Dot 3-is Mostly car fluid
Dot 4 or 5 have a HIGHER boiling point. All my bikes have dot 5. (Spend the few $$ and get the better. You be more comfortable on your ultra loaded up with everything including the kitchen sink, your spouse barreling down a hill!)
Basically most fluids are the same stuff but with something?? to make the boiling point higher.
Now, you can use SILICONE, that doesn't ever absorb water, (ALL brake fluid absorbs water in the air, never keep an open bottle @, when it's yellow, it got water in it)
Silcione is great, once replaced, forget about it.
You gotta clean the entire system out, no old brake fluid though. I swapped regular fluid for silicone and just flushed it good.
The seals won't matter if you use 3-4-5 or Silcone.
If it won't gravity bleed or you can't get a good brake pedal,
What I do is
I have a "Brake syringe" I bought at a auto parts store, (but I've used a turkey baster with a clear hose)
Fill with fluid and WITH ALL THE AIR OUT, ( get you syringe lower than your calpier and hose higher a bump to push the fluid out, (like a nurse giving a shot), gently push fluid from your caliper TO your master cylinder, (this is called reverse bleeding).
Get you master empty before hand.
This pushes the air from the calp to the master.
Or you can PULL the fluid out by placing the hose on the bleeder pulling the plundger out,kinda "sucking" the fluid out.
Sometimes you gotta reverse bleed to get the air out as it get's stuck in the someplace.
You can do the old pump the piss outta the lever crack the bleeder and SQUIRT fluid all over, but that's hard on your master cylinder. This is a much better method.
Get you master higher than your calp and you'll gravity bleed.
Once you do it, you see it's not hard. Just DON"T GET ANY AIR in your lines. Air can't compress and you'll go "*&%%^$$@@#!!!!" when you try to stop.
When you done flushing the system, gently pump your brake lever and it should all be good!
Joe