Brakefluid to blame?

[Belgium R/T -68]

The innerpistons on both my new mastercylinders (Dorman M75818) keeps hanging in the inner position. I don't see any damages in the bore
or on the rubbersealings so that's why I wonder if my oil, which is DOT4, can damage the rubber? I always believed that the only difference between
DOT3 and 4 was the boilingtemp. Please correct me if I'm wrong.

Per

[tan top]

all compatible, can mix both without problems

[ChargerST]

Why do most repop master cylinders say "DOT 3 only"?

[maxwellwedge]

Because that is all there was back then.

[ChargerST]

I know but I'm talking about new master cylinders - like the MP Brakes master cylinder. These master cylinders are not a correct reproduction but a functional reproduction. So no point in reproducing a correct cap which says "dot 3 only".

[b5blue]

They are both poly-eth blah blah-glycol based....DOT 4 has a higher boiling point then 3, DOT 5 is silicone based, Dot 5.1 is like 3-4 none should be your problem. You may be thinking of British cars that used "Gurling" type fluid, Dot anything would melt the natural base rubber seals in them rather quickly. I know I use to work restoring and repairing them.