hi all, no matter what I change in ball and spring, even a bigger ball my pump gravity leaks into the crank

the check ball rubber seat must be the culprit. Today I pulled it out and it measures 
7mm dia
4mm thick
2mm hole at the bottom of the internal cone
material rubber
t seems to be the opinion that the pumps have a metal housing seat and not a rubber one.
can anyone enlighten me on this and where i can purchace a new seat.
pics of seat attached
 

You might check with DEET and see if he has something like that.

Not sure if i'm on another planet-"but likely" lol. I thought the ball & spring seated on the pump body alloy on the outlet up to the cylinder on 'any' yamy pump. Do you have a new fangled external one.
Ok, i googled & found this. rdlccrazy.proboards.com/thread/47403/str...il-pump-check-valved might be interestng to read. Don't think i'd sleep at night if a had something stopping flow on the inlet from tank to pump but that's just me!!. If it opens by vacuum from pump suplying cylinder & vacuum pulling on the valve, then if your small ball & spring is faulty on the outlet side--or even worse--missing--would that create a vacuum even without motor running & pull the fangled new valve open to carry on oiling the motor as you describe. Sorry if i'm over-complicating the story--my specialty 'that'.
Just re-reading what i wrote earlier & think i meant--does oil moving into the pump cause a pull on the ball in the inline stop valve if fitted to the line between tank & pump. Umm--Never mind lol.

Could run a vacuum fuel tap inline from tank to pump like scooters have for a fuel supply from petrol tank. Just more dodgy ideas from me. Ignore it.

I was always under the assumption that the ball rests on the housing funnel
this seal sits on a ridge above the housings taper at the bittom, so must of been fitted by yamaha
bike is a 75 DT400b

i have now seen the external check valves and if I cannot locate a new seat then I will have to go this route

Again, if anyone has an answer it would likely be DEET. 

I think Yamaha also made a check valve for the inlet side...  I couldn't find the post.  DEET or Enduronut would know.

(The check valves shown so far go on outlet side of pump...  used on snowmobiles and jet skis...  plastic and metal versions available...  not sure if hose size is the same.)

I personally would fix the pump properly, knowing me I would be halfway down the street when I realize “oh shit” and hurriedly pull over and turn the check valve on, by then it may be too late for your engine, unless you opt for the vacuum activated type. I know I can raise my hand when asked have you ever forgotten to turn on your fuel petcock only to realize it after your bike stalls at a intersection and the people behind you are honking their car horns at you. 

Hi Guys
ive now tried to seal against the housing taper cone inside but this is also letting oil past. The internal casting for the seat is quite rough, meening a bad casting or machined surface. I assume thats why the rubber cone was fitted.
as this is the 11th time ive had it apart, so ive ordered the external check valve and will do my testing with that.
ive checked them and they dont have a tap, so should work like the internal unit does, will see when it arrives. Yamaha says its still made

Here's a link to a pic of an RZ500 with outboard ones fitted, I stupidly thought someone had mentioned one that fit between tank & pump which i couldn't get my head around so far as how it could possibly work. Anyway, one sure way would be an inline tap "petcock" & something to remind you to turn it on each startup. Just don't forget!!.
www.rzrd500.com/500phpBB3/viewtopic.php?...7a57041768284f496f5e

Well, I’ll look at my stuff, if you find a DT250a oil pump would works, and I can get mine out.. we can talk
Sneezles61