Deadheading a pump is when you block the hyd fluid from moving from pump to the tank or cyl which will use it to develop pressure and produce work. The result is something breaks.
A hyd pump will try and pump as long as it is turning. The fluid must move
A relief valve is a must on a hyd system to relieve the pressure caused by a restriction.
Be wary when using QD's in the hyd path.
If you do deadhead a flow path, damage might occur in the pump, hose, fittings, valve, etc.
Most expensive is the pump.