When ball get's to those bends it is dry? maybe you need to use some water or someting.with placingboom  do you not push water from  hopper to endhose?

You have to blow placing boom out with air compressor from boom back down to pump and blow it back in a mixer via diversion valve to stand pipe

having 26 placing boom jobs under my belt i went with last load was pea rock blend mix and never had a promlem

I have never heard of someone sucking a ball back on a placing boom. You need to blow it out with air.

The method I have seen: blow the first sponge back with air compressor, first sponge is either slightly damp or wrapped in a plastic bag and no water is placed in front of it, after the first sponge comes through a second round of sponge/water/sponge (no plastic bag) is blown through, and lastly the diversion valve is switched back to the pump and a single sponge is sucked back to complete the process. Putting water in front of the first sponge can lead to seggregation issues.

Fix the line where the elbos are,even if you add another 20ft of system.So nothing is uphill.

What kind of elbows do you have at the bottom of the stand pipe, I have had nothing but issues with plugging pipeline with short radius 90* elbows, Can you change to med to long radius 90's ?

This would help with pump pressure and blowout problems. There are a lot of other potential issues, the slump, the Mud can all play a role in the ability to cleanout, I have had Hiteck chem mixes that are total problem children pumping and blowing out.

If you HAVE to suck back I would recomend blowing a good ball with a bit of water 3 to 5 gallons after the line is empty to ensure a good clean. Either up or down what ever fits the site needs.

keep in mind the higher you go the more trouble you will have with tight restricted bends at the bottom of the stand pipe.

sucking back causes cavitation and segregation. leave system full. add ball and controlled air through a diversion valve back into truck if possible. if not reverse stroke slow , use air pressure as head pressure. Whenever poss. wet mud before blow back. do the math. "you can pretty much just count strokes when you prime to know" so you dont put wet mud on deck. Basically use controlled air.

Pretty sure I have posted this link before, but hear it is again.

http://www.youtube.com/watch?v=Hz5Z0YULdrk

+ some pics because I cannot type 1000 words;-)

Next time consider to buy another pump e.g. Spanish SEBHSA(search in Google,pls) with water tank of 600 litres, High-pressure, high-output air compressor, driven by direct hydraulic motor, for cleaning delivery pipeline and 150 litres compressed air tank!

These stationary pumps have also: Fully hydraulic automatic control system without stroke terminals, electrovalves or other electrical switch devices. Foolproof operation in any climate.

Here in Poland i know a few clients who operate with these type of pumps

Operators also can do it with only air:

http://www.ioperator.eu/viewtopic.php?t=189