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]

I think there are other aftermarket valves available, for instance at this link here.

All these valves (proportioning, biasing, regulating, whatever they call them) effectively reduce fluid flow to the rear brakes, although the real goal is to reduce the increase in the rear brake circuit pressure with respect to the increase in the front brake circuit pressure. They don't limit the absolute pressure to the rear brakes. That's what a pressure-limiting valve does, and I've never seen one of those used in any passenger car worth driving.

This valve in the 914 was made by Bosch in response to EU requirements of the time for improved braking distribution. I think all these valves, which were developed before automotive ABS caught on, work pretty much the same way, generating a curve like the one in the page from the 914 service manual that Eric posted. For a 'cleaner' description (ie written by someone who spoke English as a first language) see this attachment lifted from "Safety, Comfort and Convenience Systems" by Robert Bosch GmbH, June 2006. This description makes it much clearer how the valve is supposed to operate.

 Bosch_ISBN_0_8376_1391_4.pdf ( 1.92mb ) Number of downloads: 382


Actually there were also load-dependent valves that connected to the suspension to sense changes in suspension deflection (see Figure 3 in the attachment) and so have two inputs (master brake pressure and vehicle attitude), I once had a FIAT 124 that had one of those. But I digress.

I've never had problems bleeding systems with these valves in them before, so I'm a little stumped about why I would be having this problem now. I don't think I'm having problems with the rear calipers, which by the way were recently rebuilt, because when I take the valve out of the system I can bleed the brakes just fine.

I really want to keep the stock valve. I'll take advantage of the advice of the group here and try to enlist the help of a friend or try the speedbleeder idea.

Thanks to all! This site is an amazing resource to have for these cars. (IMG:style_emoticons/default/cheer.gif)

-Damon