power brake booster observations

[Z's r Best]

Got the 327 running in the RS. Didn't have much in the way of brakes (basically none). You could push so hard on the pedal you could see the firewall flex from the engine side. The car had been sitting over a year and the front calipers were rusted up so we put new calipers on the front and bled the brakes. Went ahead and bled the rears too just to get all the old fluid (and hopefully any water out of the lines and brake cylinders). Better (barely) but still bad. Now for the odd part. The engine (due to the cam) is only pulling about 15 to 16" of vacuum (not bad, but not like the 21" the IROC TPI is pulling at idle). We hooked up the vacuum tester to the brake booser on the IROC and about 3 or 4 pumps would run the vacuum ga. up to about 20". You could hook the pump/tester up to the RS and pump all you want and it would only get to about 3 to 5". We went to the S10 Blazer and hooked the tester up to it and again 3 or 4 pumps would put the needle on the ga. up around 20". Decided the brake booster was bad so I went out a bought another one while the kid did the dirty work of disconnecting the one in the car. Got the new one home and hooked the tester up to it and got the same result as the one we'd just removed. No real clue why the brake boosters in the IROC and Blazer give such radically different results than the old one in the RS and the new replacement. Kept playing with the new one and slowly (over the course of an uncounted number of squeezes on the testers hand pump) the vacuum reading slowly built up to around 20". We released the pressure and the next time the needle jumped to about 10" on the first pump. We cycled through the vacuum build up and release several times and put it in the car. Took a test drive and now have brakes in the RS. I thought I had a good trouble shooting method for checking a brake booster but now I'm not so sure. Anyone else been through this and found a good way to check the boosters?

[l_dis_travlr]

Obviously, you can't get more vacuum into the booster chamber than
the engine produces. There's probably a minimum threshold necessary,
but I don't know what it is. I'm not sure I understand the usefulness
of evaluating booster state by comparing how many strokes are
necessary to achieve what level of vacuum, especially between
different boosters. The booster vacuum chamber volume will determine
how many strokes, etc. All that being said, I just recently tested
some boosters for the 90 Formula, & determined that the most useful
thing the hand pump will reveal, is whether the unit will HOLD a vacuum
for a reasonable length of time. Otherwise, it will have the same effect
as any other vacuum leak. Sure enough, only one of the units I tested
held vacuum. Good luck,

kk