340 issue after camshaft change

With 60psi of oil pressure, a 3/8 diameter plunger would only have a out 6-7 pounds of force against it, plus the lifter spring. A 1/2 plunger would see about 12 pounds. The small bleed hole and check valve prevents the lifter from collapsing under load during operation.

A properly operating hydraulic lifter cannot open a valve spring under normal conditions. The only way to hang open a valve with a hydraulic lifter is if you float it and the lifter plunger raises above its preload setting. That shouldn't occur at cam break in rpms!