Hi Alan, I'd be unable to suggest a cause without doing some research. Do you mean the seal on the pipe connection to the base fitting, or somewhere else? I walked down to look at the pump on mine ... I can't see a thing because I made a protection cover which bolts on in front of the pump and radiator, and can't see through it. It is 1/8" plate perforated with a honeycomb of 5/16" holes. There was nothing there when I bought the machine and it worried me having the radiator and pump completely unprotected - very reassuring to have it there. I can't find a clear photograph of it, so I'll try and remember to take one to post.
I'm also wondering if the pipe has an end fitting with the wrong diameter, or you're being supplied the incorrect O-rings. Are you sure it is the O-ring that's leaking ... maybe a crack in one of the fittings? What exactly do you have to disconnect to renew the leaking O-ring?
If yours is a 3cyl, it seems it is an MF40 like mine. TractorData shows the 40B has a 4D-203 of 59HP in both Industrial and Loader-Backhoe versions. That's what I used to identify mine - it no longer has an ID plate so I was in the dark. Also, what style of bonnet does yours have? Mine is the typical Industrial style - has no resemblance to 35, 65 or 100-series styling.
Here's a link to it - a very useful site:
TractorData.com Massey Ferguson 4 B backhoe-loader tractor photos information
Alan, would you mind doing a big favour for me by taking some detailed pics of the backhoe mounts for me? I've not seen one close up at a show day and am interested in how to mount the backhoe assembly to the frame rails under the rear axle, and the steady bars to prevent F-R tilt reaction - I believe they come forward at an angle to the holes beside the clutch and brake pedals. Can you also please include a shot of your loader controls so I can see where to tap off a supply hose? Many thanks in advance for your help Alan ..... the only other thing I would love to have here is your machine!