Had our pool for 25 years. Due to broken cleaner we had to vacuum for the first time after opening pool this year. Set it up properly and vacuum plate was sucked tight in skimmer and we were on full skimmer. Plenty of vacuum suction but filter pressure dropped to almost zero as soon as we started to vacuum. We expected to see filter pressure go up as dirt filled the filter grids so it was hard to figure out when to pause vacuum to clean the grids. The grids were filthy as we expected when we opened tank. We do not have filter to waste option unfortunately Is it normal for the pressure to drop like that when first starting vacuum?