I'd suggest you upgrade your shoes to OEM Yamaha shoes .
I'd originally fitted Premier shoes in my DT1 , which I was told was an EBC subsidery .
When originally fitted the Premier shoes were very spongy in feel , and the braking was poor .
It did improve over time , as more area made contact . My hub was out of round .
I've done nearly 2000 miles , and still not all of the shoe was contacting .
Recently , I bit the bullet and bought a set of Yamaha shoes .
I ran them in over a 55 mile loop , and already they stop better , and solid feel at the lever .
Light years better than the Premiers .
 

Sorry, not read back on post so might be duplicating others. But if for some reason things aren't bolting up square it doesn't take much to get a spongy brake. Accidents within reason shouldn't cause that even if the forks a bit bent so long as the axle lugs are square to each other. [maybe i just rubished my own theory there] anyway. You could sit the bike on a box with wheel just touching the floor & revolve the fork to see if bent. & if they are, then time them to both to sit either forward or back but keep them in line. You can pretty much eyeball them & turn them 360 degrees & go--There--.Then bolt the wheel in & with the nut 'just' nipping spin it & whack the brake on hard & hold it on while tightening axle. Then go try it. Oh, & tighten clamp too if it has one on the other axle end. Also tighten tripple clamps, but you shoulda done that at the point of timing the fork bend. But wait there's more--On one of mine some bugger [probably me] had way overtightened things & half the circle of the speedo drive extension in the brake plate was broken away outboard of the circlip holding the speedo gear. Maybe a bit less than half but enough that when tightening axle the plate tilted [only a little] by force & total stuffed the brake--just a useless sponge. Ok another essay but can't think of much else except bad shoe radius or wrong shoes. Is cam the same shape both sides, not square cornered on one flat & round edged on other so only goes one way. Don't think Yamaha do that but have seen it on something.
How much movement is there on the arm before the shoes come into contact. Text that with wheel off the bike because it obviously changes when bolted up.

Hi Jammyyammy, I was involved in a thread a few months ago regarding drum brake performance and the general consensus was that Yamaha OEM shoes were definitely superior to other brands. The other learning point for me involved the quality of the brake cable too.  The failings of inner cable stretch allowing too much lever movement is fairly obvious BUT equally annoying is a cheap outer sheath spiral that 'crushes' when you put load through the overall cable.  Like the shoes it would probably be best to go OEM on the cable replacement to ensure quality. That said I would also think a reputable supplier like Venhill would be a good place to buy quality components and then you could make the cables yourself. 

Thanks for all the info. I’ve never had any luck at my local Yamaha dealer ordering parts for my DT400. They never have anything. We’ll see how I get on with the brake shoes and brake cable tomorrow.
My aftermarket front brake cable was from MP (motorcycle products).
Has anybody had any luck with Ferodo brake shoes and Yambits brake cables? - I may have to give them a try if I can’t get OEM.
One last question…. IF I was to use sandpaper on the braking surface of the hub, what size of grit would I use?
Thanks,
Phil

"Thanks for all the info. I’ve never had any luck at my local Yamaha dealer ordering parts for my DT400. They never have anything."

I wouldn't expect them to, other than maybe plugs and oil.

What I would expect them to do is order from the wholesaler (usually Fowlers) for you. My experience of delivery times is 2-5 days if in stock, or 2-4 weeks if has to be back ordered. 

If you grab the arm without a cable does it feel positive or spongy. Then slide a steel tube over it with a bit of length & try that again. Should feel quite positive for on & off feel if everything is as it should be. I remember nearly 60 years ago at work when the boss showed me a trick [well i saw him doing it & he didn't want to show me lol] on his B32 Gold Star Scrambler he flattened off the located end of the shoes so they could float to find the perfect spot when applied. It worked good too. Shoes post & maybe the cam too had a head on it so shoes couldn't escape.