Hello, Papa Hajek,
I also have an air bed that uses the Gast C-5B-103 compressor. The bed had recently not been inflating despite running for tens of minutes. I have taken apart the air pump unit and done some rudimentary testing. While inflating, the compressor will turn off periodically (probably to get an accurate pressure) and the air seems to leak back out through the pump unit somewhere. The solenoid valves seem to be operating correctly and do seal off completely, so I'm suspecting the compressor. If the compressor still puts out enough pressure, I think the addition of a check valve between the compressor and solenoid block might, at least temporarily, fix the problem. The seals on the compressor will eventually need to be replaced. I may crack it open tomorrow and take a closer look.
I have temporarily added a quarter turn tubing valve and short length of polyethylene tube where I can manually blow up the mattress while doing this testing. Surprisingly, it does not take very many puffs on the tube to get the mattress to a satisfactory setting. I call this the "redneck" sleep number bed.
Ha ha! Fortunately, the air bed itself seems to hold air for at least a week or so with no noticeable loss, so we're still sleeping on it without the air pump attached. My wife puts up with my tinkering, as sometimes it actually does provide her some benefit.
The good news is that I'm pretty sure I was able to fix the compressor by just cleaning out some "stuff" (for lack of a better word) that was stuck in the flapper checks of the compressor. Here's my rundown on the process:
1. Remove the four long screws holding down the black plastic cap
2. Remove the black plastic cap and attached gasket (gasket seal is not critical)
2. Remove the two screws on the side that secure the electromagnets
3. Remove the electromagnets and attached wiring from the main body of the compressor
4. Remove the eight screws (four at either end) that hold the reciprocating compressor mechanism
5. Remove the reciprocating compressor mechanism as one piece, being careful not to stress the diaphragms at either end of the center reciprocating linkage
6. The compression chambers at either end of the mechanism are actually two separate pieces. The center section will pull out and contains the two flapper checks. The other section remains attached to the linkage via the diaphragm
In my case, the "stuff" that I found in the compression chamber was holding the flapper checks open. But if the diaphragm tore, you're probably better off getting a new unit. I guess you could try repairing it with some kind of plastic glue, but given the stress from the rapid reciprocating action, it would probably fail again soon.