldom sees 3000 rpm's, would really like a 2.20 diff to lower the cruising RPM's, etc."
<strong><...>
Anyone know a simple test to determine that the rears are doing their share?
Over the years I'm going through approx. 3 sets of front pads to each set of rears. Seems like the rears should be doing more work than they currently are.</strong><hr></blockquote>
Nothing like a wet or icy offramp at speed, stand on the wide pedal and see which end of the car smacks the guardrail first...
Looks like the differences between fronts and rears are mostly limited to piston size. Looks like the pads are different size too, but not by much.
Based on the piston diameters (looking at my S4 specs...) the fronts have about 65% of the total available piston area. The fronts have 59% of the total available pad friction area. The difference in effective rotor diameter is negligible. There's more to the calculation than these factors, of course.
In addition to that basic stuff, the factory includes a pressure limiting valve on the rear brakes. When brake system pressure exceeds the threshold of the valve, no addition rear braking is available. Several list threads have been devoted to the relative merits of swapping in larger valves to give additional rear brake support. My opinion is that you should not even consider a bigger valve for a street car unless you have working ABS. The factory valve is 18bar, and there are 33bar and I think 55bar available readily from the usual suspects. Unless you are driving at the limit on dry pavement, you might never know the difference. I'm sure there are more opinions. (see guard rail bias test in the first paragraph before responding...
) My front and rear pads are wearing pretty evenly, by the way. The original pads lasted less than 30k, of which 22k was the original owner. In contrast, the metal-master replacements have the same miles on them now, and are about half gone. Differences in driving style I'm sure. PO: "I can't keep my foot out of it!" dr bob: "Tach seldom sees 3000 rpm's, would really like a 2.20 diff to lower the cruising RPM's, etc."