The SHurflo 4048 is recommended as 'direct replacement' for the Shurflo 5.7 Variable Speed pump
It is rated for 4 gpm at 55 psi
It has an internal bypass valve to control low flow shutter (allows pump to run continuously during low flow demands vs stop/start/stop/start action)
As for that Aquajet 55.........
Might be an installation issue, air leak or controller
Installation...Operation Manual
Troubleshooting starts pg. 16
Is it time for your medication or mine?
2007 DODGE 3500 QC SRW 5.9L CTD In-Bed 'quiet gen'
2007 HitchHiker II 32.5 UKTG 2000W Xantex Inverter
US NAVY------USS Decatur DDG31