I suspect one of the rubber cups is broken/cracked and causing the binding or locking up. I think a rebuild should be investigated. There are instructions here and on line of how to rebuild those pumps. Sprkplug a member here does rebuilding of those pumps. Here is some additional information. Sprkplug and I have rebuild kits available for those GAST pumps. The rebuild procedure for Gast and Thomas rocking piston compressors are similar.
https://www.youtube.com/watch?v=JvVv2iRDVz8
http://forums.pondboss.com/ubbthreads.php?ubb=showflat&Number=390615

Last edited by Bill Cody; 07/02/15 10:58 AM.

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