Thanks for stopping by please provide advice if you are well versed in clutch and transmission issues, please no nonsense comments proper advice only. Thankyou. I've done thorough searches on the web and on CT however for such a specific problem i've had trouble identifying my correct solution. If you'r well versed in how transmisisons work you can probably help me out because it's my lack of full understanding thats holding me back from accurate diagnosing - I have it booked into a shop on thursday this week but still would like possible expert advice. I have a manual rb30 in an r31, it was originally converted to manual from auto and the original conversion box worked fine for years. however after gearbox issues (unrelated to current issues) i had the gearbox replaced with a second hand (believed to be, rb20 double synchro gearbox, mated to a 5 puck pbr clutch installed by a local shop.) Anyway after a few weeks of the install of the most recent gearbox,i hit a problem as follows; At one point of normal driving went to shift from 4th to 5th, and now am faced with current issue; With the engine running, from neutral, i cannot select any forward gears, no grinding noise is made and it can't be forced, it simply walls, and will not move all the way to mate gears up - It almost feels as if i can feel the gears on the engine side spinning but its just not mating. Reverse can be selected with a savage crunch (presumably because no reverse synchros.) With the engine turned off, with clutch in or out regardless, gears can be moved into etc as per normal, it does feel slightly rough though when moving into gears but it moves into all gears fine and holds them. All gears can be accessed without issue including reverse, with motor switched off. I found that if i put car into first or second with the motor off, clutch pedal all the way in, and started the car, it would start, however car also lurched forwards. I managed to get car home, and i found that a few of the bell housing bolts were missing, and when clutch pedal was pressed, it was moving the bell housings apart. I replaced the bolts, dropped new oil through (no chunks of metal or filings in oil) and bled the clutch. It drove mostly flawlessly, however after i let the car sit for a day, when i tried testing again, same issue. I checkd BH bolts again and were okay this time. I re bled the clutch but that made no difference when i tried testing it. I Let the car sit for a day, changed nothing but tried it again, and it now does move into gear from neutral - I'll sit it in the garage till thursday when i drive it straight to the shop. From this i have concluded it's probably hydraulics related, mainly due to the fact bleeding it seems to have had a (delayed) affect and allowing it to work, and a (slow leak perhaps) is why it worked for a few hours then went back to the same problem as before. My -Presumption- is there is not enough push (either because of piston degredation or lack of pressure etc) and the clutch is not seperating from the engine enough to allow the synchros to engage. If it is not hydraulic related, or clutch pedal height or piston adjustment related, i am led to believe there is another possible cause (this is where my expertise stops, because i do not understand the gearbox function beyond the slave piston and how this interacts with the gearbox internals.) If i understood the gearbox internals more i could diagnose further, however basically i hit the wall of my knowledge at the clutch slave piston. I am to understand other potential causes of my problem are, that the ?internal?/external? drive shaft is remaining spinning for some reason and since the diff/tyres etc are weighted, it won't select gear because the load is too much for the synchros etc. How this would be caused, how to accurately diagnose, or repair though is unknown to me. I have essentially ruled out actual gear damage or syncho damage etc, because when i've got it driving, its all worked pretty much perfectly and if this was caused by synchro or gear damage i presume i wouldnt have been able to drive it flawlessly on recent test drive. So, any ideas on what this could be, if its not a clutch related issue (ie. hydraulics, leak in system, inadequete "push" or clutch seperation etc.) Other possible causes i've found are something to do with the spiggot, or as noted the input or output shaft seizing or something? Basically any info how the gearbox interacts, and what else could cause this would be much appreciated Please advise, thankyou very much in advance.

im gonna put my money on slave cylinder being ****ed

If it is not the slave cylinder, it might be the clutch fork pivot. Its a long shot I suppose, but it has happened to me. You can tell because the clutch fork will have a lot of movement in it. Or by having a look with a torch. It should look something like the image below. Mine broke on the base of the rounded area. I just removed both pieces, drilled, tapped and put a countersunk m6 bolt in there with locktite. Never caused a problem again.

Initially, I would have suggested synchros, but that's ruled out by the difference in operation after the BH bolts were replaced and clutch re-bled. The conclusion you've come to (slow hydraulic leak) I agree with; my only other suggestion would be a failure of the pressure plate. This would be more consistent though.

My money is on the Malaysian PBR 5 puck clutch plate trying to weld itself to the flywheel/pressure plate. Also do you have a chromoly flywheel or chromoly pressure plate face?