I have end float in the main shaft so 3rd 24T must be free to revolve freely. However on the last rebuild the gear cam shaft did not have the 0.002" end float and did not revolve so I fitted a 0.030" for this reason where as when I had stripped down initiallt there was no inner case gasket at all.
However it should be noted that inner cam steel bush had worn considerably and the PO had shimmed out with hardened? washers and no doubt also omitted the gasket to take up end play. This all being negated by the newly machined inner steel bush, it is worth remarking that there is notable hydraulic damping when the gear cam shaft is pushed into the steel bush and this has to be compressed to bleed out trapped air.
Reading around the issue and taking advice here I shall check the plain plates for flatness, all measure at approx' 0.06" thickness. The fibre plates are all approx 0.17" thick and I shall check for flatness, scrub up on coarse sandpaper and clean with petrol or acetone. If this doesn't cure the issue I see a big spend and m/c off the road for foreseeable future 8-(
If perchance the there has ever been ATF in the primary case then you will most likely need to fit new friction plates as ATF contaminates the friction plates causing stiction that can never be fully resolved.
When I assembled my cases I thought I had it all wrong, but after a few minutes the camshaft overcame the hydraulic lock you mentioned and all was then well.
When I look at the design around the mainshaft/third gear/ small bearing and k/s ratchet it seems to me that if you pull the k/s ratchet nut up tight there is no way there can be endfloat on the mainshaft. There is a shoulder on the mainshaft that the 3rd gear abuts then the rest of the parts I just listed are a slide fit on the mainshaft - ergo tighten the k/s ratchet nut and they all get clamped together (unless I have missed something).
So to get main shaft endfloat and thus a free revolving 3rd gear that nut needs to be NOT tight. To have it not tight, but secure I assume one needs to use either (or both) loctite or punch locks.
Could this explain why during the teardown I found that nut punch locked?