Petits are high flow demand configurations. With 2 cat 3535 pumps per auto. I suspect your loosing water pressure on busy days because the recovery flow rate to you water supply tanks on the Petits are exceeding the flow rate of your city water supply and or softener (assuming you are softening high pressure rinse water feeding the 3535 pumps). Try reducing this by throttling back the supply feeding your pump supply tank(s). You will need to find a setting that allows for your tank to fill just in time for the next high pressure demand. My tanks run down during a wash and are adding water the whole wash and finish filling at the completion of the next cars pre soak passes. This helps keep your building pressure up.I'm in a similar situation. I have a similar 200,000 grain twin alternating softeners but with Fleck heads. I have a busy 4/2 with dual Petits and noticed my pressure levels have been less during times of higher demands. At peak I can lose water completely at some smaller water supplied line stuff ( office upstairs). Could this be due to the water softener system ending its life? I have what appeared to be a very knowledgeable water softener guy come out and he said I had about 75 to 80% resin left in my tanks