Rear brake pressure regulator valve, Brake repair Options

[DamonsCarrera]

The brake pedal on my 914-6 is very spongy, even after repeated bleeding with a pressure bleeder. I isolated the problem to the brake-proportioning valve or pressure regulator. I cannot seem to get the brakes properly bled with this valve in the system.

I've read other posts by some here and it seems this valve is a source of some debate . Is there some trick to bleeding the brakes with it? Do I need to bleed the brakes the old-fashioned way, have someone push on the brake pedal?

[DamonsCarrera]

@ 914 teener

I'm not sure how you mean to use the word linear here. None of these valves makes a linear relationship of output to input pressure. For instance, if we look at the Bosch valve in the following graph (I'll call it Figure 1)



we can see that the output pressure has a kink at the threshold pressure, after which the output pressure increase is reduced for each increment of input pressure. Compare the resulting pressure curve to the linear relationship shown for reference, which just keeps going up at a constant rate after the threshold pressure is reached. So the Bosch valve is NOT linear, and in fact all of these proportioning valves create a non-linear relationship of front-to-rear brake force distribution.

Actually, the engineers (of which I am one, by the way) are entirely concerned about load, since the ability of the tire to generate lateral (cornering) and longitudinal (braking) force is reduced with decreasing load, and load decreases on the rear tires as weight transfers to the front tires under braking. Thus, the ability of the rear tires to generate cornering and braking force is reduced during braking, and that is what can cause a spin to occur. The 914's lower moment of inertia in yaw does complicate matters somewhat, but all cars suffer from the problem of optimum or ideal front-to-rear braking force distribution, not just the 914. That's why most performance-oriented European cars, not just the 914, came with these proportioning valves before the advent of ABS.

The later versions of these proportioning valves came with the Dynamic feature you are talking about, as a way to further optimize the rear brake performance. Let's be clear, the "Dynamic regulator valve" differs from the "Static regulator valve" in that the Dynamic regulator valve changes the threshold pressure as a function of the loading on the tire, while with the Static valve the threshold pressure is constant (unchanging). This Dynamic valve was adopted even on the Fiat 124 that I mentioned, even though it had a high yaw moment of inertia, because the ideal brake force distribution (the pressure you'd like to have going to the rear brakes if you could get exactly what you wanted) changes with load. Note also that the lowly Fiat 124 had the Dynamic valve while the performance-oriented 914 did not! Why would this be? Hint: the 124 had a huge trunk that you could load with heavy stuff, thus changing the load on the rear tires more dramatically....

To see how load affects the need for rear brake pressure, look at Figure 2 ...



and you can see that the rear brakes can take a higher pressure (more braking force) at a higher load (max load) than at a lower load (min load). But the Bosch valve shown in Fig 1 is a Static valve (the threshold pressure does not change). The Bosch valve doesn't know what the load on the tires is, so its curve must necessarily be more of a compromise than if it were a Dynamic valve. Translation: the brake force curve will be closer to the "Min load" line so that the unwitting driver won't get in to trouble in conditions of high load transfer. This results in the compromise of the rear brakes being underutilized most of the time. You find this compromise on nearly all cars built before the advent of ABS. The engineers of that day tried to deal with this issue as best they could by introducing the non-linear force distribution curve with the kink in the curve at the pressure threshold point. The ability also to change from the max load case to the min load case with the Dynamic control feature came along a little later. Relatively few cars were equipped with dynamic regulator valves, mostly just the performance oriented ones. The rest just dialed back the rear brakes way way back. And then, ABS came along and changed everything.

Now look at Figure 3



to see how the Tilton valve can be adjusted to perform the same as the Bosch valve, with the same threshold point. There is, of course, no way to precisely adjust the Tilton valve to hit the desired threshold pressure exactly. I just found the midpoint of the adjustment range of the valve, then turned the adjustment knob a little bit more counter-clockwise. It's not exact but I think it's in the vicinity of being correct. As I said, I'll check it once the weather gets better.

This is a complicated topic and I'm sure I have already bored most of you to death. I apologize for the length of the post. But for those who might be having the same problem I'm having with the Bosch valve, hopefully you find this info useful.