Well still having a few teething problems with the 76 CB750F1, but everytime I have a problem I either do a search on this forum or post my problem, don't think I could have got this far without the advice and encouragement I have recieved.
The latest problem was the tacho drive oil leak, it got worse, so I did a search and found the answer on the famous JohnK6 "How to rescue a CB750 from the garbage dump" thread, boy that should be saved somewhere.
Used a wood screw as advised and out popped the oil seal, 10mm 1/4 drive socket and in went the new one, as stated 5 minute job.
As to the Master cylinder problems I had, see the pic below of what happened after I accidently put fork oil instead of DOT 4. I could not believe the seal would swell to this extent, but the eagle eyed amongst you will spot the piston marks on the seal that have grown as well.
Cheers
Den
Removing the tacho seal
Contaminated seal and old seal