Hi all,
I’m trying to work out why my front brake on my DT400B is very poor (I don’t mean poor compared to a modern bike, I mean poor for a 1970’s hub brake). Visually, the hub braking surface is fine. I fitted new brake shoes (Yambits). However, when I adjust the cable (so that the shoes are just not catching when I spin the wheel), the handlebar lever can be pulled to the handlebar grip and there is only a mild braking effect. I was hoping that it was a case of the new shoes just needed bedding in, but I’ve done over 10 miles now and there is no sign of improvement. I set them slightly ‘on’, otherwise it would have been a bit dangerous. I cleaned out the brake dust (and used brake cleaner) and put it all back together and there is no improvement.
It has been suggested that the hub may not be perfectly round (I don’t know the history of it and the rim was trued recently, which may have caused the hub to distort??? EDIT - the handlebars and forks were bent when I got the bike, so it could have been in an accident - so it may be distorted because of that???) so was thinking my next step is to dismantle, check and get the hub machined if necessary.
However……, there COULD be one more explanation. The operating lever COULD be too long.
I would be eternally grateful if someone with a DT400B could measure their front brake operating lever (pivot point to pivot point). The measurements on mine is 94.5mm. It doesn’t have to be that accurate. If yours is significantly shorter, then maybe I should change my operating lever before resorting to machining my hub.
Sorry for the long explanation there….

 
 

New shoes may be undersize and probably need to be arced to the drum...  a lost art...  but I would imagine is still available in the UK someplace.

Scootern29 has posted his homegrown method of matching shoes to drum

Another common issue on front brakes is cable and even cable routing. 

Thanks,
it wouldn’t surprise me if the shoes were undersized. There is a certain supplier in the UK where I get most of my Yamaha bits from (I won’t name the supplier) and a lot of it is crap (poor copy of the original part). I end up chucking them in the bin….
Swing arm bushes, indicator lens gaskets, fuseholder and fork dust seals come to mind..
I’ve always found aftermarket parts very poor quality (thinking back to when I played with air cooled vw’s).

"There is a certain supplier in the UK where I get most of my Yamaha bits from (I won’t name the company)"

You named them in your original post!

I always try for original Yamaha parts first (and note that they aren't beyond superseding to parts that don't actually fit!  ), then Yambits or Sumo, and I've not really had any problems.

Yamaha still supply the shoes, albeit with the part number superseded a number of times.

Was there a change to a conical hub at some point and did the shoes fitted change accordingly?

I’m confused now because I’ve just had a look at the aforementioned suppliers website and they list two options:
1. ‘DT400 1975-1976’ - EBC & Ferodo front brake shoes are available
2. ’DT400 USA (twin shock) 1975-1976’ - nothing available.
I don’t remember there being two different options when I bought mine this time last year, so I have probably ended up with non USA spec shoes (I bought the EBC ones) on a USA (import) bike.
Hmmmmm…..

I had trouble with EBC shoes being undersize on the 175's.  They were very helpful and sent me a different part that was correct size.  Yamaha shoes are still available and never had trouble with them being undersize.

But all shoes often need "arcing"...  used to have it done all the time in the days of drum brakes. What you may have a very small contact patch of shoe contacting drum. 

Something else that can happen is backing plate not centered in drum.  With axle loose, apply the brake and hold it on while tightening axle.

The best brakes during the drum brake era were the Yamaha conical hubs. First thing I would get the shoes arced in for full contact which takes time and some plumbers sand paper. The second thing may be a cheap cable that stretches.  If set up properly, when you pull in the lever hard enough it could throw you over the bars.

Yes - I have an aftermarket brake cable which is probably made of rubber…
I can’t remember where I bought the brake cable from now, but if anyone can recommend a uk supplier for a good quality DT400B front brake cable - it would be much appreciated…
Oh, and if anyone could measure the length of their DT400 brake lever arm (see post #1) - that would also be much appreciated…
Thanks for all your help

I have the same issue with my 74 DT360 and could use some advice. The brake is "mushy" and i can pull the lever to the bars even when it is set to drag slightly. My Brake Arm is 96MM long, I am using Vesrah shoes that I have arched with sand paper. I tightened the axle with the brake engaged. Ive broken them in for 100 miles. 
A friend of mine told me he thinks these enduro bikes are supposed to have bad brakes so they dont lock up easily in offroad use but I dont buy that excuse, I'm afraid to ever go 2-up on it cuz I wont be able to stop even at normal speeds. At this point I suspect my cable.

I am using a "Parts Unlimited"  K28-3521 Front Brake Cable, its 2mm steel cable.
Does anyone know of a brake cable that they can verify is thicker and where to get it? Or can measure an OEM brake cable wire size for me?

 

My 76 DT400 measured about 3.705" center to center of the front brake arm or about 94.107mm. So about the same I think.