Work for a construction equipment rental company and for 3 years I have had to kick and scream to get repairs done to my truck ! But while no repairs get done I'm still always expected to get in and drive ! What should I legally do? Thanks! This is an 18 wheeler !

The only legal method I can think of is to contact a state police truck division (sometimes called Commercial Vehicle Division) if in the U.S. State Police agencies normally have a Truck Inspection Division. But that will probably be the end of your job. Also, in at least some states, weigh stations administered by those same SP Truck divisions randomly, or if defects are apparent at the scales, inspect trucks at those weigh stations.
In one instance in MD that I am aware of, a dump truck driver who crashed into several personnel on a state construction site due to faulty brakes, was criminally charged. As you are the operator, you have certain liabilities but I claim no expertise in that.
Your best remedy and safest approach is to find another job with a better quality company. I certainly hope that you don't operate anywhere I have to drive.
Good Luck.

Do a safety inspection and keep a copy for yourself, turn it in to the company. If any thing happens you have proof of the defects, to cover you for any claims against the company, accidents

What type of repairs are you talking about, and to what level of safety.

If serious enough, have you looked for another job ?

Also, of course you will be fired, but you can refuse to drive it, if the problems are serious enough.