Vacuum pump for cruise control?

[Barry Babcock]

Hi All-

Still trying to fix the ailing cruise control on my 87 5kcstq.  Tony 
Hoffman suggested that my vacuum pump might be weak, because it won't 
pull the throttle body linkage all the way open--  This sounded like a 
very reasonable suggestion, so I tested it. (thanks Tony).  The linkage 
will open all the way using a hand pump- it takes between 5 and 6 in Hg 
to pull it all the way open (Is this right?).  The servo isn't leaking, 
I let it sit with the linkage retracted for quite awhile and it held the 
vacuum just fine.

I'm using a new, leak free vacuum hose for all these tests.

I went to a junkyard this afternoon and pulled two other vacuum pumps 
from wrecked 5000's, one turbo and one nonturbo (part number is the same 
it appears).  NEITHER of these pumps was able to do any better than the 
one that was previously in the car.  Maybe I just got two other bad 
ones, but I did the "run soapy water through the pump" trick and got the 
same results.
The sides of the servo "suck in" but the linkage doesn't move.  I don't 
think it's a sticky valve-- if I compress the servo with my fingers, 
then release it, the spring will pull the linkage closed even with the 
vacuum pump running.

Do these little vacuum pumps fail with alarming regularity like that? 
 Were they not up to the job to begin with?  They don't suck, and yet at 
the same time it appears that they do suck very much.

Do I need to just bite the bullet and pay Rod at "thepartsconnection" 
the $140.00 for a new pump?

Thanks for the help,

Barry