hi stephen try this first, on the pump assay in boot you will find a small brass exhaust nozzle on end of compressor head,remove that , i think its 13mm hex, run with that removed,if that outlet becomes partially blocked its unable to release pre pressure before starting any height corrections,theres a mod in the US for a couple of dollars but its a pump replacement in uk,unless its changed.The last one i did i drilled small hole right through it & has worked well for a couple of years,worth a try hth keith.