Brake booster vacuum deficiency

[Bonnewagon]

My cousin has a Malibu with an over-cammed Chevy 350. He gets maybe 5.5" manifold vacuum running and about 10" decelerating with the throttle closed. Obviously the power booster is starving for vacuum. He disconnected the manifold vacuum. He has a vacuum reservoir next to the booster and he has an electric vacuum pump in the trunk next to the battery. Brakes still suck. I advised a new dual diaphragm booster and that may happen. Our question is this: what size line would you run from the trunk to the booster? Large or small diameter? He started with 1/2" and it still sucked. His brother, an engineer, suggested a smaller line so the pump can suck the booster faster. I say an even larger line is needed as it adds volume to the system, and to plumb the manifold vacuum into the mix as well just to get even more volume. I also question the pumps volume capacity. Thoughts?

 

[565bbchevy]

I have 8 AN from my engine mounted pulley driven vacuum pump to the canister and from there to my booster but my pump self regulates at 28 lbs, I don't see the need for larger than 8 AN even in a trunk mounted unit.
It sounds like there might be an issue with vacuum bleeding off, I would start by making sure the check valves on the booster and the one on the canister are holding vacuum.
Does he have the pump wired to just be on when the engine is running or does it have a vacuum source to trigger it?

 

[Bonnewagon]

He has it set to maintain 25" vacuum at all times. But it takes 15 seconds to get back to 25" after one application of the brakes. I think the vacuum pump is a POS but we'll see. If, after eliminating all possible leaks, it won't get better. I think he may need a Hydro-boost or something. But what about the tubing diameter question? Would a small tube pull vacuum from the reservoir faster or would a larger tube add to the total volume of vacuum, like having more reservoir?

 

[bracketchev1221]

Like was said. Check for leaks first. My Chevelle had crappy brakes and a solid roller cam of unknown size (seems tiny compared to my old 565). Anyway, the power booster would not pull a vacuum, and then when I pulled on the can it would not hold either. Engine vacuum was 7". I replaced the power booster first, now that held. Then I sealed the fittings to the vac can and now that held. That made the system tight.

Next, I would look at the engine itself. 5.5" is pretty low for any engine. I was able to run a 355 with a 250/260 @.050 cam in my 78 with the factory power brakes with no can with a 106 LSA. So after I did the vacuum system in the chevelle, I raised the idle timing as high as I could. Locked out timing would be best, but I got it to 22-24 initial with 36-38 total. This allows the idle to come up and then close down on the throttle blades to establish idle. This raises vacuum. Then I loosened the valve lash by .004 to make the cam look smaller. After I was all done, I got the idle vacuum to 10" at 1050 rpm. The brakes work fine for around town driving now. Stop and go traffic is still not great but the car is usable.

 

[565bbchevy]

He has it set to maintain 25" vacuum at all times. But it takes 15 seconds to get back to 25" after one application of the brakes. I think the vacuum pump is a POS but we'll see. If, after eliminating all possible leaks, it won't get better. I think he may need a Hydro-boost or something. But what about the tubing diameter question? Would a small tube pull vacuum from the reservoir faster or would a larger tube add to the total volume of vacuum, like having more reservoir?

As far as line size I think if the pump is already struggling then going larger would probably make it worse, at this point I feel it needs more velocity rather than volume.
He might want to put the pump in the engine compartment to bring it closer if everything else checks out, even though I have heard the electric ones are noisy.
By comparison my pump will pull vacuum fast enough to stay above 25 lbs even after I stop so there could definitely be an issue with his pump also.

 

[64nailhead]

Something's wrong. If he has at least a 1 ft^3 reservoir with a functioning check valve between it and the pump, then it should be plenty of vacuum along with what's in the booster, especially at 25". I hate to ask the obvious, but I will - is the booster any good? And can he maintain vacuum in the reservoir with the vacuum pump not running?

 

[Bonnewagon]

Yes, my other cousin also said to go smaller on the line. Velocity being what he meant. And yes, he made a test with both 3/8" and 1/2" lines. The smaller line refilled the vacuum reservoir about 10 seconds faster. So I was wrong on that. I also told him to suspect the booster first. He had rebuilt it a long time ago and it is probably leaking or just old. Right now he is trying to get this abomination somewhat street-worthy. He built it back when you could make money street racing so he has something like 13 to1 compression and 5.47 gears. Think late '80's street stupid. He does not have power steering and likes the idea of Hydro-Boost just for the brakes. Thanks for the feedback! EDIT: he tried plumbing the intake manifold vacuum into the mix and it was a total failure. There is so little vacuum in the motor it robbed the booster.

 

[565bbchevy]

I am guessing with those 5.47 gears he doesn't do much freeway driving.

 

[Bonnewagon]

Ha Ha! No, he said it accelerates so fast that most opponents just shut down and he doesn't even have to wind it out. It was built for street racing but I keep telling him to de-tune it and enjoy it for a change. To him dropping to 4.11's is de-tuning. With manual steering he could use a power steering pump just for a Hydro-Boost and end all this.

 

[64nailhead]

At 13:1 he better not be doing too much cruising without methanol or race gas. Must be a big tire car?