T fitting instead of prop. valve, what is the rational? Options

[DanT]

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 (IMG:http://www.914world.com/bbs2/html/emoticons/unsure.gif) (IMG:http://www.914world.com/bbs2/html/emoticons/wacko.gif)

[McMark]

The prop valve limits the amount of hydraulic pressure going to the rear calipers. The T fitting doesn't. (IMG:http://www.914world.com/bbs2/html/emoticons/biggrin.gif)

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.

[rick 918-S]

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 (IMG:http://www.914world.com/bbs2/html/emoticons/aktion035.gif) (IMG:http://www.914world.com/bbs2/html/emoticons/driving.gif)

[Allan]

Jeez!!!

















(IMG:http://www.914world.com/bbs2/html/emoticons/smile.gif)

[McMark]

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.

[DanT]

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. (IMG:http://www.914world.com/bbs2/html/emoticons/smilie_pokal.gif)

[ejm]

The prop valve gives the pedal a springy feel under hard braking.. larger front brakes and a T are the way to go (IMG:http://www.914world.com/bbs2/html/emoticons/aktion035.gif)

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 (IMG:http://www.914world.com/bbs2/html/emoticons/happy11.gif)

[McMark]

The P-Valve Cutaway

[McMark]

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.

[J P Stein]

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? (IMG:http://www.914world.com/bbs2/html/emoticons/biggrin.gif)

[GeorgeRud]

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.

[Carlitos Way]

Minor Hijack: Where can one get a T fitting?

[McMark]

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

[Dave_Darling]

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

[J P Stein]

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

[Lou W]

(IMG:http://www.914world.com/bbs2/html/emoticons/popcorn[1].gif) This is gud stuff (IMG:http://www.914world.com/bbs2/html/emoticons/smile.gif)

[DanT]

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.

[rick 918-S]

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 (IMG:http://www.914world.com/bbs2/html/emoticons/aktion035.gif) 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 (IMG:http://www.914world.com/bbs2/html/emoticons/happy11.gif)

Ed! (IMG:http://www.914world.com/bbs2/html/emoticons/wavey.gif) That wa a great time wasn't it?

How were my brakes anyway? (IMG:http://www.914world.com/bbs2/html/emoticons/cool_shades.gif) (IMG:http://www.914world.com/bbs2/html/emoticons/happy11.gif)

[DanT]

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? (IMG:http://www.914world.com/bbs2/html/emoticons/smile.gif)

[Brando]

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.