oil pressure

Check the oil pressure with a mechanical gauge,if normal replace the oil pressure switch. If abnormal could mean oil pump and engine bearing.

It already has a mechanical Gauge. The oil pressure slowly gosa way after I drive for 10 or 15 minutes. If I turn off the engin then restart it the oil pressuer come gack.

Could be the gauge itself, oil pump and engine bearings get it checked by a professional

I have disconnected the oil gage at the engine and there is nothing coming out. Changed the oil pump, filter and changed the oil. This started after I changed the head gasket and the old one may have been letting some--not much--coolant in to the oil.

Check what drives the oil pump and also did you prime the oil pump

The oil pump is working fine. It is external on the front of the engine and driven by the crankshaft.

The oil pressure goes to zero and oil pump is working fine something don't jive here-could be a clogged oil galleries.

What readings did you get when you hook-up the gauge while engine is running? No reading best check the oil pump again

It starts at 100 PSI and slowly fades to about 5 psi then go to zero.

Is the engine knocking-100psi sounds too much of a pressure points to oil pump and engine bearings for low pressure-Get it checked by a professional-I don't think I can help you further