No the mast dosent need to be removed to get to the brakes on these trucks....sounds to me like you just removed the outer plantary hub and not the whole axle end....the brakes are internal to the drive housing and actually face backwards from what they are on most other teucks and vehicles....to remove the axle end there will be a seies of bolts and studs around the perimiter ofmthe axle end...bolts on the forward portion and if you raise the mast and look at the back side you will see the studs and nuts that come thru to the back....these axle ends weight upwards of 100 plus pounds and are eccentric so will be a bear for one person to man handle.
First remove the bolts that are on the fwd front/outboard section the heads of the bolts will be in the neighborhood of 1 inch plus...then remove the nuts on the backside with the mast raised AND BLOCKED to keep it from falling....
In my opinion if youve never done a brake job or work on the front end of one of these it would be best to have a technician do it....

Also if you do try and go it by yourself...onlynremove one side at a time...removing both sides can cause issues with the center section dropping out of place.....