updated at the bottom. i recently rebuilt a stock 350 (only thing not stock was the RV cam) for my cousin, dropped it in and fired it. when i was primming it, it had 25 psi pressure. when we fired it, it had 25 psi pressure, once it warmed up, it went to 0. we shut it down. a little history. when we bought the motor we were told that it was rebuilt and never fired (missing valve covers and intake no gasket materal on any of the surfaces, also looked very clean inside). once i got into it i knew that that wasn't true. coolant was in the block and the bores needed a hone bad. from the parts we took out, it looks like it was rebuilt at one time. the bearings were down to the copper, but clean. it almost looked as if they didn't use any lube or primed it when they fired it. ive been racking my brain on why it has 0 pressure. we used the stock crank, it was in good shape. the bearings were stock that we removed, i put stock bearings in (dumass me didn't check the clearances ). the cam bearings looked new, as do the frost plugs. so we did not replace those either. new melling oil pump (standard), hardend pump shaft, welded on screen. it is slinging oil off the crank, shown on the dip stick, we have a mechanical guage plumbed in near the distributor. i have not pulled the valve cover to see if there is oil to the rockers, there was when we primed it. this motor is in a s10 blazer 4x4, i really don't want to pull it, it took 2 weeks to get it in. the really stange thing is the 4.3 that we took out of it, the oil pressure was the same exact way, 25 when cold, 0 when hot. the only thing that is the same is the pump pickup and the guage. when we pulled the pan off the 4.3 to make the pan for the 350, the pick up was laying in the bottom. with the deep sump of the 4wd pan, i though i figured why the pressure was that way. (ive known the blazer for 4 years and the pressure had never changed, never heard a rod knock in all that time, and lasted all this time) we took a break today on it, tomorrow we are going to dig into it and find whats wrong. first thing im going to do is replace the line (have spare) and bleed it, if no change, move it to the plug above the oil filter. if we have nothing there, we are pulling the motor as something is wrong with the pump. in the back of my mind (because i didn't check the clearances on the bearings) im thinking the crank was turned and they put the stock bearings in, and then i did the same and thats where all our oil is going. is it possible to loose all the oil pressure through too large of bearings? any other suggestions?