Brake Proportioning Valve

geoffbaltz · geoffbaltz's Garage

Can anyone educate me on what the numbers on the brake proportioning valve mean?

From the 944/Turbo Factory Manual:

A braking-force booster is installed for the rear-axle braking circuit of the 944 S and 944 Turbo.

The changeover pressure differs.
944S: 33/5
944S2/944 turbo: 18/5
924S and 944 are not fitted with braking-force regulators.

Identification
Changeover pressure and reduction factor stamped on regulator.
33 or 18 = changeover pressure in bar 5 = reduction factor 0.46
(check correct correlation).

Notes on Assembly
It is essential to ensure that correlation is correct. Under no circumstances may braking-force regulators with a different reduction factor (e.g. 3 = 0.3) be installed.

****************

I have been told to go with the 33/5 on my turbo instead of the 18/5 for more breaking to the rear of the car.

What does the changeover pressure and the reduction factor mean?

Thanks!

-g

arnebret · 12-28-2001, 03:19 AM

I can't help you with the manual, but i also heard from a couple of people that the 951 is very much balanced to the front, concerning brakes. Almost everybody has to change rear-discs every 3 front discs.. Meaning they almost wear down at 3 times the speed of the rear disc. Of course, brake balance in a normal car is to the front, but in the 951 it is pretty extreme, and i'm really thinking about changing this with a new valve...

geoffbaltz · geoffbaltz's Garage

I believe the 33 means that the system needs 33 bar before sending brake boost to the rear.

Anyway, I got the new valve from Paragon for $71 - Sorry Wayne, the wife got it for me for Xmas.

I will be putting it in and bleeding the system soon. After that, the brake project is finished. Pix to follow of the new setup soon!

-G

arnebret · 01-20-2002, 12:36 PM

I bought the same toy as i was noticing extreme wear at the front compared to the rear.. How is it installed and do you notice any difference? I will be installing the valve when doing a rotors/pads/bleeding job within a few weeks.

ninefiveone · 01-22-2002, 03:30 PM

Using the 18/5 as an example, the numbers mean:

- Up to 18 bar of pressure, the rear brakes are fed the same pressure as the fronts
- Abouve 18, half as much additional pressure is fed the rear brakes. (i.e. 26 bar of presssure means only 22 bar to the rears)

I've changed to the 33 valve on my 951 to balance out the braking. I've yet to test it out at the track with my normal pad setup but so far I've been very pleased on the street.

The install is very easy on non-abs cars but a bit more involved on ABS cars. On a non-abs car the valve is right off the master cylinder. On ABS cars the valve is located behind the fender well lining. Remember to bleed to brakes fully after the install.

-Hugh
'86 951

Mike B · 01-23-2002, 09:30 AM

ninefiveone...Did you use the 928 brake proportioning valve or go with an aftermarket variety?

ninefiveone · 01-23-2002, 10:11 AM

MikeB,

Yup, I went with the 928 valve from Paragon.

-Hugh
'86 951

Chouse · 01-24-2002, 09:15 AM

Has any one tried an adjustable proportioning valve such as those used on a track car? I beleive they use a twin master cylinder, like Kokeln makes. I was just curious as to how hard this would be to install, or if it would be worth it for light track use. I have seen some pretty cool setups on race cars, that have two gauges with a knob to adjust the pressures. It just seems like it would be the ultimate setup to solve brake bias problems, but it may not be worth the trouble to install if it is only used once or twice.

Cyrus

Pilot_951S · 05:40 PM

There are two ways to get adjustable bias.

The cheap and easy way is to put an in line manually contorlled proportioning valve on the rear brake line where it comes off the master cylinder. Willwood has a decent one.

The Expensive way is the Kokeln dual manual master cylinder with a bias bar. This is cockpit adjustable. The bias bar is the better way to go (because it literally balances brake line pressure at the cylinder rather than limiting pressure in a line) but is many times more expensive than a simple proportioning valve, and requires much greater leg effort.

The proportioning valve can also be moved to the cockpit but it requires rerouting the brake line for the rear brake circuit through the passenger compartment.

Chouse · 02-03-2002, 05:50 PM

How much is the Kokeln unit?

Pilot_951S · 02-05-2002, 10:45 AM

$1500

http://www.lindseyracing.com/944sus7.htm

Chouse · 02-05-2002, 12:06 PM

Thats definately the expensive way. Do you have any experiance with it? It looks like anyone with some time, experiance, and access to a decent machine shop could make the braket and buy the master cylinders. Or maybe even modify a wilwood or other comparable unit to work in a 944, although I'm not sure that they are any less expensive.

Pilot_951S · 02-05-2002, 02:15 PM

I've seen it in action. I've never installed one, and the guy that had it went from that to a stock 968 MC and booster on his '86 951. He has Big Reds up front.

Anyody with the tools and know how could build it. It isn't a complicated piece. You'd definitely want to see one before you tried to make one though.