I think what you are seeing as a line lock us a sequencing valve. If you push the left pedal pressure is applied to the inching slave cylinder THEN the piston onside the sequencing valve is pushed against the fluid in the right side circuit- this applies the brakes. Can you post the full SN and I can coach you through troubleshooting?

doesn't this unit have a master cylinder on both sides? the one on the right side is for the brakes only, while the one on the left side is brakes and inching valve (transmission decoupler) and what you may be calling a "line lock" might either be a crossover, or someone installed a mico-lock kit (which may be part of your ?problem?). the brake fluid from the left side master cylinder will be the one that pushes on a valve on the side of the transmission (under the drivers foot area).

Thanks for your reply, yes there is free play on the master cylinder.
If I have this right, the master cylinder is at the bottom right side and has a rod back to the peddle linkage. The brake line comes from back of master cylinder up to a line lock and then to wheels from what I can see.
does it go to gearbox at all?

chips.

at the "at rest" position of the inching side of the brake master cylinders, is there freeplay in the shaft that pushes the spool of the master cylinder? if not, adjust the linkage to allow about a few millimeter of freeplay