Had the same problem...many times...95% slave is at fault!
fought slave cylinder issues for many years on my Backdraft. My first failed third day of ownership! LOL I did not find heat being an issue, hell std brake fluid boils at 450*F...that is hot! anyway
IMHO, It is a slave cylinder that only uses a single U cup seal. Switched to a dual seal on the slave piston (one U cup and one O ring) have never had to bleed it in 4, maybe 5 years! B4...had to bleed 2 - 3 times a year. You can probably machine an "O" ring groove the the piston you got. The other issue is the bleed, at least on mine was on the bottom...
so, I did a new mazda slave that had the bleader on top. I think I put the P/N on this site.
worked 4 me...but one man's food is another's poison...
found my post for 2007...the unit is a 1988 mazda L4 2.2 liter is the one that came on car #260, (advanced auto parts #0728342, this was the number on 9/18/2006, also, the 2.6 liter is a top bleeder version).
My slave failed because of overstroke, according to the Backdraft boys in BB, Fla. They sent me a new one, this ones seal was in bad shape, didn't even use the new one. My failed seal was in better shape. Honed the slave cylinder because of a sever out of alignment of the cylinder to the fork,...this caused the push rod to rub on the bore, plus the over-stroke...caused leakage, draining all fluid overnight. Adjusted the stroke and its been 6 months on the re-aligned slave and honed unit...still good. BTW...stroke adjustment are the nuts on the shaft from the clutch petal to the master cylinder. If you get a replacement, I'd dissassemble it look at the seal to make sure it has a good lip and the bore looks good. Hope this helps.