Another mysterious clutch/transmission problem

[huggy59]

I hope you will be able to help me...sorry for poor technical english

Following a MC and SC change because of leaks my problem is: shigfting gears up is no problem and easy but once in 4th at normal speed I'm unable to down shift into 3th (or any gear) until I stop car
Once car stoped select a gear is now difficult but once selected I'm able again to select 2/3/4 before been unable to down shift

However if I cruise at 25mph I'm able to select any gears up and down: No problem

I tried 2 differents MC (repro and rebuild Girling), 2 differents SC (repro and rebuild Girling) and even mixed them for tests, slave rod have 1mm freeplay, MC rod is adjustable, hose is braided, brake fluid is silicon.

....I'm totally lost: how engine rotation speed could change gear selection ?
Thinking about an hydraulic problem as originally problem was a leak but I now start to think about a clutch mecanism problem: could it expend or retract with speed ?

Advices warmly welcome !!!!!!!

[ill_will]

Did you solve this in the end?

When you say you can't downshift, does the lever jam and prevent selection or does it scrunch or make another noise at all?

[gjz30075]

Why silicon brake fluid? Flush and go back to DOT3 or 4 (might be labeled differently in Europe) and
see if it makes a difference.

[baileyman]

I have a problem shifting when the transmission gets warm, which I think may indicate I need a different oil.

Is your problem the clutch will not operate as a function of RPM? JOhn

[Craven]

Increased temperature in slave cylinder and pipe line from close proximity to the exhaust can move operating point of clutch.