Remove the round end cap and see what you think of the thick black diaphragm.....it'll have impressions from the pin and spring transfer heads. Is it still flexible?
Check the large plastic washer.
Get some silicone grease on the threads of the spring IP adjustment screw.
Next comes the difficult bit......remove the yoke nut, it's been on there for decades and will be a pain......if it's really seized, don't worry, as it really doesn't have to be removed to do the next step......(your 14 won't be a problem as the yoke is held on with a circlip).
Take out the guts under the sintered filter and get some christoube or silicone on the shaft of the seat and work it into the balance chamber, reassemble and probably find it's working fine and no parts are needed.
2nd stages with silicone diaphragms almost never need any parts, even the seat.......I'd just take it apart, clean and lube the orifice nut and clamp screw. I wouldn't even flip the seat.
Parts need for a full overhaul:
First Stage.......new style grey thin diaphragm, large white plastic washer, sintered filter and circlip, balance chamber o ring and HP seat. (don't remove the white packing washer under the o ring in the balance chamber)
Second stage......LP seat, orifice nut static o ring.
Plus o rings for each end of the hoses.
Vintage Double Hose or eBay.