You might want to check that you have your front wheel axle assembly correctly put together.  On my DT1 I had a washer/spacer in the wrong position so when I tightened the axle it put the forks in a bind and they would not return, just like you described.  After studying the wheel and parts pics avialable on line I moved/added the spacers/ washers around and sloved the issue.   Went out and took another look at my front wheel and I did indeed move a washer from one side to the other.  So, my wheel is not assembled according to the parts picture but the forks are not binding.  I have been riding it that way for a couple years.  Guess the next step is to take some time and figure out the issue.

 

Loosen everything, make sure its not binding. Fork braces can cause binding of not installed properly. 

Check your fork stauchions are straight .
If there is a bend in there , depending on where the staunchion is sitting ,
the staunchion and slider could bind . Rotate each staunchion through 360 degrees
at 90 degree intervals . If there is a bend on one or both tubes ,
the distance between the sliders at the axle will change .

Are you expecting the forks to be "topped out" unless you hit a bump?  That's not really good suspension.  Some sag is beneficial.  

You haven't said anything about how much travel you're talking about here?  A few mm?  Half the fork travel?
 

Possibly your  springs are just suffering from age.  One suggestion is to install some spacers above your springs, which may take up the slack.  Your springs are the same as my DT1-250, 214-23141-00-00 which have a standard length of 438mm.  You can pull yours to measure and see if they are not to length.  Then fabricate a spacer to compensate for the shortfall.  Attached is a service bulletin for DT1's, & RT1's that use the same spring which might be of some help, if you do not already have it.

Hi Mauser, you mentioned it's quite difficult to gauge the amount of unwanted sag on your front forks (as Mark T said a little is healthy). You could try putting a couple of cable ties on the stanchions just above the dust seals, then when you depress the suspension you'll get an idea of the 'rebound' sprung distance. Also when you then assist the return by lifting the handlebars up you can see/measure the remaining travel in your forks. These observations along with measuring the spring lengths will help decide how long you need to make your fabricated spacers. Hope this helps and good luck with it all.

Mauser, from the info you've supplied you are saying the springs return 50mm of the travel and you can then lift the bars and get a further 50mm of travel ? If so then that doesn't sound right to me. I would expect it to be more like 85mm rebound from the springs and approx 15mm 'extra' when you lift the bars. You don't mention whether initially you had overhauled i.e. stripped the internals of the forks prior to this problem occurring? Have you got the right viscosity oil and correct volumes in each fork? If all this is/was correct then consider rechecking the fork replacement section of your manual, as my manual was very specific about the order in which you retighten the various nuts involved presumably to ensure 'binding' doesn't occur. Good luck with it all

Just a small point here. If you've changed the fork seals and not used genuine or high quallity ones you can get a surprising amount of sticktion .
Couple that with old springs and you lose some boing from the springs action. I would agree too with around 15 to 20 mm for front sag. Don't fret too much if it's not bang on as you're not giving it a handful of brake before hitting a bend at 85 mph as the rear shocks are more akin to a space hopper than real suspension.

Hi again Mauser, I've done some testing on my DT250B forks and have the following results: 84mm was the amount of rebound on the forks when I  held the brake on and pushed my 12st through the bars. When I then lifted the bike the 'extra' extention on the forks read 92mm i.e. a further 8mm of travel.  Hope this helps. Andy.