JC-4 Vacuum Pump??

[slc]

The question was "Just for comparison what is nominal engine vacuum in a braking situation?"

I am pretty sure unless you are very good at being a mechanic, that you may wonder the same thing.

I read a message a time back where someone was taking readings and said with the vacuum pump
running, the system would hold 20lbs vacuum.

But he also suggested that pressing the brake pedal, with the engine not running, and holding the
pedal down, does not hold the psi, but the vacuum leaks off, ever so slowly. You will eventually,
and it seems in a short time lose any vacuum you have stored if you have no vac pump operational when
the engine dies.

food for thought.

Just what I read.
--
GatsbysCruise. \
74GMC260 Former Glacier Model style. \
Waukegan, Illinois \ Keep those MiniDiscs Spinning \ MY GREYHOUND IS FASTER THAN YOUR HONOR ROLL STUDENT \ WindowsXP-Win7-Win8.1-UBUNTU STUDIO -
UBUNTU VOYAGER - Berzin Auto Center

 

[RF_Burns]

I posted this about a year ago on the operation of the JC-4 vacuum pump. Its not a great pump, about 8-9" of vacuum is all you can expect from it.

I bypassed the internal circuitry so now it only acts as a vacuum pump. Then I wired it through a relay controlled by Dave Lenzi's vacuum switch
assembly. Best I can get is about 15" of vacuum. Not great but better than nothing.

http://www.gmcmhphotos.com/photos/data/6785/JC-4_vacuum_pump_operation.pdf

--
Bruce Hislop
ON Canada
77PB, 455 Dick P. rebuilt, DynamicEFI EBL EFI & ESC.1 ton front end
http://www.gmcmhphotos.com/photos/showphoto.php?photo=29001
My Staff says I never listen to them, or something like that

 

[RJW]

> I posted this about a year ago on the operation of the JC-4 vacuum pump. Its not a great pump, about 8-9" of vacuum is all you can expect from
> it.
>
> I bypassed the internal circuitry so now it only acts as a vacuum pump. Then I wired it through a relay controlled by Dave Lenzi's vacuum switch
> assembly. Best I can get is about 15" of vacuum. Not great but better than nothing.
>
> http://www.gmcmhphotos.com/photos/data/6785/JC-4_vacuum_pump_operation.pdf

I might have kept my JC-4 pump had I known about the mod to use an external switch and some sort of reserve vacuum storage container. I thought a lot
about the possibility of losing brakes while I was driving down to Death Valley from California and was happy I had the upgrade over the JC-4 pump.
That trip caused me to decide to go with Dave Lenzi's disk brakes (I already had a 4-bag system). During the installation of the disk brakes I found
out that I had a badly leaking drum brake cylinder that may have contributed to my less than great brakes during that trip down to the valley.

--
Richard
76 Palm Beach
SE Michigan
www.PalmBeachGMC.com


Roller Cam 455, TBI+EBL, 3.42 FD, 4 Bag, Macerator, Lenzi (brakes, vacuum system, front end stuff), Manny Tranny, vacuum step, Tankless + OEM water
heaters.

 

[RF_Burns]

Richard,
Have you read DaveL's article in the Spring GMCMI magazine about the proportioning vales. If you have replaced your OEM with a brass proportioning
valve, you may have limited rear brake pressure.

The "Mod" of the JC-4 wasn't really a high tech "Mod". I believe the internal control circuitry pulled one side of the motor to ground through a
Power FET to turn it ON. I just moved wire on the switched side of the motor to the JC-4 ground pin and ran that pin through the relay to ground....
I'd have to look at it again to be sure.

--
Bruce Hislop
ON Canada
77PB, 455 Dick P. rebuilt, DynamicEFI EBL EFI & ESC.1 ton front end
http://www.gmcmhphotos.com/photos/showphoto.php?photo=29001
My Staff says I never listen to them, or something like that

 

[RJW]

> Richard,
> Have you read DaveL's article in the Spring GMCMI magazine about the proportioning vales. If you have replaced your OEM with a brass
> proportioning valve, you may have limited rear brake pressure.
>
> The "Mod" of the JC-4 wasn't really a high tech "Mod". I believe the internal control circuitry pulled one side of the motor to ground through a
> Power FET to turn it ON. I just moved wire on the switched side of the motor to the JC-4 ground pin and ran that pin through the relay to
> ground.... I'd have to look at it again to be sure.

I never changed out my OEM valve to the often touted brass proportioning valve because I never got around to it. As it turned out, Dave showed me his
cutaways of various valves and explained what he had learned about them during a visit to his place in March. Consequently, I'm happy I never got
around to switching to a brass valve. I've never had better braking, since I installed his disk brakes, the newer 4-bag suspension, 80MM calipers in
the front and a still working OEM proportioning valve. However, I don't have a working parking brake.

--
Richard
76 Palm Beach
SE Michigan
www.PalmBeachGMC.com


Roller Cam 455, TBI+EBL, 3.42 FD, 4 Bag, Macerator, Lenzi (brakes, vacuum system, front end stuff), Manny Tranny, vacuum step, Tankless + OEM water
heaters.