If it has antidive valves for the front suspension, then the valve and piping might be blocking the return fluid flow.
Old brake lines can fail with the internal hose swelling and blocking under pressure. The brake dragging could have a dud line.
There is little value using DOT 4 fluid, after a few months its deteriorated to no better than DOT 3. DOT 4 is really most usefull on racing systems.
Are the hydraulic lines seperate off the master cylinder or do they have a single line into two
2001 Yamaha Zuma, DR Evo 68 cylinder, Leo Vince TT derestricted, Delorto 17.5 (#98 jet), standard aircleaner, RMS fan, Race CDI, Standard oiler and premix @ 75:1 Shell SX2, Doppler SR3 variator, RMS clutch, RMS R rear shock absorber and Pirelli SL26 tyres.