JC4 auxiliary vacuum booster pump
Today I  had 3 conversations about the JC4 pump and how to use it as the auxiliary vacuum source for our brake systems.  I guess the GMCMI news letter has created some interest, so I thought I would give the link to the information on how to buy and use this pump for our brake application.

This pump is still available new and like the 119 pump, requires no external components in our application.  This link shows how it fits into our application, vs, the other pumps available.

Here are pictures of a  mounting of the JC4 pump using a bracket made by the Car guy

here is a link to his products

Here are pictures of using the Electric wiper Modification from the "Wiperguy" to mount the JC4 pump

Here is the wiper kit

Excellent price on ebay , $79  plus $13  freight

The main article shows the models of the cars that used these pumps, but, they are still hard to locate in a Junk Yard. Look at the following pictures, they show where the pumps are located and what they look like.  There are words on the pictures that show the different types of pumps and how they are mounted.  It is importiant to look at the insides of the pump before you leave the junkyard.  The pictures show how this is done.


1: Look for a car with a transverse engine, an engine that is mounted across the car.
2  Look at the driver side front, under the hood for a  battery  holder.
3  Look at the rear of the battery holder for two rubber vacuum lines that run next to the fender.(see picture)
4  If these lines are there, a pump is located just below the battery, in front of the wheel.
5  You have to remove the plastic wheel liner to see the pump.
6  You can check to see if the pump is good buy removing the cone shaped cover over the diaphragm.
7  Use a small pair of vice grips to bend down the tabs.  Remove the cover and a spring will pop out.
8  Remove the diaphragm by pulling on the center pin.
9  Inspect the diaphragm, if there are no holes in it, the pump is good and can be used.
10 Remove the pump and the bracket and when you get home, replace the diaphragm.

Many of us have worried about using a Junk Yard Dog vacuum pump on our GMCs', especially since only 1 in 10 of the pumps from the wrecking yard have a working diaphragm and there are no repair parts available.  The diaphragm is clearly a weak link in this pump.  Manny Trovao has provided us what appears to be an excellent solution to this problem.

The Cadilac car of this generation, had a different pump.  This pump does not have the features of the ones in the Xcars, Power boost relay, Min runtime, check valve.  However, they used a piston rather than the diaphragm.  This Piston will work in the Xcar pump and seems to be easier to find.  They do go bad just as often so take them apart and  look carefully inside the piston at the 4 rubber flapper valves.  If these are out of place or have a hole in them, move on to the next car.  The pictures are in the group shown above.



This is My Story
It was a dark and rainy night in Morro Bay Ca.  In traffic I turned up the street going away from the bay.  As I reached the top of the hill, the front wheels lost traction  and started to spin and smoke on the wet pavement.  I jammed on the brakes, and the engine died when the gas ran to the rear of the tank. There were cars behind me honking, the engine would not start, it was dark, it was wet and I had to back down this hill about a block, stopping many times to make the cars behind me go around in the busy traffic...

The vacuum pump ran the whole time, and saved my bacon.  It took me about 10 min. which seemed like 10 hours to get on the flat where the engine would run...... I would not have wanted to look for a switch in the dark in situations like this when I am in full panic with out brakes......

Hallelujah brother, let it run, let it run. gene

On the other hand several of us know of an owner whose coach ran out of power attempting to climb a narrow, winding mountain road just outside Albuquerque who would have given her 1st born child for a backup brakesystem. The coach rolled back down the road to the first bad curve afterwhich it ended up in a ravine. The tow company further destroyed what thecrash itself didn't accomplish. Owners survived. David
Ken B story
I put a new gasoline selector valve on my coach and drove it down the street.  About 1.5 miles from home the engine quit.  I called my wife to bring the blazer and tow me back home.   It is rural here and I could not turn the coach around.  She towed me home which was about 5 miles around to total circle to get back.  We did it at about 15 mph.  Try stopping the coach when being towed without vacuum.  I ended up using the emergency brake every time we need to slow down or stop.  It was not fun. 

The next week a new found friend that I met here (even though he picks on me at times) sent me a vacuum pump in the mail from west coast.  It got installed immediately.   Do not leave home without it.  BTW the selector valve had a crack in it and it was sucking air. KenB
Testing the newly installed distributor.  Low on fuel, but not that low. Head to my favorite test hill.  Steep thing that.   I can check for "ping" to get my advance set as best I can.  Wife is driving and I'm listening near the hatch as we go up the hill.   Did I mention it was steep?

Carb backfires once and then the engine dies.  Turns out that I was very low on fuel and the steep hill put all the fuel away from the pickups. 
The vacuum pump that I installed at Gene's urging started humming.  A nice sound.
I took over the helm and backed us down this hill to a flat driveway about 1/8 mile down.  Fired the coach up and drove to a gas station.

On the way down, I just HAD to see what would happen if I turned off the pump.  Since it was wired to the key, that was easy.  Took but a few pumps and there was no way I could slow the coach with foot pressure. 
Once again silently thanked Gene for his insistance that I should install the pump.

Yes, I could have set the parking brake and left the coach on the hill. 
Would I have tried to back it down, thinking I could do it?  Quite possibly.  At the time I really wouldn't have known better since I was pretty new to the rig.

Can't for the life of me figure out why there is such an aversion to this mod.  Fairly simple, takes up minimal space and works perfectly. 
Whatever.  It saved my butt   Kelvin
Regarding the power brake system, I was leaving my sons house in my 77Kingsley at Big Bear Calif when I ran out of gas on the way down the mountain.  I had just gotten the coach and was trying to figure out how the gauges worked, which I learned on this trip.  Since it was down hill Idecided to see how I made out going down since there was absolutely no whereto get gas on the way down and I didn't want to call my son and have himrescue me with a 5 gallon gas can.  I drifted 11 miles into a little towncalled Mentone, right into a gas station and within 10 feet of the gas pump.The station attendant and I pushed it the other 10 feet so I could fill itup.

On the way down I had no power steering but I had brakes because I have the aux vacuum pump out of an old gm car that most have used.  It was a little hairy on the tight turns but as long as I had brakes it didn't bother me.  I can't think of ever wanting to drive my coach without that vacuum pump.Bob
I have a hydroboost on my Blazer that uses power steering fluid.  The accumulator is good for maybe two low speed stops after the engine dies.  I would never install one on my GMC.

I installed a Powermaster on my GMC and it has worked great for the first 1000 miles or so.  It is totally self contained and as long as there is 12 vdc to it and the ignition switch is on I have power brakes.  I haven't made a panic stop, but it seems to develop more stopping power with less pedal pressure than the vacuum booster it replaced.

While I got mine from a junk yard and rebuilt it, Powermasters are available from all the Auto Parts Houses and someone sells an adapter to install them on GMC's.  I used an adapter and Powermaster from a '87 Chevy Stationwagon and had to shorten the actuator rod 1" and enlarge the adapter mounting holes through the dash since the adapter uses metric studs that are slightly larger than the 3/8" ones used with the vacuum booster.  Everything else was a direct bolt in. The wiring for the vacuum pump is the same as the Powermaster wiring.  I just cut the leads to the vacuum pump and spliced them to the Powermaster wires.  Also I can remove the spacers I installed when I installed a P-30 master cylinder. Jim
(Kills a Vacuum tank Idea)

Well, I might have known, Gene & Emery are RIGHT!  Another of those ASSumptions, or believing something heard!

I connected a vacuum gauge to the accessory port on my vacuum reservoir. Cranked the engine and got 20"Hg.  Shut the engine down and waited about 3 minutes -- still had 20"Hg.  Put my foot on the brake.  The vacuum dropped about 2"Hg immediately, then continued to drop at about 0.5"Hg per second! Mind you, that's with the 4"x30" reservoir!  Without it the rate would be much higher.

Among the variations of the test I conducted was to run the engine, put my foot on the brake, and turn the Cadillac suspension compressor/vacuum pump on.  The pump WAS able to hold 20"Hg vacuum and to recover it if I cycled the brake.

With neither the engine nor the pump running, the pedal rose slowly, almost imperceptibly, as the vacuum decreased.  I was, frankly, surprised on the first trial, to find that it had moved:  I thought my foot was at the same level as when I first applied the brakes, until I released and reapplied the pedal and found that it went down only a little way.

I'm disappointed and baffled by this situation.  For all my power brake driving years -- at least 50 of them -- I've thought the vacuum was trapped with the pedal down and would hold the brakes indefinitely.  'Tain't so!

The need for the reservoir is now more acute than I had any idea -- that controlled leak means that the fabled 2-3 brake applications must be quick, short ones because one long one will deplete the vacuum.  And "long" will be pretty short.

An electrically driven pump also makes even more sense than I realized -- that's the only way to extend the boosted brake application time beyond about 30 seconds! :-(

Unless all of us have defective boosters.  KenH


To plumb in the pump,  remove the vacuum line from the brake booster and connect to the filter.  The other side of the filter goes to the  pump exhaust. A new hose goes from the suction side of the pump to the  brake booster.

Installed this way, the engine sucks through the filter, through the pump, to the booster.  The only time the pump comes on is when the vacuum is too low and then it helps the engine vacuum.  If the engine stops, (goes up to ambient air pressure) the pump provides all of the vacuum.  If the pump fails the engine provides all of the vacuum.

Some owners plumb the  pump  Tee connected between the engine and the booster, a check valve is needed between the Tee and the engine.  The pump will not be able to draw a vacuum in this configuration if the engine stops because the engine will bring the booster up to ambient air pressure.  A check valve is necessary to block the engine.  I do not recommend this configuration. Gene

Pump Orientation

The pump must be mounted with the rubber mount bracket on the top.  There are drain holes in the bottom of the pump that remove any moisture that might have collected inside the pump.  If the pump is mounted upside down, it will have a short life. Gene

Pump Wiring

Here is some information from Jeff Diamond

Here is the place to get the Acc power for the control of the pump

Pump Second Source 


We show these sources to indicate where we have looked

The vacuum pump I used is a complete unit including the pump, vacuum switch, and resilient mounting bracket --- everything you need except a filter.

 Pump:  MP (Master Power) Brakes ---(704)664-8866 Part AC2723
  $269.95 plus $20.30 shipping from NC to OR.  Mounts easily on the
firewall "box" by the steering post under LH access panel.

Vacuum Filter
        NAPA partnumber.#675-5136, retail $10.60, Net $7.99.
        Delco 1#17070814.Larry
BTW, in the process of checking out my vacuum pump installation I found
that I had a small leak in the original vacuum line to the
booster.....probably never would have found it until complete failure
had I not installed the pump.Gary
This is available here from the local AC/Delco dealer for around $127..the pump# is AC215-119
The AC2723 number above is the Master Power number and I had the AC/Delco dealer  xref the Gm# 22034995.
I found the 4-connection connector at Pep BoysCBWood

Surely in NJ there are some AC/Delco dealers.  The pump number is 215-119,
just as you mentioned.  The list price is $197.36 and the wholesale price is
127.33.   Ask for the wholesale price.  You want a new pump.  You may have to
go to a junk yard to get the mounting bracket and an electrical connector for
it.  I could not find a source for the electrical connector, except for the
junkyard.  JR Wheeler
Thank you for inquiring with Auto Parts West.  The vacuum pup 215-119
will cost $125.68.  The freight via UPS ground is $5.00.  If
you wish to order please use the order form at
Ron and Julie
I bought the connector and the pins from NAPA - the lock tab on the
connector is slightly offset - I didn't have time to send it back or look
for one in the junkyard so I used a nylon cable-tie to hold it in. Works for
I bought a new pump from a GM dealer, I think it was around $150, NAPA
wanted $200 ( we don't have an AC/Delco store out here, we're lucky to have