Clogged SCR Decomp Tube

On city buses after a certain amount of engine hours the DPF is removed and replaced. When it’s removed you have to look down the decomp tube and many times they are filled with hardened DEF and it looks like giant crack rocks. It’s a pain in the but to remove the decomp tube and clean it out and replace the dozer. Is it just a faulty dozing valve or does this happen due to engine RPM and and not enough exhaust blowing through. The DPF swap is easy but it sucks when you have to remove the decomp tube. Any suggestions. Sometimes if it’s not to bad you can chisel the hardened DEF fluid and vacuum it out and then replace the dozing valve with the tube stil installed.