Slave cyl. leak is most likely due to rust caused by moisture that's been absorbing into the fluid for years. Doesn't take much.
"Brake & Equipment Warehouse" in MN can reline M. Cyl.'s with a stainless steel sleeve. They can probably do a slave cyl. the same way and you would never have a rusty cyl. again. They have done excellent work for me on a couple of odd projects. They seem to be able to do almost anything involving brakes.
(Ph. # 612-378-3141 Rob Anderson)
ROB@BRAKEPLACE.COM