I have an 02 chevy suburban it dropped oil pressure I replaced the oil pump but am still not holding oil pressure after it warms up good the pressure drops to zero. What else could it be causing lose?

What tests have you run? How did you run them ? what engine are you working on ? If you are following a diagnostic, does it tell you where to sample the pressure? where ? and then what was your reading ? What is your specification ? at what condition ?

If your readings are below spec, the next test, having tried a new pump already, is to put induced oil pressure in the engine, provided you are using the A/C delco parts and you have a good filter and your pump has a good relief valve. Now I must ask... while the pan was off, did you pull some mains ?

Suburbans also are known for the oil sensor leaking . It is located on the back side of the motor by the firewall. Usually will leak down the driver's side on to the filter area and down to the bell housing. Between that and the crankshaft sensor which is located above your starter which if it's leaking you will see oil on the bolts of your starter and around the starter also. Not always a rear seal to start. I would check those items first then move to oil pan gasket next. Rear seal would be absolutely last thing I'd go for unless you see oil in the check port hole around the flywheel. Good luck all.