I bought a rebuilt tandem pump from MBpumps in Las Vegas. Small outfit, I think I sent him a check first, installed the rebuid, and returned my core.

(702) 481-1312. About $200 in 2008.

And, if you have to supply air pressure to the low pressure supply input, it would seem effective to put air into the fabric covered low pressure supply hose. Again, this hose can clog.