Brake Proportioning Valve Question
07-31-2005, 08:50 AM
#2
Kelly,
Should be very easy, need to remove the old one and replace it with a machined straight through fitting. Only problem is that I cant remember where it is on the 993. I will find it and let You know. You will find that the car has much more of a tail happy/trail brake attitude if You do it. Not necessarily bad btw but can create havoc for an unprepared driver...
Hope this finds You well!
Glen
Should be very easy, need to remove the old one and replace it with a machined straight through fitting. Only problem is that I cant remember where it is on the 993. I will find it and let You know. You will find that the car has much more of a tail happy/trail brake attitude if You do it. Not necessarily bad btw but can create havoc for an unprepared driver...
Hope this finds You well!
Glen
07-31-2005, 01:20 PM
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Kelly:
The 993's PV's are on the ABS unit. There are two of them. You would need to bypass both valves as each one goes to one rear wheel caliper. FWIW, there are two pressure rated versions found on 993's, a 40BAR as on C2's and a 55 BAR found on the C4S and turbos. To bypass, you would need what Glen mentioned, a male/female brake line flare straight through.
I considered playing with the idea of swapping/removing the PV's on my turbo but abondoned the idea onve I learned of posible issues if keeping the stock brake booster/ABS system. From what I learned when looking into this, the PV is a specific value as tuned into various aspects of the braking system, most importantly the ABS computer progrmamming. Most of the brake "gurus" felt it better to tune the system with pad compounds if I wanted to keep the ABS system working properly at the "limit". Most folks suggested that if I played with the PV's, I also needed to look at the ABS program to also match. I did this and looked into other ABS computer boards from stock P-Car and the motorsports ones offered. The various ABS computers were big $$$ and too much for me o consider as a hobbiest...
$0.02
The 993's PV's are on the ABS unit. There are two of them. You would need to bypass both valves as each one goes to one rear wheel caliper. FWIW, there are two pressure rated versions found on 993's, a 40BAR as on C2's and a 55 BAR found on the C4S and turbos. To bypass, you would need what Glen mentioned, a male/female brake line flare straight through.
I considered playing with the idea of swapping/removing the PV's on my turbo but abondoned the idea onve I learned of posible issues if keeping the stock brake booster/ABS system. From what I learned when looking into this, the PV is a specific value as tuned into various aspects of the braking system, most importantly the ABS computer progrmamming. Most of the brake "gurus" felt it better to tune the system with pad compounds if I wanted to keep the ABS system working properly at the "limit". Most folks suggested that if I played with the PV's, I also needed to look at the ABS program to also match. I did this and looked into other ABS computer boards from stock P-Car and the motorsports ones offered. The various ABS computers were big $$$ and too much for me o consider as a hobbiest...
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07-31-2005, 01:45 PM
#4
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Glen and Jim.. thank you both for your input. As you might have guessed, I'm looking to do this as a brake caliper switch is in my near future from a TT and have read where removing/bypassing the PV was an easy/cheap way to regain the "proper" f/r balance. I know the RS rears are the way to go, but I don't track the car and will not be using the car at it's limits or abusing the brakes, hence looking for a cost effective solution.
Heck, I may not even need to reproportion the balance for the intended usage (heavy canyon runs, daily) but was curious as to what was involved to bypass it. I read a thread in the 964 forum where someone had taken the guts out of the valve and just used the body as the bypass fitting. Is this even possible/recommended in a 993? Sorry for the naive questions... just playing in an area I know nothing about and am trying to learn my options. Is the 993 PV valve/s located the same as the 964?
Also, if I do decide to play with the valve, do I run into the problem of having to bleed the ABS for air? If yes, can that be done w/o using a hammer? (I have no access to one.
)
Heck, I may not even need to reproportion the balance for the intended usage (heavy canyon runs, daily) but was curious as to what was involved to bypass it. I read a thread in the 964 forum where someone had taken the guts out of the valve and just used the body as the bypass fitting. Is this even possible/recommended in a 993? Sorry for the naive questions... just playing in an area I know nothing about and am trying to learn my options. Is the 993 PV valve/s located the same as the 964?
Also, if I do decide to play with the valve, do I run into the problem of having to bleed the ABS for air? If yes, can that be done w/o using a hammer? (I have no access to one.
)
07-31-2005, 06:16 PM
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Kelly:
From knowing many folks who have run the TT rears instead of RS rears with the "big red" fronts, most have achieved a nice bias by using a slightly different pad for the rear. I could offer a suggestion if I knew what brand of pad you like. For most folks doing street driving the TT F/R combo works just fine. It's a little too much front bias, but on a narrow body street only car, that might be alright for the owner.
The 993 ABS/PV setup is very similar to the 964. On an early C2 , it might be identical. It's hard to tell for sure as there are a few variants for these cars, i.e vacuum power assit, hydro assiist, etc.
If you do the PV's I would plan on a complete system bleed. The PV's are right off the ABS valve/servo, there is a good chance of trapping a bubble there.
Hope this also helps.
Regards
From knowing many folks who have run the TT rears instead of RS rears with the "big red" fronts, most have achieved a nice bias by using a slightly different pad for the rear. I could offer a suggestion if I knew what brand of pad you like. For most folks doing street driving the TT F/R combo works just fine. It's a little too much front bias, but on a narrow body street only car, that might be alright for the owner.
The 993 ABS/PV setup is very similar to the 964. On an early C2 , it might be identical. It's hard to tell for sure as there are a few variants for these cars, i.e vacuum power assit, hydro assiist, etc.
If you do the PV's I would plan on a complete system bleed. The PV's are right off the ABS valve/servo, there is a good chance of trapping a bubble there.
Hope this also helps.
Regards
07-31-2005, 06:31 PM
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Jim,
I run PBR/Axxis Ultimate brake pads. They are a bit more aggressive pad than the stock OEM Textars I was using. If I were to try tuning using different pad materials, which end should the more aggressive pad be mounted, front or rear? I've read conflicting setups.
I will, of course, try the least invasive course of action first, which would be to do nothing and see what needs to be improved as I learn the differences.
"If you do the PV's I would plan on a complete system bleed."...
would you mind defining "complete"? I would like to know if I have the resources to do this job and if it involves special tools.
thanks again for your input.
I run PBR/Axxis Ultimate brake pads. They are a bit more aggressive pad than the stock OEM Textars I was using. If I were to try tuning using different pad materials, which end should the more aggressive pad be mounted, front or rear? I've read conflicting setups.
I will, of course, try the least invasive course of action first, which would be to do nothing and see what needs to be improved as I learn the differences.
"If you do the PV's I would plan on a complete system bleed."...
would you mind defining "complete"? I would like to know if I have the resources to do this job and if it involves special tools.
thanks again for your input.
07-31-2005, 06:58 PM
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A couple clarifications about fooling with 993 brake systems:
4 channel systems have two proportioning valves, one for each rear brake circuit. If you wish to eliminate them, you can order the lines from a 993 cup, they are a little longer. All 4wd 993s and 2wd 993s with optional ABD have the 4 channel system.
3 channel systems have just the one valve.
You can get valves in all sorts of different pressures, since you can use 993, 964, 928 and 944 valves since they are all interchangable.
Basically, you can afford to run quite a bit more rear brake than stock once the car is lowered. With the lower Cg, weight transfer is less, so the rear tires don't get so unloaded when you are braking at maximum. I usually run RS calipers since the change afforded by the different valves is very small. I then choose the valve with a switching point just under the pressure which will get the car into significant ABS under straight line braking. This pressure depends on the tires and car weight etc.
Then I use the driving technique of using a great deal of pedal pressure while straight line braking, insuring that the rear brakes are in ABS. As I transition to trail braking, the pressure drops below the switch point, and the rear brakes now do somewhat less, so I have a stable rear end as I turn down toward the apex.
By doing this you can avoid the compromise of max straight line braking vs. good stability under trail braking.
I don't like to run different pad compounds front to rear to acheive bias. With this the rear brakes never release the same as fronts and it makes the final brake release (just prior to throttle application) that much more tricky. I suppose that this compound difference COULD be made into an advantage. Usually though you are looking for more rear brake, so you run an aggressive pad on the rear, which usually has a more sudden release, which isn't what you want.
To tune the ABS system itself, really the only option is to the use the motorsport control unit. The stock control unit leaves much to be desired if you are running on R tires or slicks. The motorsport unit really transforms the feel of the system. The drawbacks are that it only works on four channel systems and it costs about $1500. Also it is not a plug and play swap. There is some additional work necessary to make it work.
The bleeding of any of these systems is a whole different conversation. If no air is introduced prior to the ABS unit (such as a prop valve change or caliper swap) then bleeding is straightforward. If there is air in the ABS unit then you need a means of activating the valves in the ABS unit.
Chris Cervelli
Premier Motorsports
4 channel systems have two proportioning valves, one for each rear brake circuit. If you wish to eliminate them, you can order the lines from a 993 cup, they are a little longer. All 4wd 993s and 2wd 993s with optional ABD have the 4 channel system.
3 channel systems have just the one valve.
You can get valves in all sorts of different pressures, since you can use 993, 964, 928 and 944 valves since they are all interchangable.
Basically, you can afford to run quite a bit more rear brake than stock once the car is lowered. With the lower Cg, weight transfer is less, so the rear tires don't get so unloaded when you are braking at maximum. I usually run RS calipers since the change afforded by the different valves is very small. I then choose the valve with a switching point just under the pressure which will get the car into significant ABS under straight line braking. This pressure depends on the tires and car weight etc.
Then I use the driving technique of using a great deal of pedal pressure while straight line braking, insuring that the rear brakes are in ABS. As I transition to trail braking, the pressure drops below the switch point, and the rear brakes now do somewhat less, so I have a stable rear end as I turn down toward the apex.
By doing this you can avoid the compromise of max straight line braking vs. good stability under trail braking.
I don't like to run different pad compounds front to rear to acheive bias. With this the rear brakes never release the same as fronts and it makes the final brake release (just prior to throttle application) that much more tricky. I suppose that this compound difference COULD be made into an advantage. Usually though you are looking for more rear brake, so you run an aggressive pad on the rear, which usually has a more sudden release, which isn't what you want.
To tune the ABS system itself, really the only option is to the use the motorsport control unit. The stock control unit leaves much to be desired if you are running on R tires or slicks. The motorsport unit really transforms the feel of the system. The drawbacks are that it only works on four channel systems and it costs about $1500. Also it is not a plug and play swap. There is some additional work necessary to make it work.
The bleeding of any of these systems is a whole different conversation. If no air is introduced prior to the ABS unit (such as a prop valve change or caliper swap) then bleeding is straightforward. If there is air in the ABS unit then you need a means of activating the valves in the ABS unit.
Chris Cervelli
Premier Motorsports
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08-01-2005, 02:38 AM
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Chris,
Whoa!! Thank you for the information... much more than I was looking for, but good stuff to know if I need to delve deeper into the braking system and for a fuller understanding on how to work on it. It is indeed a pleasure to see you posting more often and hope you will continue to lend your technical expertise and insight to those of us with far less time and experience w/ these cars. Your contributions are greatly appreciated.
"If no air is introduced prior to the ABS unit (such as a prop valve change or caliper swap) then bleeding is straightforward. If there is air in the ABS unit then you need a means of activating the valves in the ABS unit."
What are symptoms of air getting in the ABS unit? If I do somehow botch it up and get air in it, what is needed to/how do I activate the valves?
Thanks again!!
Whoa!! Thank you for the information... much more than I was looking for, but good stuff to know if I need to delve deeper into the braking system and for a fuller understanding on how to work on it. It is indeed a pleasure to see you posting more often and hope you will continue to lend your technical expertise and insight to those of us with far less time and experience w/ these cars. Your contributions are greatly appreciated.
"If no air is introduced prior to the ABS unit (such as a prop valve change or caliper swap) then bleeding is straightforward. If there is air in the ABS unit then you need a means of activating the valves in the ABS unit."
What are symptoms of air getting in the ABS unit? If I do somehow botch it up and get air in it, what is needed to/how do I activate the valves?
Thanks again!!
09-28-2009, 12:38 PM
#10
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A couple clarifications about fooling with 993 brake systems:
4 channel systems have two proportioning valves, one for each rear brake circuit. If you wish to eliminate them, you can order the lines from a 993 cup, they are a little longer. All 4wd 993s and 2wd 993s with optional ABD have the 4 channel system.
Basically, you can afford to run quite a bit more rear brake than stock once the car is lowered. With the lower Cg, weight transfer is less, so the rear tires don't get so unloaded when you are braking at maximum. I usually run RS calipers since the change afforded by the different valves is very small.
Chris Cervelli
Premier Motorsports
4 channel systems have two proportioning valves, one for each rear brake circuit. If you wish to eliminate them, you can order the lines from a 993 cup, they are a little longer. All 4wd 993s and 2wd 993s with optional ABD have the 4 channel system.
Basically, you can afford to run quite a bit more rear brake than stock once the car is lowered. With the lower Cg, weight transfer is less, so the rear tires don't get so unloaded when you are braking at maximum. I usually run RS calipers since the change afforded by the different valves is very small.
Chris Cervelli
Premier Motorsports
Does anyone have the part #s for the 993 Cup lines that Chris is referring to for the 4 channel (C4S/TT) P/V delete?
I think the lines in question are the ones I have circled, but the Cup version rather than these ones shown in PET.
Thanks
Last edited by Tuner1; 09-28-2009 at 02:44 PM. Reason: pic
09-28-2009, 04:45 PM
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I apologize and don't mean to hijack Kelly's thread. What difference (other than greater straight line braking capacity) should I expect if I replace my calipers (C2S) with "Big Reds" all the way around and use the same pad compound front and rear?
Thank you.
Thank you.
