Pneumatic Sequencer Valve
by Mark Bellis July 2008
Based on real hydraulic valve functions

The valve assembly (Pic 1) consists of:
A non return valve (NRV) made from a 2-way valve (dark grey) driven via an extended lever by a small cylinder, which pulls against two (in this case K3) belts.
An additional pilot cylinder that acts upon the 2-way valve lever in such a way that it pushes the lever when extending but can retract without pulling the lever.  The pilot cylinder pushes against one belt (in this case K3).
The air input from the system control valve (the old light grey one) is connected to the 2-way valve, the pilot cylinder base and the NRV cylinder top.
The air output to the base of a large cylinder connects to the 2-way valve and the base of the NRV cylinder.
The valve is wired so that extension of the pilot cylinder and retraction of the NRV cylinder pushes the valve to connect the input to the output.
When the valve cylinders move the other way, the valve is prevented from moving over its centre position to the other open position, lest it should leak air.  It remains in the all-ports-closed position.

How it works:
Step 1 (Pic "Pneu Seq Vlv2").  The valve is at rest with input and output depressurised and the valve closed.
Step 2 (Pic "Pneu Seq Vlv3").  Air enters the valve.
The NRV cylinder contracts because the input pressure on its smaller piston area is greater than the back pressure from the output cylinder on its larger piston area.  The pilot cylinder expands because the inlet pressure on its larger piston area overcomes the belt force.  The sum of the excess pressures overcomes the two belts on the smaller cylinder.
Step 3 (Pic "Pneu Seq Vlv4").  The valve has now moved sufficiently far to the left for the air to escape to the output cylinder.  Once the air has escaped, the pressure in the two valve cylinders is no longer sufficient to overcome the belt forces and both cylinders return part way to the valve closed position.  Movement all the way depends on belt forces relative to the force required to overcome the valve detent.
Step 4 (Pic "Pneu Seq Vlv5").  Further air enters the valve, expanding the valve cylinders fully and raising the pressure in the output cylinder to that of the input.  Typical pressure 25-30psi for LEGO pneumatics.
Step 5 (Pic "Pneu Seq Vlv6").  The control valve is reversed, producing a major leak in the valve input.  The belts and back-pressure in the NRV cylinder cause the valve to shut quickly, keeping the air pressure up in the output cylinder.  To enhance the valve shutting speed, the pilot cylinder was modified by using a 1.4mm screwdriver to bore out the restrictions in the nozzles, enabling its air to leak quickly.
The output cylinder is an unmodified type, so the air release from it (and the effect on the load) is slower than that from the modified one (and the effect on the pilot), so the pressure in the load cylinder is preserved against leaks of the input.
The effect depends on the relative air movement rates from the valve and load cylinders.  For a significant load, more cylinders would be used, requiring more air to move through the output pipe, keeping the effect when the load is greater than that of the valve belts.  The NRV cylinder can only help to shut the valve when the valve is already partially shut and output cylinder air is not leaking so fast that it cannot apply pressure to the NRV cylinder.
In a real hydraulic system the fluid is ideally uncompressible, so calibrated restrictions in pipes may be used to enhance the load release protection.  The NRV cylinder is also tailored much more to the function, rather than being a generic cylinder design.  The belts are replaced with springs and the forces inside the valve chambers are allowed to be much higher.
NOTE: All modifications to LEGO parts are done at your own risk.  Special care must be taken if boring out the top nozzle because the cylinder top seal may be damaged if the tool is inserted too far into the nozzle.  Never modify a part unless you have a spare one!
Step 6 (Pic "Pneu Seq Vlv7").  Air is again applied to the input.  The valve cylinders act to open the valve, with the input pressure being lower than that in the output cylinder.  When the valve opens sufficiently, air leaks out of the output cylinder until its pressure has fallen to that applied to the input.  In this way the valve prevents air escaping from the load cylinder unless the supply pressure is above a certain threshold, but releases air down to that threshold when air is supplied to the valve.  This protects the load from a loss of supply pressure, even when the load cylinder is driven in the other direction.  This can prevent the load cylinder from contracting too far or too fast if too much pressure is applied to the other end.
Step 7 (Pic "Pneu Seq Vlv8").  The air is released from the valve input and the valve closes, with the residual air pressure in the output tube remaining just below the threshold at which the valve will open.