Hydraulic Brake Fluid.

[Boilers2000]

I have a '92 eagle trailer with a hyrdraulic brake actuator. Manual says you can use Dot3, 4, or 5. I'm pretty sure Dot 5 isnt being used since it's a boat trailer. Does it matter if I top of the master cylinder reservoir with Dot 3 or 4? Google searches haven't been conclusive? I thought Dot 4 was compatible with Dot3, but just talked to a trailer store to confirm and they said I would be fine putting Dot3 in even if there is Dot 4 already in the reservoir. Anyone have any input?

TIA!

 

[Scott Danforth]

You can put DOT 4 in place of DOT3 but not the other way around. DOT 5 is not interchangeable or compatible with DOT 3, 4, and 5.1 fluids and can cause catastrophic system failure.DOT 3, 4, and 5.1 are glycol ether based. They are compatible, DOT 5 is silicone based

so if you dont know, either flush and fill with DOT 3 or top off with DOT4

 

[Boilers2000]

Thanks Scott...that is what I thought!