1. Sequence Valves

2. A sequence valve symbol is similar to a relief valve symbol
 A sequence valve symbol is similar to a relief valve symbol. The main difference is that a sequence valve always has an external drain line — and often has a bypass check valve for reverse flow.
A sequence valve is a pressure-operated, normally closed, poppet or spool valve that opens at an adjustable set pressure. 
A sequence valve always has an external drain port to keep from trapping leakage oil. 
Trapped fluid modifies set pressure at best or keeps the valve from opening at worst. 
For reverse flow capabilities, use the integral bypass check valve shown in the symbol.

3. Main idea
Fluid at the inlet port of the valve cannot pass to the secondary circuit or outlet port, until reaching set pressure
 Upon reaching set pressure, the valve opens enough to let excess pump flow pass on to the second operation

4. in some circuits the pilot signal to open the valve is from a source other than the line feeding it.
An external pilot-operated sequence valve opens and allows flow when a remote operation reaches a certain pressure.

5. Counterbalance valves