Based on what I just measured, on my 318, pistons come to TDC in pairs, and both their valves are closed at that point, so a leakdown test can be made on both (one after the other) without having to move the crank. Those pairs are 1/6, 8/5, 4/7, and 3/2. Now maybe that's a function of the cam shaft, and some cams don't behave that way. I don't know. But I did verify I can pressurize those pairs. (I'm not saying they're both on a compression stroke. Just saying they're at the top and their valves are all closed).
Yes, I guess unscrew the 4 rocker bolts and close all the valves at the same time. I don't have a torque wrench handy to screw them back in so I didn't do that. I also wanted to see if anything was going on with lifters and rods so ...
I only have a 5 gallon tank that I can pressurize with an air compressor. I charge it to 100 psi then connect it to the cylinder with an inline valve. Turn the valve and use a stop watch to time 10 seconds then turn it off, take note of tank's new pressure. I'm seeing 5 to 8 PSI drop in all cylinders except 7. I see 12 - 15 psi drop for 7.
Compression test (engine cold, mid 60's ambient temp, engine not run for a few days, throttle jambed open) gives 150 - 160 psi range for all cylinders. 7 comes in at 150. By the third crank they're all 140, even 7.
I don't think that's too bad for a cold engine, but I'm wondering if 7 is causing my rough idle, vacuum needle really bouncy.