Re: F-150 will not go into park, NEED HELP!
The cable may need adjusting/tightening. This is not uncommon with higher mileage vehicles or ones that are constantly shifted. I'm not sure if the piston you mentioned is operated by transmission fluid (hydraulic pressure) or not, but if you haven't had a transmission service done in a while you'd be wise to do so. Dirty fluid can wreak havoc on components. Valve body (control plate) has holes that as small as a pen tip and can affect pressure if they get dirty or clogged up.
'11 Ram Quad Cab 4x4 SXT 5.7L HEMI (stock); 48,500m
'06 Pontiac G6 GT 3.5L 220hp; 64,000m
'12 Chrysler 200 Limited 3.6L 283hp; 28,000m
'99 Taurus 3.0L 2V Vulcan 145hp; 154,300m - Traded
Amsoil in all vehicles!