It sound like you have little option but to strip the trumpet housings off and have a look. These oil immersed disc brakes (if the correct axle oil is used and changed at the correct intervals) are normally good for the life of the machine. Are the brakes noisy and is there traces of the discs in the axle oil? Also you should check the axle oil level as if the piston seals are leaking this adds to the axle oil. If the axle oil is at the correct level, the chances are that the piston seals are ok. Has the axle oil been changed regularly? Also has the correct oil been used? Sorry i can't be of more help but i have nothing in writing to send you.