Power Brake Problems at Low RPM

[mosel70]

My problem is I have power brakes well above idle but no power brakes at idle or slightly above.
I installed CPP's disc brake conversion kit (6370SWBK-6D), all new front brake lines, and put in Summit's disc/drum master cylinder with booster (SUM-760197) and CPP's proportioning valve (PVKS-2).
I'm running 396 engine with Edelbrock 600 (1406) carb. My vacuum line runs direct from manifold port on back of carb to brake booster, a dedicated line. I checked all vacuum lines for leaks with carb cleaner and no RPM increase, no leaks. I checked manifold pressure and found 8 inches at idle and 17-20 inches at higher RPM (say above 2000). So after reading that many on the forum said a vacuum canister might help, I installed Summit's vacuum reservoir (SUM-G1464B) and put a vacuum gauge on the canister. I now have 17-20 inches at idle and 25-30 inches at higher RPM. But, I still lose power brakes after one, at most two pumps once rolling at low RPM.
I'm stumped. Anyone out there have a great idea to bail me out of this??

[mosel70]

It's in a 1970 Chevy C10 stepside, 3-speed automatic

[LongBox]

You idle vacuum is way too low. It should be up between 15 and 20 at idle. The reservoir can hold enough vacuum for a couple of applications, then the low idle vacuum problem comes back to haunt you.

Does the engine idle smoothly? Does it have good power? Is the valve timing chain set up correctly? I'm thinking that the problem is not brake related, it's got something to do with the engine set up.

[Boog]

Yours would be a good candidate for a hydroboost setup. It operates off power steering pump. Pressure at any engine speed.

[geezer#99]

You might need a vacuum pump.
Like here.
https://www.summitracing.com/int/sea...m-pumps-street

They're a bit pricy but you can find them on most diesel trucks at the junkyard and adapt them.

[Lattimer]

Swap to hydro boost or install and electric vacuum pump. Your particular engine sounds like it has a pretty lumpy cam, which keeps it from making good vacuum at idle. As took engine would make 15-20 inches of vacuum.

My small block has a moderate cam in it but I still make about 12, and with a vacuum canister like you have it all works fine.

[mosel70]

I was told when I bought the truck this summer that it has a 3/4 cam. It runs good though the Edelbrock 600 Carb is a mismatch for the TorkerII intake manifold. On the street we put it in drive and at idle released and applied brakes and it stayed above 15-20 inches pressure on the canister gauge but still would not produce power brakes past first brake pump or two.

[mosel70]

Great suggestions btw, thanks! My idle timing is 12 TDC. I'm looking for that possible fix before I go to an electric or hydroboost.

[geezer#99]

What's your idle rpm when you check your timing?

[B. W.]

What does the gauge read on the vacuum canister after a couple of pumps? If it reads low you will need to install a hydroboost, vacuum pump or change camshafts. (assuming the engine is in good condition & properly tuned). It is possible that the 396 is tired. The Torker II is a single plane intake, the engine will perform better with a dual plane for daily driving. If you are still in touch with the PO ask him if he has the cam specs.
If it were mine, after checking everything else, I'd replace the cam & intake. You can live with that intake but it'll already be off. (it's real easy for me to spend your money!)

[mosel70]

Idle RPM is ~600. After it warms up I notice sometimes it wants to idle at 1000-1100 in park, strange. I'm going to Edelbrock RPM Air-Gap intake with an Edelbrock 750 carb, just not there yet. The 396 (402) is the original and I was told by owner (through classic car dealer) it was recently built. Overall runs good. I recently rebuilt carb which helped.

[geezer#99]

If your idle rpm is 600 when you check your timing, what is the idle rpm in gear?

Your sometimes higher idle in park says your tach might not be accurate.

[HO455]

Originally Posted by LongBox

You idle vacuum is way too low. It should be up between 15 and 20 at idle. The reservoir can hold enough vacuum for a couple of applications, then the low idle vacuum problem comes back to haunt you.

Does the engine idle smoothly? Does it have good power? Is the valve timing chain set up correctly? I'm thinking that the problem is not brake related, it's got something to do with the engine set up.

LongBox is correct. Anytime vacuum gets down to about 10-12 inches your standard power brake operation gets dicey. Larger volume vacuum canister (s) will help. More volume=more pumps. You can find factory vacuum pumps in older Caddy's at the junkyard. I went that route on a Firebird I used to own.