It's not worth the effort to try to fix line cards... usually. If a line won't go on hold, make sure your A-leads are connected properly and you have a line light when you go off hook on that line. From the sounds of it, you've done this already and have tracked the problem to the card itself. It is more than one relay that makes it put the line on hold, I believe. The other relay that could be faulty is the L relay which is in the metal can. You can't open it, nor should you. That's where the green goop lives. If the problem is indeed the line card, the thing to do is just replace it.
Be wary of the the ebay seller, Cliff. I've bought line cards from him and also asked specifically for Issue 15. My request went completely ignored and I got everything but a 15. He's on the up and up, otherwise. Always ships pretty quick, and usually has some good deals on old 1A2 gear. He probably sold out of Issue 15's ages ago. I only have 5 Issue 15's and they're in use, or I'd offer them to you.
There is a really simple modification to make any other issue of 400D into a 15. It only works with other 400 "D", none of the other letters. But 400D's are a dime a dozen. I got a schematic from another member on how to make the modification, and it involves desoldering one end of one component and adding a single resistor. It takes literally 2 minutes to do and it is completely reversible should you want to. I have to ask if I can share that. I'll let you know.
