I just measured cylinder leakdown rates on my 2007 Tacoma V6. 5 out of the 6 cylinders had retention rates of 75-85% (leaking 15-25%) but my cylinder 4 (left engine bank in the middle) was leaking about 75%, retaining only 25%. The easiest of the several places to check for air movement coming out was its intake manifold shaft (circled yellow) and I could sense air coming out. Of course, it could too be leaking on the exhaust valves, piston rings and the head gasket but those were not accessible to check easily. At least one possible outlet has been diagnosed positively.
My guess is that, when I did my head gasket job some 6 years ago, and lapped all the valves, I may have not done it right on one of those valves or both (2 valves each side). What are some possible effects of a cylinder (one out of 6) that leaks that much through one or both intake valves? Can I drive with so much leak or do I need to lift up that cylinder head?