Cant select gear sometimes on my 1987 t25 1.9 dg

possible actuator not being fully engaged
check for vacuum leak or faulty unit

sounds like its bulk rings in gear box, they spin the gears ready as u change gear when they wear the gears are not spun enough to select u probably need to over hall gear box , to help u change gear for now double d clutch, whats this as u want to change gear push clutch down move out gear release.. clutch pushdown again put in to gear use this with each gear un need to change what this does is spin gear ready for u as if bulk ring had done it for u if u are good u can just change gear but have to b good for this

old car. 5th gear? is this M/T? 1987 ?
A/T? is 4speed

what does stopped working mean .
1: shifter fails to select 5 gear H pattern slot.
2: grinds?
3: cant force it to 5th with clutch depressed.? is free play at spec?
4: raise front wheels off ground, can you get in to 5th now? keyout and running?
5: can you get into 5th parked engine off, and select 5th and then start car in 5th with clutch depresses first step. then slowly very slowly attempt to move car starting in 5th, but dont finish, just feel the gear, feel that its there and not neutral.

not working ,says nothing at all.

master cylinder for sure possibly due to fluid contamination. DOT3 and Dot4 fluids cant be mixed . causes softening of seals in cylinders much like if contaminated with mineral oils

Change or atleast check trans fluid level. Could also be a seizing bearing which means a transmission rebuild is going to need done.

The reservoir should be under the hood beside the brake master cylinder.

is yours carb,if not 23- manifold air temperature (mat) sensor or circuit (1990 and earlier models) 23- intake air temperature (iat) sensor circuit (fuel-injected models) if its carb,,,,,23- mixture control (m/c) solenoid or circuit (carbureted models)

Sounds like 5th gear selector gone in gearbox,dont know what car you have do all other gears work normally?

Limited adjustment under dash, most likley slave cyld. or worn clutch and pressure plate.

you have to put the #1 piston at tdc, then put the cam in at the mark.