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]

Don't know what the argument is about. It seems to me we're wanting to limit pressure (and flow) to the rear brakes to more closely follow the ideal braking force distribution (The attachment below helps to see why this limiting is a good thing). If we didn't do this limiting to the rear, the rear brakes would lock before the front brakes under heavy braking. And doesn't the Tilton valve offer the same characteristic as the Bosch valve with an appropriate adjustment, from looking at the pressure curves of both? It looks the same to me.

Anyway, I got to thinking about GeorgeRud's comment about the high point of the brake system, and so I made some measurements. Not including the master fluid reservoir, the high point in the hydraulic system is the T-fitting coming out of the pressure regulator valve. This point is a good 3 inches higher than the master cylinder or the top of the loop of hardline attaching to the rear caliper, which itself is another inch or so higher than the rear caliper bleed valve. That T-fitting is also higher than the hard line from the master cylinder to the valve. So, even though the master reservoir at the front of the car is higher than that T-fitting, air can still get trapped in that T-fitting because fluid comes to that point from a line that is lower.

So is air getting stuck at that high point, and that's what causes the spongy pedal? Maybe that's why people are saying to crack the line there to try to bleed that part of the system. Hmm, guess I'll think about how to put a bleed valve there to test the idea.

 AuE850__Lecture_Set_6__Stability_Under_Braking.pdf ( 1.83mb ) Number of downloads: 191