I do an ebike commute here in NZ with a steep downhill at the end of the ride at night and a longish downhill in the morning.Brakes are Shimano Hydraulic MT200, 180mm Rotors and pads were Shimano resin brakes which I bought in bulk as I found I was wearing them out quickly. When they ran out, tried other pads-BBB BBS-53E and also have Kool Stop KS-D620E which haven't tried yet. Have bled the lines twice-but not since Feb-though have bled at brake levers front and rear recently...use Mineral Oil for the Shimano system but last week lost all braking capability on a hill, pumping worked to no avail-which usually works-and am picking the oil got just too hot so loss of pressure. Pads were new on front and good on rear. Anyone have any words of advice to alleviate this potential overheating? Would a more regular full line bleed be recommended? Or would a full line be rendered hopeless because of the level of overheating? Many thanks...in anticipation!
pumping worked to no avail-which usually works
This is ringing alarm bells for me - it implies you encounter brake fade on a semi regular basis.
This is almost certainly down to poor braking technique (dragging the brakes excessively). You need to learn to let the bike pick up speed and then brake hard for a short period and repeat. This approach gives brakes time and airflow to cool, and causes more of your energy to be lost to aerodynamic drag.
On a steep descent you can do things like sitting up tall and sticking elbows and knees out to noticeably increase your drag.
You might also like to investigate swapping to a sintered brake pad which will A) be more durable, and B) tolerate heat better. You mentioned using resin pads which are known to perform poorly when they get hot. Here is an article discussing the pros and cons of different brake pad materials: https://off.road.cc/content/feature/which-disc-brake-pads-are-best-for-mountain-bikes-sintered-or-organic-1901
First, I'm sorry you had what sounds like a nasty crash. I've been there, a couple of years ago and it's no fun.
I'd be looking hard at the pads. In my case it was mechanical disc brakes so there was no fluid to boil, and the descent was so steep and twisty there wasn't time to let the brakes cool much they were resin-bonded ceramic pads (sold as simply "ceramic", but that's only 70% true). The resin holding the ceramic together on the back melted, and after the event I could pick bits of it out from the vents in the rotor.
The front, in my case, failed for another pad-related reason - very fast wear on the pads and rotor over the course of the day which I didn't detect until too late (or I might have been able to adjust them). Hydraulics should be self-adjusting, but that's only true up to a point.
Sintered metal pads are better at dealing with heat than other types, and wear better than resin pads. I'd expect them to conduct no less (even maybe slightly more) heat to the fluid than resin though, so if it was fluid-related fade they wouldn't help.
With mineral oil (as opposed to DOT fluid), it's not likely to be the oil getting too hot, so much as a tiny volume of trapped water, which doesn't mix with oil and so stays in the caliper and boils. That could be addressed by changing the oil, or at least much of it at the bottom. Bleeding only from the top won't help at all, and could conceivably make things worse (or at least the need to bleed frequently could, as it would mean air getting in and air is humid).
After a crash you'll naturally be more cautious on descents, but the problem is that dragging the brakes generates a lot of heat. In my case, keeping my speed down to 30km/h on a 16% downhill works out to dissipating about 1.4kW of heat, and that's got to go somewhere. Going far slower helps; you lose cooling airflow but put much less heat in to start with. But this really does have to be a lot slower, like walking pace. A complete stop does allow the brakes to cool, but it has to be several minutes to help much, as heat will continue being conducted to the fluid from the hot pads. Going faster helps in a couple of ways: increased airflow over the brakes (even a tiny bit between pads and rotors assuming they're released completely), and air drag increases rapidly with increasing speed, meaning some of the braking power doesn't go near the brakes. If you're using one brake heavily, you really need to be distributing the heat more between the brakes, and the front will cool better.
Some brakes can take pads with cooling fins, though the benefit isn't massive. Bigger rotors can help too, if you can fit them. New pads need to be bedded in, but I'd probably replace the rotors after an incident. These are among tips listed for MTB disc brakes, along with bedding in pads properly, and bleeding really well. If you're looking to replace your brakes, 4-piston systems should be less sensitive to fade, spreading the heat over more metal from which it can reach the air, and more fluid.
Getting the weight down can help - can you store more in work, especially lock(s), heavy clothes/shoes, laptop. I'm heavy and my 2 locks for commuting add up to 2% of the bike+rider weight, for example.
If you're overheating them on anything than very steep descents, you're doing it wrong. Use air braking - that is, let the bike get faster until you reach the equilibrium and don't accelerate any more. When you do need to brake, do it in short sharp impulses - brake hard(ish) for about a second, then let it accelerate again, brake, let it go... Waver around the correct speed. DO NOT drag the brakes to keep you at a constant speed. No brake in the world can deal with it. You may be putting little heat in compared to hard braking, but you do it constantly. Gradually the disc, the pads and the oil will get so hot that the brake will fail. With interval braking, you put some heat in and let it cool down. If you are riding very steep hills, use metal (sintered) pads, they're a bit more resilient. Shimano do a version of some pads that have a titanium back plate and don't transfer heat that much (expensive as hell though). Shimano also do pads with cooling fins on them (and discs with cooling as well), perhaps you can get them for your version of the brakes. For oil, you already have the best with shimano fluid.
If the brakes are fading so badly that you need to pump them, THIS IS NOT NORMAL. You need to do something against it, pronto. Change your technique, change your pads, change your discs, if it doesn't help, find another route. Having to pump the brakes should by no means be an everyday occurrence.
Your ebike needs regenerative braking. The situation you're describing is a perfect opportunity to reclaim the energy. The biggest advantage of e-vehicles is using the brakes only for emergency stops, downhills are for charging. Ebikes without regenerative braking are rather unsuitable for hilly terrain, as the extra mass of motor and battery puts extra load on brakes meant for lightweight bicycles.
The kind of braking you're describing should not happen in the first place.