I have the TST 507 system with flo thru monitors. I'm going to replace with the standard monitors. On my trailer with metal stems they stick out past the rim. I have hit road alligators(recap treads) twice and lost a flo thru both times. Right now I only have 3 working monitors. I think I'll just replace the whole unit since 4 monitors will cost me $200. I'll stick with the TST though.