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

The difference between a static valve and a dynamic valve is:
* Static valve regulates output pressure as a function of input pressure only.
* Dynamic valve regulates output pressure as a function of input pressure AND a measure of vehicle load or rate of acceleration.

I haven't seen any aftermarket valves for sale that are dynamic regulating valves. The last time I saw such a valve was on a Fiat 124 I once had. That valve worked like the one shown in the Bosch document: there was a lever arm attached from the valve to the axle which moved up or down depending on the pitch angle of the car. That was Fiat's way of measuring load on the rear axle. That was a great car, wish I still had it.

So to answer your first and second questions: The Tilton valves are all static valves as far as I know. Certainly the one that I bought is a static valve.

Also, the Bosch valve that came on the car originally is a static valve.

And to answer your third question: Yes I use the stock 914-6 master cylinder and stock 914-6 calipers front and rear. I do drive the car on the street. I put maybe 2000 or 3000 miles a year on the car at most.

@ Bob L.

That's a good idea. I may do that if I can't figure out a way to test the valve off the car. I'm going to explore the idea of building a hydraulic test bench with which I can apply measured loads to the Bosch valve, and recreate the curves in the Factory Service Manual. This shouldn't be too hard. I'll need a pressure source (hydraulic pump capable of creating up to 2000 psi), a regulator, some pressure transducers, a couple of flow meters and a flow control valve. I haven't looked into it yet, but those things should be readily available.

The problem with re-installing the Bosch valve is it takes up a lot of space in that area and doesn't leave much room for anything else. The Tilton valve is tiny by comparison, and leaves room for the second T-fitting with easy access for bleeding.

But I do want to get the Bosch valve back in the car if I can, if for no other reason than keeping originality.