2002 Silverado

[reh76]

Today I did a some work on my Silverado...now my oil pressure gauge is acting up. When the truck is running the gauge reads normal around 40 psi at idle and maybe up to 70-75 while driving. The problem is when I turn off the engine the gauge stays between 30-40 PSI, if I turn the key back to on it drops back to zero...turn it back to off and it starts creeping up. Here are the things I did.

- replaced oil level sensor (was telling me to check oil when the level was fine)
- changed oil & filter (new brand of oil)
- removed battery to fix a small leak on the wiper fluid tank

Anyone have any experience with this? I kind of think it is the oil pressure sending unit, but I don't know if it would be working fine when the truck is running. Thanks for any help