Your pressure should be about the same across the three ( about 150 to 175 psi hot) as "m" is main and f-r is.....well fowrd and reverse and "c" is converter. Not going to worry about " c " port as you have good reverse. First get the main pressure and record. Place the guage on the f port and with drive tires up and park brake set, select forward and watch gauge. It should build slowly ( 1 to 2 seconds) untill the accumulator in the valve body is bottomed out and then pressure should spike rapidly to near main pressure. Do the same with reverse port and compare the three. Check the selector shaft adjustment for full travel into foward. If the tranmission is engaging harshly then more than likely you have broken accumulator spring,disc material missing from friction plates or drum has plate grooves. A stall test would not hurt at this point. Stall should be around 1900 to 2000 rpm forward and reverse. I would remove the top plate of the tranny if after doing the pressure test, linkage adjustments and stall test had no results. Disassemble and clean. After clean and reinstall and is the same, then i am afraid you will have to go deeper and do a complete trans teardown to find the problem. Could be a simple lip seal or drum shaft seal. But if you go this far then i would build or replace the tranny.