I ran the 292/108/508 cam with power brakes and a manual trans. I had no trouble with the booster I pulled from a 73 Dart. Yes at first start there was no assist.But with couple of blips, it came on line. With a manual trans I am always downshifting so the vacuum always stays high enough for full assist.
Your reservoir should have enough vacuum stored in it for at least one full assist from 30 mph with the engine shut off.
If you are running a lo-stall TC the engine is forced into an rpm where it makes very little vacuum. If you are also running a very small rear gear, then the engine is further forced to stay there continually.
If your timing is too retarded, the engine will not pull a good vacuum until a little later than it should.
Try this.
Put a vacuum gauge on the intake.With the engine warmed up, rev it up until the vacuum peaks. Note the rpm and the max reading. Run the car up to 30 mph, in whatever gear will get you at least close to that rpm of peak vacuum. For example if your engine vacuum peaks at 2200 rpm, stick in first and run it up to 30 mph. If the Rs are over 2200 great! If it will do it in second use that gear. Shut the car off and brake to a stop. If the assist goes AWOL your booster is not working right. You could; Beg borrow or steal another for a test. But before you do, pull the M/C away from the booster until you see the little plunger in there.It is adjustable. Figure out what size wrenches you need and make it one turn longer.Then try it again.Watch out! It should take a bit less pedal pressure to affect the same stop as before. Remember to bring up the vacuum so the darn thing can work. If there is no difference, there is something wrong with it. But just in case the adjustment is waaay out to lunch, make that plunger 1 turn longer and try it again. You can continue in this way until the brakes stick on, then the plunger is too long and the fluid is failing to return to the reservoir thru the compensating port; you will have to shorten it one turn.
Now, the longer you make that rod the sooner and more violently that booster should assist. To the point that your body will be thrown ahead when applying the usual pedal force. If it doesn't, then either the booster it is not working, or someone has put a smaller M/C in there. You can measure the M/C by pulling it away from the booster and sticking a caliper in the back of it. The Power M/C is usually about 17/16 while the smallest is a 15/16( I think).
If you have drum front brakes as you come to a stop you should have to reduce pedal to stop smoothly.NOT press harder.
If you have disc fronts,it should be one continuous same pedal pressure, with perhaps a tiny bit more pressure as the car comes to a smooth stop.
