One thing you need to do is see if any valves are stuck, tap each valve gently with a rubber hammer or end of 2 by 4 to make sure it is not stuck, oil stem first, if stuck, gently work it until it moves, if not, a pushrod may be bent-not good.

Since you seem to be turning it over, still need to make sure all valves are working properly. So do above procedure to each valve.

Ed

Last edited by EdPruss; 07/29/2018 3:30 AM.
'37 GMC T-18 w/ DD 4-53T, RTO-610, 6231 aux., '95 GMC running gear, full disc brakes, power steering, 22.5 wheels and tires.
'47 GMC 1 ton w/ 302, NP-540, 4wd, full width Blazer front axle.
'54 GMC 630 w/ 503 gasser, 5 speed, ex fire truck, shortened WB 4', install 8' bed.
'55 GMC 370 w/270, 420 4 speed, grain, dump bed truck from ND. Works OK.