So in case of pump failure I can use enough water to flush out one side but is there any way of swinging the tube manually in order to flush out the other side

I would shut the truck down, put the truck keys in my pocket, and using mixers water crawl in the hopper and flush the other side out.

I always carry an air line, and an Air chisel on-board too.

I dont know if that answers your question.

There are manual over-ride switches built into most pumps so you can bypass the electronics side and manually pump. But if your shift fork or something fails and you loose all hydraulics, theres not much you can do besides chisel, wash, blow a ball through boom, ect..

If you have access to plenty of water you can push a hose up your swing tube and keep flushing Untill you have nothing but clean stone left. 

Disconnect hydralic lines on rams that swing S-tube keep a concrete pin on your truck and swing tube by hand thats how i did it a few weeks ago to make sure barrels are clean take a pic with your phone

First and foremost is NEVER get into the hopper or work in the hopper with the truck running and making sure the Accumulator Pressure is gone from the system.

With this being said, the Hydraulic valve responsible to shift a PM S Tube is Open center, meaning both A and B ports are open to Tank.

So as long as there is no other binding due to wear parts binding or bearing “issues” IE lack of grease you should be able to pry the tube over to center position to allow cleaning of both cylinders,  this will take a sizeable pry bar, You can test this in the shop before having problems to find out how well it works.

Remember the only thing in the hopper with the truck running is CONCRETE