Correct me if I am wrong but pressure could not be building in that cylinder b/c the valves are not closing fully, i.e. valve train issues rather than a broken piston or connecting rod, therefore during compression stroke air is just pushed out through said open valve rather than being compressed. Thus, I would concur with the others: pull valve cover and take a look. Then, if you really think you have a bottom end problem drop the oil pan and look in it (metal bits) and at that cylinder.
Alex Jones
Atmore, Al
'64 SIIA 88
'79 MB 240D daily driver
'80 MB 240D donor
'59 MB 190D "rutso-ration in progress"
'00 Chevy Silverado 4x4 "rescue vehicle"
"Glad I ain't haze grey and underway!"