Ok, I know this has probably been discussed in many threads before. But, what is the reasoning for replacement of the Prop. valve with the "T" fitting for the rear brakes. Does the prop. valve cause more line pressure to the fronts or the rears when it is operating properly? I want to understand the engineering aspects of this modification.
I will be running "M" calipers with mythical billet hubs in the front and stock 914-4s in the rear.
thanks

The prop valve limits the amount of hydraulic pressure going to the rear calipers. The T fitting doesn't.

Long answer, the prop valve has a spring in it. For the first part of the pedal travel the hydraulic pressure is less than the spring rate. As you push the pedal farther the hydralic pressure eventually equals and then surpasses the spring. As the spring moves it closes a valve which completely cuts off any more possible pressure to the rear calipers. Once you release the pedal, the spring decomresses and the valve opens and the rear calipers release. That spring/valve action is one of the reasons the rear calipers/ P-valve are so hard to bleed.

Here's my spin on the deal. I'm running larger diameter tires with a wider contact patch. The added unsprung rotating mass puts undue stress on the stock brakes. It has nothing to do with a larger motor, more HP or any other HP mod. In my opinion it's mass rotating vs applied force. I converted to BMW 2002 4 piston calipers in the front on the stock solid rotors. I added a 19mm 911 master cylinder to make up for the added fluid volume and reduce pedal travel. I eliminated the "P" valve to increase the pressure to the stock rear brakes. I like the feel. I've never felt as if I had too much rear clamping force. With less mass and a smaller contact patch a brake up grade isn't nessesary. IMHO

one observation, at Willow I had visable brake dust on all 4 wheels for the first time in my life that I can recall. I guess they were working

Jeez!!!

Oh, and for the lurkers who might be reading this and don't know:

Locking up the rears before the fronts equals spinning car. So the P-Valve prevents that.

My car will be a dedicated track/Ax car so I wanted the vented front rotors.
I will be running 205 sticky "R"s. Longevity with good pedal feel is the reason behind my brake upgrade.
I don't like spending my time between sessions bleeding the brakes.
This is great information guys.

The prop valve gives the pedal a springy feel under hard braking.. larger front brakes and a T are the way to go

QUOTE (rick 918-S @ Dec 1 2005, 10:33 PM)

at Willow I had visable brake dust on all 4 wheels for the first time in my life that I can recall.

sorry rick

The P-Valve Cutaway

Once the P-Valve closes, it should basically become just like a capped off brake line. So a P-Valve, theoretically shouldn't cause spongey brakes. Of course, if air gets trapped in cavity #6 (image above) then the air will compress as you push the pedal farther, giving a spongey feel.

The adjustable prop valve gives you options in case your computations in your math formula are....incorrect...perish the thought. You do have computations, right?

I've known of and used 914-6s that have had only the T fitting in place of the prop. valve for the last 20 years, and it seems to work fine. The brake pedal feel is much better in my opinion. The comments on rear brake lockup are correct, but I've not experienced it in either my stock 914-6 or my conversion car with Carrera front brakes. It's an easy enough conversion, and the T fitting is certainly cheap and easy enough - give it a try and see what you like.

Minor Hijack: Where can one get a T fitting?

VW shops have them. I think aircooled.net sells them as well, if you're so inclined.

If you check your factory manual (if you have one available), you will see that the P-valve does not completely limit the pressure going to the rear brakes. Rather, it reduces it by a percentage once the main system pressure rises above a certain point. There's a graph that shows this; there's a 1-to-1 correspondance between input pressure and output pressure until a given point, and then about a 3-to-2 correspondance past that point.

Anyway, the idea behind the P-valve is to keep the rear brakes from locking up under hard braking. If you swap in a T, the rear brakes are now doing more work than they were with the P-valve. This is good if you still lock the fronts before the rears, but bad if you lock the rears first. Removing the spring-loaded valve from the brake system results in better pedal feel and easier bleeding of the system, as well.

It may be prudent to replace the stock P-valve with an aftermarket adjustable one. That way you can fiddle the brake balance until you get something that you like, but that is still safe. Brad used to tell me that "everyone who uses one of those things winds up with it full-open", but the guys at Grassroots Motorsports supposedly wound up backing off from that point when they did this swap.

The worst-case braking (in terms of rear-wheel lockup) is with a full tank of gas, a nominal cargo load in the front trunk (e.g., spare tire and maybe a bit more weight), no load in the rear trunk, no passenger, on a slight down-hill, with your bestest grippy tires in a high-traction environment. That will give you the greatest transfer of weight off the rear wheels, giving you the greatest propensity to lock their brakes.

--DD

Mine is now one turn less that full open.....a butt sensor induced adjustment that I am pleased with. A trail braking issue.

This is gud stuff

I am sure the reasoning behind the T is that putting larger front calipers while keeping the stock rears, statically makes the proportioning due to size differential...with stock front and rear the rears were too closely matched to the fronts, yielding rear lock up with out the proportioning valve. The factory needed to use vw parts that were available and the P valve was a cheaper easier fix than changing brake calipers.
This same process is used on some late model carreras like the C2 when you upgrade to big reds or big blacks...all of the brake gurus say remove the stock P valve.
My buddy did this on his C2 during a Big Black upgrade. It worked fantastic. The C2 (964) like the 914 had too large of a rear caliper in relation to the fronts, stock from the factory, necessitating the use of a P valve. By the way a big black upgrade on a 964 put the caliper that comes on the front of a C2 onto the rear with the 928 S4 front up front (same caliper as a 993)
The T fitting makes sense in this scenario as well.

QUOTE (ejm @ Dec 1 2005, 06:55 PM)

The prop valve gives the pedal a springy feel under hard braking.. larger front brakes and a T are the way to go QUOTE (rick 918-S @ Dec 1 2005, 10:33 PM) at Willow I had visable brake dust on all 4 wheels for the first time in my life that I can recall. sorry rick

Ed! That wa a great time wasn't it?

How were my brakes anyway?

QUOTE (J P Stein @ Dec 1 2005, 08:52 PM)

Mine is now one turn less that full open.....a butt sensor induced adjustment that I am pleased with. A trail braking issue.

JP,
what brakes are you running front and rear?

Also keep in mind that with much better gripping tires your chances of locking the brakes are reduced as the tires on the wheel rotating will grip better than the caliper can clamp onto the brake rotor.

Personally if you are locking the brakes you are doing something wrong or something is not setup right. Once the brakes lock you've lost traction which means you've lost control. All very bad.

You mean, this graph Dave? It was on the next page. I hadn't turned the page.

QUOTE (Brando @ Dec 1 2005, 09:35 PM)

Personally if you are locking the brakes you are doing something wrong or something is not setup right. Once the brakes lock you've lost traction which means you've lost control. All very bad.

Yes, but locking the brakes is not always under your control, and if they get locked up somehow, you want to plan for that worst case scenario (at least in a production street car). As my dad has said a million times, "It's not if there's going to be problem, it's what's going to happen when that problem occurs."

Good thread.

I have been "testing" the stock p-valve and have a tee in tool box.

I've found the spongy feel is directly related to heat in the calipers.

With normal driving and the first few laps on track I have firm pedal.

As I develop heat, the pedal travels farther.
I don't like the feel under the foot, but the actual brake action does not suffer.
I'm trying to get used to that.

I mainly need to shorten the length of time I'm on the brakes and thus, apply them harder and later.

KT

QUOTE (Dan (Almaden Valley) @ Dec 1 2005, 09:26 PM)

QUOTE (J P Stein @ Dec 1 2005, 08:52 PM) Mine is now one turn less that full open.....a butt sensor induced adjustment that I am pleased with. A trail braking issue. JP, what brakes are you running front and rear?

S up front, M rear.

QUOTE (McMark @ Dec 1 2005, 09:42 PM)

"It's not if there's going to be problem, it's what's going to happen when that problem occurs."

Our parents wisdom!
Mom used to say "hope for the best but plan for the worst"

As Dave implies, it's the shift in weight (to the front) that creates the lock-up and that's just a matter of physics. Is that likely to happen in many "street" situations? Not all rear lock-ups will result in uncontrollable over steer, correct?

QUOTE (trekkor @ Dec 1 2005, 10:15 PM)

Good thread. I have been "testing" the stock p-valve and have a tee in tool box. I've found the spongy feel is directly related to heat in the calipers. With normal driving and the first few laps on track I have firm pedal. *Edit* I mainly need to shorten the length of time I'm on the brakes and thus, apply them harder and later. KT

TK, That's the answer we always used to give at the races...."stop using the brakes!"

I understand the fundamentals here, and any racer car I've ever driven had a brake bias adjustment. (But) What do the garden variety 911's use? I have '89 Carrera brakes, front and rear, ready to install. Did that car have a prop. valve? Do Carrera owners mess with their prop valves? Do they experience spongy pedals? Of the street 911's I've ever driven, I always thought they had excellent brakes. Anyone?

Andys

QUOTE (Dead Air @ Dec 2 2005, 05:15 AM)

Not all rear lock-ups will result in uncontrollable over steer, correct?

True. If you lock the rears while you're going in a straight line, and the road surface isn't too irregular, you shouldn't spin. If you're not near the limit of adhesion in any way whatsoever (except of course for the rear wheels sliding), you've got decent odds of not spinning.

Then again, this is a race car (or at least a track car) we're talking about....

--DD

QUOTE (andys @ Dec 2 2005, 08:52 AM)

What do the garden variety 911's use? I have '89 Carrera brakes, front and rear, ready to install. Did that car have a prop. valve? Do Carrera owners mess with their prop valves? Do they experience spongy pedals? Of the street 911's I've ever driven, I always thought they had excellent brakes. Anyone? Andys

The corelation between a Carrera brake set up and a 914 with Carrera brakes wouldn't seem to be straight across. The Carrera has more rear weight bais, bigger tires rear to front, undoubtly transfers less weight to the front during braking...The only thing that does work is a reasonable compromise between braking force caliperwise front to rear......it's close enuff to work out on a 914.....if
you have a adjustable valve as less pressure will be required in the rear on a 914
due to the reasons above.

QUOTE (J P Stein @ Dec 2 2005, 09:22 AM)

QUOTE (andys @ Dec 2 2005, 08:52 AM) What do the garden variety 911's use? I have '89 Carrera brakes, front and rear, ready to install. Did that car have a prop. valve? Do Carrera owners mess with their prop valves? Do they experience spongy pedals? Of the street 911's I've ever driven, I always thought they had excellent brakes. Anyone? Andys The corelation between a Carrera brake set up and a 914 with Carrera brakes wouldn't seem to be straight across. The Carrera has more rear weight bais, bigger tires rear to front, undoubtly transfers less weight to the front during braking...The only thing that does work is a reasonable compromise between braking force caliperwise front to rear......it's close enuff to work out on a 914.....if you have a adjustable valve as less pressure will be required in the rear on a 914 due to the reasons above.

Re-reading my post, I see I mixed a few things together, perhaps confusing the intent.

JP, you're absolutely right. Two different cars with two different sets of dynamic conditions. 911 = more rear weight bias = different brake biasing requirements than the 914. BTW, I will be running 255-17's in the rear.

Teener's generally seem to complaing about the prop valve, yet 911's use them throughout, ergo my comment "Do they experience spongy pedals? Of the street 911's I've ever driven, I always thought they had excellent brakes." So what's the difference?

Andys

QUOTE (Dead Air @ Dec 2 2005, 05:15 AM)

QUOTE (McMark @ Dec 1 2005, 09:42 PM) "It's not if there's going to be problem, it's what's going to happen when that problem occurs." Our parents wisdom! Mom used to say "hope for the best but plan for the worst" As Dave implies, it's the shift in weight (to the front) that creates the lock-up and that's just a matter of physics. Is that likely to happen in many "street" situations? Not all rear lock-ups will result in uncontrollable over steer, correct?

I think that in panic stops on the street the worst case scenario can happen very quickly: non-progressive brake actuation, slamming on the brakes, which doesn't allow for weight transfer and front tire loading to acheive maximum traction during the stop. This means the fronts lock before being loading to their limits via weight transfer and even worse the rears may lock. This is why ABS is so effective on street cars but a little less of an advantage on race cars.

Every good driving instructor I've had always emphasized not standing the car on it's nose with hard braking. The reasoning is that even on a fairly balanced car you don't want to unload the rear tires anymore than necessary in case you need to do some steering. At Willow Springs I can be reeling the "other Porsche" down from 130 and those brakes will stand the car on it's nose if I ask them to. At those speeds you really don't want to "upset" the chasis anymore than needed.

Later,
Kevin

QUOTE (andys @ Dec 2 2005, 09:39 AM)

Re-reading my post, I see I mixed a few things together, perhaps confusing the intent.

I'm easily confused so don't let that worry ya.

My old stock brake set up had a hi hard pedal. I never could figure out what folks
were talking about ...."spongy?" back in the day. See. gettin' confused is nuthin' new for me.

This is where I got my T-fitting ($8):

http://www.wolfsburgwest.com/cart/DetailsL...fm?ID=113611755

Eric...

Did you remove your reply? Inquiring minds -- that have M calipers up front but still run the proportioning valve -- want to know.

pete

QUOTE (mdkeats @ Dec 2 2005, 09:45 AM)

Does locking the rears mean you ARE going to spin....NO, just release the pedal a bit and they start rolling again. You can also use this to rotate the car, as in Karting, which has only rear brakes. Finesse is the key. The T allows the rears to actually work, which they really don't unless you really get on them with the P valve. Spinning is not inevitable. Just my 2 cents.

Some like to free the car up upon entry into a corner with some rear brake bias. It's all in how you like the car setup.

Karts with rear brakes only require some degree of forced technique, since rear brakes is all you have to work with. Ever try a direct-drive kart? Here again, it forces a technique change or adjustment. You are correct, that many often forget to modulate the brake pedal to get the desired result.

A street car needs to be capable of dealing with a very broad range of conditions. I inagine everything is designed to err on the side of safety.

Andys

First Post...

QUOTE

This same process is used on some late model carreras like the C2 when you upgrade to big reds or big blacks...all of the brake gurus say remove the stock P valve.

Carrera's went to a P-Valve in 84 or 85 (I think it was 84) when they switched to larger 43mm pistons for the first time in ages on the back of a 911. Up until 84 the piston size was 38mm. This is why I don't really recommend the 914 fronts (43mm) back there unless you've got some serious brakes up front.

With M-Calipers on the front and stock rears I would use a "T" like Rick has in the Alien. Many people have echo'd that here.

I'm not a fan of pulling the P-Valve just because the brakes are spongy. If there spongy there's probably another problem (like proper bleeding or the heat fade that Trekkor is talking about). I'd say, stock brakes, keep the P-Valve.

Pete, for-Pete's-sake, I'd probably put a T-in there.

Mine are exactly the same as JP's w/o the adjustable p-valve. I used Pagid Oranges up front and regular Jurid's in the rear. Basically a 911RS/First Year 930 system. There was another thread where Davep's buddy proved the theory that those are the ticket for the 220-250hp range.

You can get a T-fitting at www.cip1.com or from James here I believe...

Brakes......a decent buy:
http://forums.pelicanparts.com/showthread....threadid=254378

QUOTE (Eric_Shea @ Dec 2 2005, 03:20 PM)

With M-Calipers on the front and stock rears I would use a "T" like Rick has in the Alien. Many people have echo'd that here. Pete, for-Pete's-sake, I'd probably put a T-in there.

Yipes, I've been rolling this way for 14 years...

I've never run into a problem but was never blown away by the brake upgrade. Now I know why -- I'm probably hardly using the rears! I've locked up the fronts maybe twice in that time, I think. Never had a fade problem, though -- which I did with the stock rotors.

I drove a 993 with motorsport Brembos a while back. It had a lot more rear-brake bias and that thing was ~amazing~ under braking on the way into turns. Somehow, I don't think my setup will approach that -- but a little better is always, well, great!

pete

QUOTE (andys @ Dec 2 2005, 10:39 AM)

So what's the difference?

when i upgrade my brake system to a 19mm MC and mid '80s carrera rotors (vented) & calipers on all 4 corners i noticed that, with the stock prop valve still in place, the rear brakes did barely do anything.

almost all of the braking was done by the front brakes.

i replaced the prop vale with a "T" per brad's advice. he said that they had similar problems with all their race-build 914s and they first tried running adjustable prop-valves but always ended up running those on full open.
which is exactly what the "T" does, except it's 100 times cheaper than a cockpit adjustable prop-valve ...

now, with the "T" in place, front and rear brakes seem to do about the same amount of work,
with the fronts locking up just a bit earlier than the rears.
when i hit the brakes hard at speed, the whole car goes down evenly as compared to before with the prop-valve where the front would do a nose-dive while the rear got lifted up ...

i'm happy with my "T"!
Andy

Hi Dan,

I'll give you what Doris and I were running in our 914-6 (the one you went faster in than me at Laguna (shhhhh! )

911T vented rotors / calipers front and 944 vented rotors / calipers rears (no parking brake). T instead of stock prop valve. I did have a cockpit adjustable valve.

At Buttonwillow and Laguna Seca I ran the adjustable valve wide open and had not problems with brake balance. At THill (CCW) I would get too much rear brake coming hard into turn 14, so I would adjust that out. I also had the same issues at Sears, where coming hard into 7 or 11 and braking late I would get a little too much brake force in the rear. A couple of turns and I could adjust it out.

I like the 19mm m/c pedal travel as a preference because it was easier to heal and toe without resorting to blocks. I'm with JP as far as the brake bias adjustment, especially if you can use it inside your car without it adding too many points to put you in a class you don't want to run in.

Bill P.

Basicaly, what Bill, Andys & I are talking about is trail braking
into a corner. With more rear bias the car will tend to oversteer on entry....depending on how deep you like to go....or have to go on a OOOPs . You can have too much of a gud thing as Bill points out. You can either change your driving style (style is not revelant describing mine) or turn the knob... if you have one.