how do you get the back cap off? i might be in the wrong section.
Welcome to the forum. Stinkypete posted this in another thread - http://www.microprecisiongroup.com/d...aintenance.pdf. They have maintenance files for their P & K horns at the bottom of their webpage. Maybe one of them might help.
From what I’ve read, they have a clamp ring around the back of them.
There’s a restoration thread on the Horn and Whistle board which is a good site if you need to ask for help with your restoration project.
ok that helped me… now does anyone know where i can get new gaskets and the diapeams?
They’re probably custom made given the age. The Horn & Whistle board could be your best bet in finding out where and how to get a set.
Last but not least you could try contacting Ed Kaspriske’s at the Horns Inc. web site. If anyone will know it’s him. That guy has a fantastic collection and although he doesn’t sell any horns, he will probably set you on the right path if you ask nicely 
Good luck with the restoration project. The M series sounds like a nice mellow horn that’s a real historical classic. Post a new thread showing off your handy work if you manage to sort it all out.
Go on the Horn and Whistle Board and post your question then ask for Tom Aker’s info. You can probably get diaphragms through him. There is a lot of good info as I’m a member there myself. Just don’t mention about blasting people, then you will not get that far. Also they are tuned with a special spanner wrench from the back cap. Do you have the round or scalloped bell?