Adam--

I'm surprised to learn of this failure and feel badly because I'm the guy who recommended the Bosch relays back in the nineties. The part number installed in my L-235 is 0 332 002 150 with no diodes. Apparently the relays you used (model 0 332 002 156) have diodes to snub the voltage spike when the coil de-energizes, which seems like a good idea. In looking at this data sheet, though, there are some other differences between the -150 and -156 relays. The -150 relays I used have dual (Silver Tin Oxide) contacts. They are rated for 40,000 inductive life-cycles and 400A inrush. The -156 relays that you used have snubbing diodes across the coil, but only single (Silver Nickel) contacts. They, too, are rated for 40,000 inductive life-cycles, but only 250A inrush. I can't say that one version is better than the other because the specifications on both of them certainly seem adequate to handle the hydraulic pump. In my installation I installed large ZNRs across the relay contacts to snub the voltage spike from the motor when the contacts open. I also installed smaller ZNRs across the relay coils to (hopefully) protect the pressure switches. You wouldn't need the latter because your relays have the snubbing diodes.  (I'll get the ZNR part numbers I used next time I'm at the hangar.)

Do you intend to take the relay apart to verify that the contacts are, in fact, welded? Bosch might be interested as well, but I certainly wouldn't mention that you're using their relays in an airplane.