proportioning valve questions..., that a search didn't help. Options

[red914]

so, what can be done about the rear brake proportioning valve? it seems most people replace it with a "t" fitting, but that doesn't seem to address the issue. it must be there for a reason. can it be rebuilt? can it be replaced? how can one tell if it is working properly? what is the best way to bleed the little beggar? (IMG:http://www.914world.com/bbs2/html/emoticons/confused24.gif)

i appreciate all the help and information. soon i'll be able to tackle the brake project, now that the holiday vortex is loosening its grip on my life.

[Steve]

Its supposed to stop the rear from locking up during hard braking.
The biggest sign of one not working is mushy brakes.
If you need one most people run an adjustable one inside the car or in the stock location.
I'm running a T and I cannot lock up the rear brakes.
The fronts lock up first.
I'm currently running 914-6 rear brakes and 911S Aluminum calipers up front.
Yea I know the Aluminum one's flex and I should get rid of them.
I'm planning on installing 944 turbo brembo's up front but I need to fix oil leaks etc. first.

[campbellcj]

There's a lot of good brake info in the archives. In a nutshell you DO NOT want a "tee" with stock brakes. If your calipers are working properly then the rears would lock-up well before the fronts. That is not fun.

Most cars with "tees" have significantly upgraded brakes but also have setups that are well-balanced enough to get away without an attenuation of pressure at either end. For instance I have SC "A" calipers and rotors up front and 914/4 front calipers in back, with Pagid race pads all around, and I use a tee with no problems. (FWIW I also have non-stock stuff just about everywhere else which also makes a difference in weight transfer and grip).

Your best bet is probably a Tilton or Wilwood type of adjustable valve. You'll need to adapt the prop valve to our cars' somewhat unusual metric flare but the usual sources should be able to hook you up with the right bits.

[DJsRepS]

Mushy brakes if not working? how can you test if it is working and your brakes are not mushey??

[Steve]

QUOTE (DJsRepS @ Dec 31 2004, 06:18 AM)

Mushy brakes if not working? how can you test if it is working and your brakes are not mushey??

IMHO mushy brakes is a brake pedal that does not feel firm.
I don't know how you can test this except maybe bleed your brakes first and then replace the proportioning valve with a T and then bleed them again to make a comparison.
I agree the valve is there for a reason with stock brakes and you should not run stock brakes without it.
If you want to keep stock brakes I would look into an aftermarket valve.
My car started with a 19mm master cylinder but I replaced my stock brakes with S front brakes, 914-6 rear brakes and a T and noticed a big diference in how firm the peal was.

[airsix]

Even perfectly bled 914 brakes will have some spongyness due to the proportioning valve. Why? Because as you increase brake pressure you compress that huge spring/piston in that huge tube and thereby displace a HUGE volume of brake fluid which is directly translated into additional peddal travel. Since you are basically compressing the proportioning valve spring with your foot, it feels mushy. You could almost say it was designed to be mushy. (IMG:http://www.914world.com/bbs2/html/emoticons/barf.gif) Some people go with a 'T' and eliminate all of the mushyness attributable to the proportioning valve. I like to maintain some adjustable bias so I used a Willwood adjustable valve. It does the same thing as the stock valve but the volume of fluid it can displace is very small, so it doesn't feel mushy. It's a compromise. It's not as firm as a 'T', but almost. I mounted it in the stock location so I have to adjust it from under the car and then leave it the heck alone. If it was in the cockpit I'd be messing with it all the time. (Same goes for my boost adjustment)

-Ben M.

[Reiche]

Found this on the bird board. Prop valve exploded and explained. Apparently it can be rebuilt and adjusted.

airsix is right, but much of the chronic mushiness in the brake circuit can be eliminated by completely bleeding the rear circuit ahead and behind the proportioning valve before bleeding the brakes. Like most brake tasks, it is more tedious than difficult. Just make sure the valve itself is completely free of entrapped bubbles. It helps a lot.

[rhodyguy]

shane, did you solve the rear caliper issue? getting the brakes bled with two people can be an ordeal. did you purchase speed bleeders (4)for your car?

kevin

[red914]

thanks for all the input!

Kevin: i have not had time to sort out the brake problem (holidays and all), but will soon. and yes, i have ordered speed bleeders; they are enroute.

silly me, i read in the haynes manual that we weren't supposed to rebuild the prop valve. nice to see it can be done. i'll try bleeding it, first.

as always, the 914club comes through with the scoop needed to get me on track. a great way for the club to kick off its 3rd year! (IMG:http://www.914world.com/bbs2/html/emoticons/smilie_pokal.gif) thanks again! (IMG:http://www.914world.com/bbs2/html/emoticons/pray.gif)