you sure this happened after you replaced the slave cylinder?
when the clutch pedal feels spongy, try to bleed air out of the system, if not, the clutch needs adjustment
if this happened before you made your repairs, it would be a different problem, might be that the synchros in the transmission are worn out, or it could be the clutch disk or pressure plate,or thouwout bearing
if it happens only when you downshift, you need to match the engine speed close to the transmission speed before engaging gears
1st gear and reverse gear are exceptions, usually don't have synchros, so you need to slow down or even stop before you can shift, no matter how hard you push the gearshift it would not go, it would grind
Transmission fluid has nothing to do with a faulty clutch slave cylinder or transmission issues.
Well obviously if you are going 60 it won't go in 1st but Im guessing you know that. My guess is syncros are going bad. Happens to my second gear. It goes in but its not buttery smooth.
Sounds like a bad clutch or bad pressure plate. Or master cylinder, or throw out bearing.
Could be internal almost like the synchronizers are worn out.
Your clutch needs adjusting.
What can it be i changed the slave cylinder.and transmittion fluid
