[collin2]

Have a 46 Chevy pu with a sbc 400. .030 over, Crane cam, 750 vac secondary holley with the lighter yelo spring set, tuned HEI dizzy,and block hugger headers to 2-1/2 exhaust. Th 400 with shift kit. Engine pulls super.

Problem is the best vacuam I can pull from the engine with the Crane cam with a good tune is 16" Hg. The brake setup is from a donor 68 Nova, disc/drum with the 11" booster, running to a 3 year old Heidts MII power disc front end. I need 18"hg minimum from what I have read to fully actuate the booster.

I do not think a vacuam reservoir is the answer, and am leaning towards a vacuam pump to achieve the full 18". I know I can buy this setup from Jegs, Summit, etc: but the cost is a little over my wallet right now

Question is what junk yard donor vehicle has a pump setup to get me where I need to be.

Any help is appreciated.

[cobalt327]

Originally Posted by collin2

Have a 46 Chevy pu with a sbc 400. .030 over, Crane cam, 750 vac secondary holley with the lighter yelo spring set, tuned HEI dizzy,and block hugger headers to 2-1/2 exhaust. Th 400 with shift kit. Engine pulls super.

Problem is the best vacuam I can pull from the engine with the Crane cam with a good tune is 16" Hg. The brake setup is from a donor 68 Nova, disc/drum with the 11" booster, running to a 3 year old Heidts MII power disc front end. I need 18"hg minimum from what I have read to fully actuate the booster.

I do not think a vacuam reservoir is the answer, and am leaning towards a vacuam pump to achieve the full 18". I know I can buy this setup from Jegs, Summit, etc: but the cost is a little over my wallet right now

Question is what junk yard donor vehicle has a pump setup to get me where I need to be.

Any help is appreciated.

If the engine is actually making 16 in/Hg vacuum, that's enough to run a PB booster.

If the booster and booster vacuum check valve are OK, I'd be looking into what the brake pedal ratio is.

[collin2]

Pedal pumps up solid and holds with engine off, drops when engine is started.

Don't feel there is any problem with check valve, booster, or vapor trap. Booster is on it's own line from rear #8 cyl manifold runner port, no tee-offs.

Pedal assembly, booster, MC, were bolted to 46 firewall just as they came from the 68 Nova donor. Pushrod to booster from pedal assembly is in same hole, so ratio should not have changed, I would think. Prop valve is centered.

The truck stops ok at the 16"hg, but impossible to lock up brakes on a panic stop, pedal just stiffens up.

Can 2" less vacuam make that much of a difference? Aside from above or replacing booster what else is there to check but lack of vacuam to the booster?

Thanks for your help

[Blown496]

From a search for pvc booster:
I used about a 2ft length of 3" diameter PVC pipe, 2 pipe caps, and a threaded hose barb to connect the reservoir to the brake booster.

I drilled a 3/8" hole in the end of one of the pipe caps, threaded the brass hose barb in, then cleaned out the inside of the pipe and glued it up with PVC cement.

I then cut the vacuum hose coming off the brake booster (on the booster-side of the one-way check valve), inserted a T, and connected the reservoir with 1/4" vacuum hose and hose clamps.

I took it out for a test drive and went from 3-4 presses on the brake pedal to 6-8! So I've basically doubled my safety margin!

Currently, the reservoir is held in place with liberal use of zip ties. I'll probably whip up something a bit more permanent in the coming days or weeks.

PVC melts so obviously don't put one of these next to the exhaust manifold or you'll find yourself suddenly without brakes!

[T-bucket23]

booster should be fine at 16" of vacuum. What does the rest of the brake system consist of and what did the donor car have.

[collin2]

The brake setup is from a donor 68 Nova, disc/drum with the 11" booster, running to a 3 year old Heidts MII power disc front end. The Heidts setup, per the manual, uses 73-76 chevelle rotors,78-88 Calipers, and Mustang bearings and seals.

The front disc brake line (feeds both front brakes) runs off the bottom port of the larger bowl of the mc. Brake lines are per port sizes. I am not running residual valves.

My problem is not loss of brakes per se, rather the inability to lock up brakes on a hard stop. The 400 idles around 800 rpm in drive and will not pull above -16"hg even out of gear at 1000 rpm which is too high, as I then get into stall creep. Mechanical advance also starts to come in around 900 rpm, so 800 works the best for this block and cam.

The rear end (12 bolt posi) has new shoes, wheel cylinders and hardware, and are adjusted to spec.

You are all correct 16"hg does operate the brakes, but will not give me the hard stop capability I feel safe with. Whether a vac can, booster pump, or new booster is the answer is yet to be determined.

Any other ideas please send them along

Thank you all for your help.

[123pugsy]

i have the same problem on my pickup with a 350 TPI.
I changed booster and master cylinder and they felt great.
A week later same symtoms as you describe.

[collin2]

123pugsy,

Are you running the same setup: 11" booster, front disc/rear drum, 1" bore MC? Does the 350 have a cam or been modified? What are you running for manifold vacuam at idle in a TPI equipped engine?

From talking to people at the local car shows this past summer, some running dual 7 or 8" boosters disc/disc some running 7-8 - 11" disc /drum I did find out that duals require even more vacuam to operate properly. Some guys gravity bled the brakes by jacking up the rears which I have also done, no air in any lines. All bench bled the mc's and centered the prop valve as I did. Many with the problem are running the reserve tank, but this in itself would not increase vacuam to the booster without a vacuam pump to increase what the engine is putting out for manifold vacuam.

Since you replaced both booster and MC and ended up at square one, I am now even more assured that it is the lack of vacuam to the booster causing the hard pedal problem on panic stops..

Need to find a cheap booster pump, run it into a reserve tank, and get the vacuam past 18"hg to the booster.

In the meantime on to ignition rewiring before spring.

[123pugsy]

I'm running a dual booster.
Its the largest stock application I could find for a Chevy pickup.

Vacuum at idle is 18" but drops immediately upon any throttle application.
I have a non stock cam in it as the TPI was an afterthought. Its pretty mild but the EFI likes a wider lobe seperation.

When I pulled my first master off, I found it to be leaking into the booster.

I haven't pulled the new one to check if the same thing is happening.

[shine]

sliding aint stopping i prefer a system that will not lock up. if the car stops and gets to almost locking up you have great brakes. if they lock up in a panic stop you will run over something before you stop.

[123pugsy]

The OP has the same problem as me. There's a bit left between where the brakes are now and where lock up begins.

I don't mind lockup as you can back off immediately just a tad for the best braking possible.

[shine]

in a panic stop it never happens. ask any accident investigator. once locked down they stay locked down. that is the reason for anti-lock brakes. 9 out of 10 will never ease up. just human nature.

[collin2]

I'll take the chance of lock up over taking twice as long to stop with a hard pedal.

Old school nowadays, but if I have the reaction time I always pump the brakes to recover vacuam anyhow.

I guess the best way to describe the feeling is to go thru a puddle with the old non power 4 wheel drum setups and try to stop immediately after. Hard pedal but no stop until they dry out.

[shine]

if the pedal is hard you are bottomed out. even the best pedal will still move a little . check your travel . on some systems there is a small piece inside the master cylinder. same master cylinder for two different boosters. it is 1 1/2 inches long. it slips inside the bore of the master cylinder . some cylinders do not have the deep hole but many do. if it is not there you loose 1 1/2 of travel between the booster and master cylinder.

[collin2]

Will recheck, but had MC apart and booster pushrod bottomed out fine against
the end of the MC. Even tried a very thin washer between pushrod end and MC. Caused the booster to make a sound like it was bleeding off vacuam, so took it out. So figure booster to MC pushrod length and adjustment are ok as is.