The numbers refer to the square inches of the diaphragm. One of them appears to be a 24 service brake with a 30 spring brake. A 30/30 would fit, but it would would give you more braking power on the drive axle which could cause the the rear brakes to lock up on wet pavement.
I see 24/30" on German eBay pretty reasonable. In German spring brakes are Federspeicher Trommelbremse. I don't know enough Dutch to find them on eBay.NL. I am quite sure they are the same size mounting as US, but you might check. US 24 and 30 are both 4.75 or 120.7 mm between mounting bolt centers. You need to make store that you get one with a long plunger that you can saw off at the correct length. US clevis are 5/8 fine thread. The old clevises are probably rusted tight. I keep a bucket of clevises on hand and don't even try to remove a clevis that has seen salted roads.
I will be meeting a guy from Holland at an old car show in May 13.
http://www.motmontfort.nl/joomla/ He recently sold his trucking business.
The compressor looks like an old Bendix Westinghouse Truflo400. They are pretty obsolete and are usually replaced with Truflo 501 compressors. The old BW compressors usually keep on pumping air but are replaced because oil gets into the air which is no big deal.
The valve you show looks like a quick release valve. They are called a Bendix QR valve.
I also answered your PM.
