Your pricing model needs consideration. IMO .75 for 90 seconds of vac is not a good pricing strategy. By the time the customer gets the hose off the machine and starts using it they get maybe 1 minute of vacuuming. I think 2.00 for four of five minutes of vac and wash customers for $2.00/4:00 would make for happier customers. Even though you're going down on price per second I think over time it will increase site revenue. It will also deter bucket washers who want the lowest startup price possible. You may need signage explaining the math and how you've actually gone down on pricing even though the start up price has increased.
There are a couple schools of thought when it comes to pricing, one is squeezing every quarter out of your customer. I've met operators who feel like they are trying to outsmart their customers in a few ways in efforts to get more revenue. This may work if you're a traveling carnival but you need good relationships and
loyalty to your site for long term success. The other school is providing the best wash experience you can at a fair price and let the happy customers decide how much money they want to spend. Regardless of price no one but the bucket washers likes short cycle times. I'll keep my cycle times between four and five minutes and go up on price as needed to maintain a good profit.
The "customers have broken a lot of equipment"" comment concerns me. Happy customers usually don't break anything. You're either in an absolutely terrible neighborhood or people feel they are getting ripped off and taking revenge on the equipment. If you're truly up to snuff charge higher startup prices.
Also, carry a woman with you to the wash and get her perspective on site appearance, wash performance, and perceived lighting/safety. Or hire my brother as a consultant. Every time he walks onto my wash properties he finds something to criticize.