Extreme Veteran
Posts: 574
 
Location: North Wales | The brake servo has a non return valve in, so it should retain vacuum when the manifold goes to boost pressure. Even if the servo lost the vacuum it wouldn't matter, unless you were trying to brake while also having your foot on the accelerator. Once you lift off the accelerator to hit the brake the manifold will go straight to vacuum, and so will the servo.
Where the breather system connects to the manifold, it also has a non return valve. While the inlet manifold is under vacuum, the crankcase fumes are drawn into the manifold. When the manifold gets boost pressure the non return valve shuts and then all the fumes are drawn in the inlet pipe after the air filter (so that the breather system is always under negative pressure).
A piston dump valve won't work for long on a Metro because the seals will swell from the petrol that will find it way to the dump valve. You need a diaphragm dump valve.
The dump valve will open when there is vacuum on it to open it, and boost pressure trying to push it open. When the pressures are matched (or very close) it will remain closed. So, when off boost (and the manifold and plenum are under vacuum) it will be shut, and when on boost (with pressure in the manifold and plenum) it will remain shut. This why you cannot connect to the breather system as it would be open all the time. |
dump valve
