Brake hydraulic proportioning valve

[chrischrischris]

I did some auto safety training recently and this included practicing emergency stops in various conditions. The instructors informed me that my car is over-braking in the rear. They saw the rear wheels lock before the front ones, which usually induced a spin. I confess to doing something not-so-wise by installing different brake pads on the front vs rear. The front are inexpensive Bosch "blue" pads and the rear are the normal Textar pads. I have since learned that it is a no-no to have more aggressive pads in the rear. However, I don't know if the mismatched pads on the rear would induce the amount of over-braking which was observed.

I know the 951 has a proportioning valve on the master cylinder to regulate brake bias. So, this valve may be faulty. Have others have problems with this valve?

FYI: The front calipers were completely rebuilt and restored recently, so I'm not considering any fault with them.

[walfreyydo]

I would match the pads and do the test again, before changing the proportioning valve. Mismatched pads could definitely be the cause so you would want to eliminate that variable first.

If you change the valve now and then switch your pads to match at some point in the future, you could discover that now your rears are under-braking.

[chrischrischris]

Thanks for the feedback. Yes, I plan to switch the pads so they all match. I have not seen any information about how to rebuild the valve, if needed. However, if the valve is the problem, then I may try to fit the adjustable Wilwood valve which only costs $60 compared to the expensive porsche valve.

The stock valve activates at 18BAR, so I would try to set the wilwood to the same pressure.

[dr bob]

Having matched pads is pretty important for a few reasons, including the one you shared. That might be a good first step. FWIW, after few pad changes behind me now, I've developed a habit of servicing all four corners at the same time. Some look at the rears and see that they have worn a lot less than the fronts, and may economize. I have a suspicion that the worn rears then force more work on the fronts.

There's a thread here somewhere that included disassembling the proportioning valve, with pics of the parts inside. They aren't complex, but they are sensitive to cleanliness for proper function. Old tired fluid at any time in the car's history risks corrosion, with orange slime and debris (read: rust...) inside. The shell of the valve is sacrificial-cad plated, and obviously gets sacrificed before the corrosion starts. The internals are pretty simple with a coil spring and a disk-and-pin-and-ballcheck valve inside. Knowing what we do about them I'd be encouraged to look inside any that are questionable before much more.

-----

Maybe related: On my 928, the car was shipped with a very conservative bias valve for the rears. Considering the intended market for the car, and some period history with oversteer under braking in the backmotor cars, it certainly made sense. Fast-forward, literally, a lot of available braking is lost especially on dry pavement. Tire technology has improved ever so slightly too, and mine isn't driven by the "dentist's wife" target audience. Raising the bias valve threshold was an easy and very noticeable way to improve stopping, or at least the stable feel when braking especially during an avoidance maneuver.

The last element is ABS, and the decision to swap the valve in my car is conditioned on the idea that the ABS system is fully functional. Not for dry-pavement driving, but for the wet- or even snow-slick conditions the car might see sometime in the future. Or that hard braking-avoidance maneuver.

Q: How do you determine when the adjustable Willwood valve is at your target setting?

[chrischrischris]

I saw one valve that had "click" positions which correspond to different settings.
The other style of valve has an adjustment screw. I guess you could count the number of turns to get to a ballpark setting. The website recommends you do some emergency stops in a remote parking lot until you get the bias you want.