Rob
I rebuilt the reserve taps in both our MK IV’s, using Tygon tube instead of cork or o rings, with success.
Regards
David
That’s a very interesting solution. Can you recall the type of Tygon and inside/outside diameters you used?
0900766b81656fcc.pdf (rs-online.com)
Thanks,
Peter
Peter
The valve stem as I recall was about 1/4 inch and the valve internal I. D. was about 3/8, the tube used was green Tygon, petrol resistant, don’t know if it was a certain grade or if there is a certain grade.
The piece required is about 3/8” long.
The valve still operates and for 25 years so far has not leaked.
Regards
David
Thanks David.
Sounds like a really good fix.
Peter 
It’s not the length of the suction pipes that determines the volume of the reserve versus the main fuel quantities in the Mk IV. This would be the conventional method used in a normal tank which draws from two different levels, but the Mk IV design is simple and clever to ensure you always have a reserve quantity irrespective of the tap setting. Both pipes are extended into the filter screens which are integral with the drain plugs, so if you empty one side, there will be barely a cupful left in the drain plug sump. If you have removed the two drain plugs and don’t see the pipes low enough to seat inside the filters, then they have probably been bent away. These filters had two purposes - to hold back rubbish and water. If the gauze is the right grade of fineness, water won’t pass through with the low head pressure in the tank.
What determines the storage separation is the low-height baffle across the tank which acts as a weir and which has no transfer holes. There are some photographs of this in this forum from a few years ago. When one side is pumped dry, the other side holds a nominal amount on its side of the weir - apart from what might slosh over the top during driving motion. The baffle is offset, to the left of centre I think, so there is a different volume available each side. With this arrangement, either side can be the reserve so it doesn’t matter in which position you set the reserve tap, but it is normal to prioritise the tap knob in the lower position.
To replace the tap seals, I used a piece of rubber fuel hose that was, by coincidence, a close match to the tap id. The adjustment of the compression to ensure a seal was good needed a bit of trial and error. Every now and then I stroke the tap up and down to avoid the seal sticking. It hasn’t after ten years or so.
Thanks Peter. If I’d given it more thought I’d have realised that my suggestion would only have given a miniscule reserve volume. 
I’ll get my coat. 
Peter
Don’t go! Please stay and have a cuppa.
Ok, thanks, but only if I can I have it from a reserve cup.
Peter
My cork seals seem to have shrunk and gotten hard.
I guess I’ll look around for 3/8" OD fuel hose and see what I can do with it.
Found some vacuum hose slightly over 3/8" OD, cut two pieces 3/8" long, assembled the plunger, at first it wouldn’t fit in the body, but I smeared oil on it and managed to push it in. No drips. Back in business.
Rob
I am not sure what vacuum hose is manufactured from, I am concerned that it is not resistant to petrol.
The lawn mower repair shops usually have short lengths of suitable Tygon tubing.
Regards
David
Regards
David
Buna-N, very resistant to gasoline.