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]

Well, in case anybody is following this thread ...

I added a T-fitting with a bleed valve right next to the tee fitting that splits the brakes to right and left rear. This second T-fitting is at the highest point in the brake system after the line from the master cylinder comes out of the back of the car, and it is at the same height as the T-fitting that comes out of the top of the original proportioning valve, which I had removed as noted in a previous post.

To recap, after removing that valve and replacing it with just a T-fitting, the brake pedal was a lot less spongy. I thought I might have solved the spongy pedal problem but I was bothered by the idea of air possibly being stuck at that high point at the firewall before the lines go right and left to each brake caliper.

So I put this second T-fitting with a bleeder valve at that high point. After bleeding the brakes the normal way, using only the bleeders on the calipers, I found as before that the pedal felt pretty good. But then I opened the new bleed valve on the firewall, and got several inches of air in the fluid line before getting solid fluid. So air was indeed getting trapped there.

After bleeding that air out, the brake pedal now feels excellent and solid.

Then I plumbed in the new Tilton valve so I have some brake proportioning. I made a temporary bracket out of sheet metal to keep things in place while I design a nice aluminum bracket to hold the valve and the T-fittings for the long term. Rebleeding as before retained the excellent pedal feel.

So the car is back up and running, with great-feeling brakes. Next is to figure out what's wrong with my original Bosch valve and why it wouldn't bleed.

It's clear at least that air gets trapped on the output side of that valve. Some people suggest cracking the lines at the output as a way of bleeding out that air, but that's not easy to do if my stock-configured car is any indication. It's very difficult to get a wrench on these fittings with the Bosch valve installed, hard to open and then close them in a timely manner like you can with a bleed valve that you have great access to. Cracking those lines would certainly get brake fluid all over the paint on the firewall, which I want to avoid on my freshly-painted car.

Maybe there's something wrong with my Bosch valve and that's why I couldn't bleed the brake system with it installed. It's been suggested that many of these valves don't work after 40 years. Maybe that's true.

So next on the agenda, after making a permanent bracket for the Tilton valve, is to explore why the Bosch valve doesn't work as expected. I'd really like to get that valve back in the car if I can.

[Bob L.]

So next on the agenda, after making a permanent bracket for the Tilton valve, is to explore why the Bosch valve doesn't work as expected. I'd really like to get that valve back in the car if I can.

Have you had the Bosch unit in the car since you got the air out of the top tee?
I would reinstall it and see if you can get the pedal to come up.