Where to start trouble shooting power brake issue (72)

[Ekliptix]

After the 350 to 402 engine swap in my 72 C10, I've found the brakes are not very effective. The pedel is stiff, but it takes all my power just to stop the truck. So, I have not been driving it out of safety concerns.

Things I've tried:

• Added a vacuum reservoir. No difference.
• Bleed the brakes. No difference.

What other things can I check? I'm not very familiar with the mechanics of the power brake system, so maybe a diaphragm or something.

Thanks for any input. I'd like to get it on the road, without bringing it to a mechanic if possible.

[mr48chev]

With no other changes and probably not a radical cam I would think that possibly you have a restriction in the fitting on the manifold or in the line to the booster.

If you have an in line check valve in the line to the booster you didn't happen to reverse it when you put it back together? Some rigs have the one that pushes in the grommet on the booster and some have one that is placed in the line.

If not that, check the fitting to the manifold or carb base to see that it has a full size orifice and is allowing full vacuum to be pulled against the booster. Years ago I bought a 57 Chevy with a seriously stout 327 with dual 4 barrels and it would run out of gas about half way though low gear at wot, shift and it would pull for a bit and run out. Someone had used a natural gas fitting with a 1/8 or less orifice in the fuel line because the lines fit it. That would also have restricted vacuum flow on the booster.

[geezer#99]

How much vacuum does your motor produce?
You might need a pump.

[68 P.O.S.]

Check for vacuum leaks and how much vacuum you have. Here's a good troubleshooting guide for reference https://www.classicperform.com/TechB...oubleshoot.htm

[Ekliptix]

Great info y'all. I'll report back what the vacuum is. I think 11in on the gauge, but i'll double check. That link is great 69POS, thanks!

[HO455]

Have you checked to see if the diaphragm in the booster is leaking? I just went through this same problem on a 68 Lemans. Bad booster diaphragm. Got it rebuilt and now I have great brakes.

[Ekliptix]

Vacuum is only 6" at idle.
10" at 2000rpm.

I'll start using some of the trouble shooting ideas at the link above.
I do have an electric vacuum pump I may try wiring up temporarily to see if that at least makes the brakes work, as I try to find a leak.

[geezer#99]

Not enough vacuum.
You need 12 to 14 at idle.

How come such low idle?
When I lived west of Dead Rear my rowdy 350 pulled down 14hg at 750 rpm.
What cam you got?
What’s your timing curve like?

[Ekliptix]

Cam is .555 lift, 110 lobe separation. Nothing crazy. I'm guessing there's a vacuum leak. Could a diaphragm leak cause this?
Can't speak to the timing curve, but a perf shop here did the timing curve in the distributor last year.

[geezer#99]

Do you know the specs they set the timing at?
What’s initial?
How much mechanical?
All in by what rpm.

[Boog]

A hydro boost system may be just what you need for power assist brakes.

[Ironangel]

My cam has a 108 degree lobe separation with .615" lift which computes to lots of overlap and practically no vacuum at idle. I added a vacuum reservoir with gauge that indicates nearly 25" of vacuum @ 3000+ rpm. Two questions come to mind, how do you have your vacuum reservoir plumbed? And are you using fresh functioning (new) brake booster vacuum check valves? Sometimes them things get sticky and require a few inches of vacuum to open. I dont need or use vacuum for timing or for the transmission, only for that brake booster. I get excellent braking twice, after that I have to rev the motor to restore vacuum. The key is to step on the pedal and stay on it with a medium to light steady pressure and the vacuum holds...Cant be any leaks! Use new 11/32" vacuum hose with pinch clamps. PS. Plumbing should be routed from the carb to the reservoir check valve, then from an open barb from the reservoir to the check valve in the booster...Many reservoirs have three openings, one is usually an 1/8" NPT opening for a handy $20 vacuum gauge. Lets you see how much and when your motor makes vacuum through the rpm range. A tight system will hold vacuum after the motor is shut off...Good luck! "Learned everything I know from Geezer!" ...Now I am one...

[Ekliptix]

I did some testing, here are the results:
- Vac reservoir is good. Using an electric vacuum pump and gauge, I applied vacuum to the vacuum reservoir. It holds vacuum (25hg), even after the pump turns off.
- Brakes work well, if I hook the vacuum pump directly to the brake booster.
- Vacuum produced directly from the running engine is only 5Hg, using my test guage.

With the source of the low vacuum being the engine, I started investigating where the leak may be. First, with the engine on I tried feeling around the intake manifold-to-head seal, and the valley pan. Couldn't feel any sucking. Also checked around the base of the carb/efi unit. Nothing.

Next I hooked the vacuum pump up to the vacuum port on the carb. Turned it on and felt for suction leaks. Nothing. Then I used the vacuum pump to blow air into the vacuum port, hoping to feel air blowing out from a leak somewhere on the motor. Nothing. I even taped up the top of the carb/efi unit, and even the tailpipes in case the air was going through the valves/heads/exhaust. Couldn't feel any air coming out around the intake manifold or EFI base.

I've read low compression can be one reason for low, steady vacuum. I'm doing a compression check right now. I've done 4 out of 8 cylinders so far, and they're each 120psi. I'm cranking for about 5 seconds (spark and fuel disabled). 4 more to go. A quick google search says a 350 should have 175psi, so is my 120psi result from the compression test a big red flag?

[geezer#99]

120 at your elevation is not a problem.
But a low compression over cammed motor will produce less compression and miserable vacuum.
Any word on your timing specs?

[Ekliptix]

If I recall, initial timing is 32 degrees.

[geezer#99]

Originally Posted by Ekliptix

If I recall, initial timing is 32 degrees.

You maybe recall incorrectly!
Likely won’t even crank over with that initial.
Your total might be 32.
Total is initial plus mechanical (distributor) timing.

If I still lived in Rocky I’d jump in my truck and bring some tools down and get you fixed up.

[57taskforce]

For what it’s worth the hydroboost swap I did on my Truck is one of the best things I’ve done so far. It stops right here right now. Obviously this isn’t the fix for your low vacuum issues but it does make the truck stop very well.