I presume you are talking about juice brakes? If one tells where one is located, (rust belt or not) it will help. Get new wheel cylinders, it is not worth fooling with cleaning out old, corroded cylinders unless last resort. Get a kit for your master. Then bleed the system with a pressure bleeder. Old wheel cylinders may not have bleed valves that thread in properly to seal(thus new wheel cylinders), then tell us.

Ed

'37 GMC T-18 w/ DD 4-53T, RTO-610, 6231 aux., '95 GMC running gear, full disc brakes, power steering, 22.5 wheels and tires.
'47 GMC 1 ton w/ 302, NP-540, 4wd, full width Blazer front axle.
'54 GMC 630 w/ 503 gasser, 5 speed, ex fire truck, shortened WB 4', install 8' bed.
'55 GMC 370 w/270, 420 4 speed, grain, dump bed truck from ND. Works OK.