This unit has the D6 undercarriage. I've replaced on the older undercarriage but cant figure this one out and help on getting this brake ban out

The brake just stopped working as you where using the machine? If that is the case, you most likely had pin fall out in the linkage.

If you need to remove brake band to read the service manual is easy way. I believe it should be in the members only area Tech Library here to read. But in general remove fuel tank to get to the steering clutch compartment. The band is bolted together and comes out in pieces.

Still most likely a pin is out or the band is broke, if it stopped working instantly.

Yes stopped at once, became member and looked in members only Library for instruction but can not find anything. I agree it is a pin but can not feel or see location. Any help would be great. pictures even better

I have a 955E also 12A8263 (1960) the D315 is yet another project. Ray54 is right the fuel tank and if you have a ROPS they will need to be removed; also if you have a rear attachment they may also need to be removed for access. I'm sure you will need to remove the hydraulic tank (right side), and hydraulic filter (left side) to get better access. A service manual, parts manual and an Operation and maintenance manual are going to be a must have source. Each with its own information you will need. Cat is infamous for putting bolts, nuts and other things in your way and the 12A Traxcavator is three times worse.
Have fun and.Good Luck

I have not spent much time in the library section (I have the books that cover my tractors) but it was time consuming in my way of searching. But you need the Serviceman Reference Book that covers your tractor. But It might be under one of the D4 models. I am not familiar with loader to tractor correlation, so will leave that to others. No earlier than a u series D4 but could as late as a D4C or D.

Depends on how you wish to proceed, had thread about changing a steering clutch in a 9u D6 without removing a cab, just fuel tank I think. So I would guess you could get fuel tank out without removing other things. But have not done it, so I could be wrong. But loaders are noted for being a stand on your head job to work on.

A start would be removing the cover in back used to adjust the brakes and steering. But attachments do get in the way.