Porsche 914 rear brake pressure regulator

[Davidmgbv8]

The Porsche 914 had a rear brake pressure regulator that under extreme breaking would decrease the pressure to the rear brakes to keep the rear end pointed the correct direction however, under normal operation did not affect the pressure to the rear brakes. Has anyone thought of using this on a GT40? Is it even needed? Does the rear ever step out under polar movement under extreme breaking?

 

[Bob Putnam]

It could be useful if it's adjustable to the brake pressures on your car, but the effects would be fairly small. Because it optimally works with a smaller rear master cylinder diameter, manual brakes would require more travel.

 

[Davidmgbv8]

Hi Bob, it is not adjustable. It is a weight and spring type assembly that under heartbreaking. The weight moves against the spring and then restricts the pressure to the rear wheels. The more I think about it since the GT40 is a dual cylinder set up and you have a balance bar that really kind of alleviate that whole problem versus the 914 which had a tandem master cylinder with no four aft adjustment.

 

[Marc]

Frankly, I can't really agree on that and would expect you're all fitting a (adjustable) proportioning valve to the setup.

Dual MC with balance bar just gives a static front / rear brake pressure distribution. This would typically be setup to just make the fronts lock before the rears under hard braking in dry conditions. That way, the rear should not lock before the front in any scenario.

However, this does mean that under light braking (either due to low traction or due to other loads on the car, like cornering), the brake balance is way off.

Consider you'd have 50/50 weight distribution. Under hard braking of say 1G you probably go to >70% load on the front, <30% on the rear. That is how you'd setup the balance bar. For the sake of the rationale, consider the road is slippery and you can only get to 0.1G. Then you'd want 50/50 braking, but you only get 50/20. You're leaving 30% of the potential brake power unused, where you need it the most. Similar for light braking in a corner, you don't want to have that all on the fronts.