Stovebolt.com Forums The Parking Lot Big Bolts 67 C50 Pulling Vacuum From Cab Filter All-The-Time

If this is posted in the wrong spot please let me know. I recently purchased a 67 C50 farm/grain truck. Original 327 - 4 Speed - Hi/Low rear axle. I wouldn't say I am restoring it but definitely doing a re-fresh.

One thing that has me completely baffled is that the engine pulls vacuum continually from the cab filter (near the back of the seat on drivers side) regardless of whether you push on the brake pedal or not. The vacuum booster is working (pulls brake pedal down when you start the motor with slight foot pressure) and it actually has good brakes and pedal. Has original firewall mounted brass check valve. I can hear the valve under the floor switching when you throw the two speed switch up/down. Now whether it's actually switching lines I have not confirmed but it sure sounds like it is. All lines and hoses look good.

So I know this pulling of constant vacuum can't be correct because it will and does make the motor run wonky but I'm not sure where the issue is or how to correct it. My thought is that it should only pull atmosphere vacuum when stepping on the brakes like every other farm truck I have seen.

Seems like the problem is in the valve under the cab. Can someone confirm? Appreciate any feedback!

IDidlt,

First off, Welcome to the Stovebolt!

As your post is most to related to trucks in our "Big Bolt" part of the community, I have moved the post there for further discussion with folks who know best about your truck and related systems.

Am sure fellow "Big Bolters" will chime in with some helpful info and experiences or more questions to help pinpoint the issue.

Dan (Mod)

The cab filter allows clean air to the positive (atmosphere) side of the diaphragm on the booster unit. The only time you should hear air is when you release the brake pedal and the internal spring pushes the diaphragm back to it's resting position forcing the atmosphere air back out. If you hear air being sucked in, you have a leak in the diaphragm. The leak could be small enough that the booster still works, just not as good as it should.

Mike B

Hy IDidit, I believe what you have there is a Midland push through booster, mounted on the firewall. As Mike B has said with your symptoms it sounds like the unit has a leak in the diaphragm. You may be able to find a rebuilt unit, good luck.

Does anyone have a lead on where to get that midland brake booster rebuilt? I have the same truck as the OP and was gonna make another post but saw this thread already.

Whitepost restoration in Virginia can rebuild them.

IDitlt and AlexB,

I did some searching through past Stovebolt posts, using the search the Site tool found in the left margin.

This older conversation has a bunch of still useful part number and rebuilder info about the booster you are seeking.

I checked the links to resources in the last reply posted by member "Micksurfer" and found all but one (the 2nd link) are still good links. A couple are to actual folks/companies that still offer rebuilding services.

Hope this info may be of some help in getting your brake booster issues resolved.

Dan

Have you checked with your local NAPA? They should be able to send it out or take it in as a core for a rebuilt unit.

I'm not sure if White Post does big truck boosters, make sure to inquire before sending it off...

Mike B

Hy Guys, i would check with Napa, I found Napa p/n NBB5479406 that may be close to what you are looking for. Napa will want specifications to home in on what you need, things like the maximum diameter of the unit and the length from the firewall to the maser cylinder mounting surface, hope that helps.

Thank you for the feedback gentlemen. But - this is still not making sense to me. At first I though Yeah - a hole in the booster diaphragm but then I traced everything. Truck sat for ten years in a field before I got it but the original owner told me the booster was replaced shortly before he quit driving it. One of the reasons they replaced it was that they could not get it to run right. I know that doesn't mean it doesn't have a diaphragm leak but suggests they tried to correct this problem previously.

Just to detail things here a bit. (See Images) My finger points to the line that begins at the bottom of the brass check valve. This line pulls constant vacuum and there is no way it would not because it Tee's forward of the brass check and goes directly to the motor with the other part of the T going to the booster. From the brass check it drops down the firewall and enters a VALVE of some sort mounted on the frame. From there it exits that same VALVE as a different line (on the same side it enters only higher) then curves back around and goes back up to the cab filter. With this routing it IS going to pull constant vacuum unless there is something in that VALVE that is suppose to stop that.

Now I assumed that the VALVE in question was only to change which side of the rear differential two speed box it would send vacuum to. In other words activating either HIGH or LOW speed. But if this valve also somehow is suppose to cut off the FILTER VACUUM then it is not doing its job.

Can anyone confirm my hypothesis?

By the way just threw in that image of the truck. Was painting the bed yesterday.

Just to add a little more info. I cannot find a line entering the back side of the booster diaphragm. In my feeble mind that makes it a simple vacuum assisted booster (being vented somehow on the pedal side) rather than a standard full vacuum booster? If this was true then why the cab filter other than to allow atmosphere in behind the VALVE (Image 4) for the two speed box?

I try and not use full retail places like NAPA because of the expense. I have used them in a pinch but for stuff like this I use these guys. They are re-builder's - at least one of them anyway - and I have been happy with them so far. They are usually backed up and you can wait 30 days or so but they normally have a core to rebuild and then you send them your core after the rebuild arrives. I have only purchased full under the bed 2 and 3 line boosters from them. No idea if they would have a firewall mounted unit but doesn't hurt to ask.

Powerbrakes.com

Power Brake Sales

1517 McCormack Avenue

Sacramento, CA 95811

I talked to Whitepost about rebuilding the Bendix Hydro-Vac for my '57, they told me they have parts for everything and could do mine. After disassembling mine I chose to rebuild it myself.

IDitit Seems like your NAPA store could help you find your local booster re-build company so you could ask them. We have one in Evansville Indiana,I talked to the guy about my '51 original booster,he said if it starts getting rid of brake fluid daily he knew how to fix that.Find those guys in Kansas and ask them.

Hy IDidIt, I have been meaning to get back to this for days, but life kept getting in my way. After checking a loose booster of the same type as your truck has, I find that the atmosphere air enters the booster through the back of the booster where the pedal shaft goes in. My conclusion is that the vacuum two speed control is somehow allowing the vacuum to escape. Possibly the vacuum shift diaphragm has a hole in it, hope that helps.

Just wanted to get back to you guys and let you know what I found and what I did to correct the problem. (See Images) Problem was actually in two places. #1 What I will call the UNDER CAB DIST BOX. This is a mechanical over diaphragm operated box that is controlled by the two speed button on the shifter. #2 The VACUUM OPERATED TWO SPEED CONTROLLER mounted at the rear axle.

Once I pulled the UNDER CAB BOX apart it was mostly gummed up. After cleaning it seemed to operate fine on a test bench so I re-installed it but still had a similar issue. Next I went to the rear TWO SPEED CONTROLLER and took it apart. Inside was also heavily gummed up plus a hole in the diaphragm. I finally found this diaphragm on ebay through Rebuilders Interprises. Part #GM#3847058 BENDIX#374451. About $30 Shipped.

PROBLEM HAS NOW BEEN CORRECTED.

Below I will just go ahead and do a copy and paste of the file I am building on this project. It may help others. Since I am not done with this project some of it may turn out to be invalid but here it is anyway. Thanks Guys - Much Appreciated!

*******************************************************
1967 Chevrolet C50 Grain Truck
327 Small Block V8 - 4 Speed Trans - 2 Speed Rear Axle - 14.5' Knapheid Bed - Anthony Lift - Original underhood items like Air Cleaner - Govenor'd Carb (Disconnected) Distributor - Firewall Mounted Vacuum Booster.
VIN CE537J141691 - Mileage 58,173 (4/15/23)
Engine: Fuel Pump M6624 - Dist Cap DR429T - Condenser DR70T - Points DR22780PT - Carb - Autoline C945 - OEM Part Numbers 7013353, 7015019, 7020109, R22G Rochester 2 Barrel;

Distributor Delco Remy 1112796

Paint Color: THECOATINGSTORE.com Habenero
V-Belts Water Pump/Alternator 17560DR 56017DR x 2
Bendix 2501422 GM# 3959747
2 SPEED REAR AXLE DIAPHRAGM GM#3847058 BENDIX#374451
Eaton 47638 takes a GM 2398285. Bendix 374451 which takes a 3847058 GM
Transmission - Muncie P21XC (M21)
REMOVING THE GOVERNOR OFF CARB: On the passenger side of the carb is a diaphragm assembly. On the back side of diaphragm is a removable steel tube that goes into a T that enters the lower part of the distributor. Your distributor is mechanical advance so vacuum is not required. On the T you will find a wire attached to a spade terminal and solenoid. Additionally there is a small filter screwed in the side of the dist. These can be left in place if you want. This carb mounted diaphragm pulls constant (metered within distributor) vacuum. If you simply block that vacuum by removing the tube and plugging its entrance to the diaphragm then the motor will no longer rev up. Just forward of the diaphragm is a small flat plate held on by 4 screws. Remove this plate. Inside you will find a spring attached to a rod that goes up into the diaphragm. First remove the spring then the nut that holds on the "rotator". Now you can turn this by hand until you can line up the slots and remove it from the rod. Don't try to remove the rod from the diaphragm because the brass rod stays in place but it is now no longer affixed to the rotator plate. Put the small rotator plate back on and affix with the small nut and then re-attach the spring. Next make sure the rod is not binding up anywhere by hand throttling the carb. If everything looks free then go ahead and put the outer plate back on and you are done. For ease of removing the distributor or other wise working on it then it is advisable to remove the steel tube entirely and plug both the diaphragm and the Tee to prevent dirt from entering. Now you have accomplished two things: You have removed the governor and also removed what is mainly just a vacuum leak at the source.

IDidit, I'm curious why you want to disable the governor, it was intended to protect the engine from overspeeding. If the governor is in working condition it has a couple of adjustments to get it into spec. My '57 2 ton has the original drive train, 283 2 bbl, SM420 trans, single speed 7.20:1 rear end. The factory specs for my application and many other of our vintage trucks is a 'no-load' governed speed of 4000 RPM, I wouldn't want to run this engine any faster even though it is a 'modern' high speed V8 with a short stroke. Your 327 and my 283 are cut from the same cloth, same stroke, 3.875" bore on mine and 4.00" bore on yours. I would be cautious of too much RPM unless the engine has been rebuilt and balanced.

78Buckshot. Born in 56 on a farm in SW Kansas. Was driving both tractors and vehicles by the time I was 7. Had to use both feet to engage the hand clutch on our Minneapolis. Depending on the vehicle it sometimes took my brother to run the gas/clutch/brake while I steered because couldn't reach both. Farmed about 900 acres and custom cut/chopped another 1,500 acres of customer alfalfa. 12 hour summer days - 7 days a week. During this era my brother drove the tractors and my job was driving the trucks. Did this for 9 years til my dad died.

To shorten this up. Would say I had a heavy foot as always needed to get back to the field before the next truck was full. Never blew a motor that I can remember. Never needed a Governor on our trucks. Same with this one I guess.

OK, just thought I might save someone a costly project. We turned the governor up on the "Sreamin' Jimmies" back in the '70's and '80's, loaded at 80,000 lbs every night coming out of the brew house in Columbus and back to Michigan. We could hit 3400-3600 RPM on those, took a little gettin' used to the slow turning Cummins but cubic inches and the long stroke way made up the difference. I made some repairs on the city route trucks, those were gas 366 and 427 with Holley 4 bbl and the same basic governor as yours. I didn't dare turn those up 'cus the city drivers would have scattered the engine.

The 5th port on your vacuum selector switch (not labeled on your paper towel) goes to the speedometer to control its 2-speed reduction unit which keeps the speedometer reading correctly in both high and low range.

Mike B

Mike - Saw that small port on my unit. While the port tube was cast it was never drilled through into the housing. I stuck a drill bit in there and it is solid at the bottom. In other words blocked and nothing was ever hooked to it that I could tell. I made an assumption that the box was dual purpose and they either drilled it out for one purpose or left it as-is. I had the speedo gauge cluster out of my truck but didn't see anything there that vacuum would hook up to nor can I figure out - if it did - how that would actually work. The correction of speed would have to be accomplished in the mechanical cable somewhere between the speedometer and the transmission. Does that sound correct? Speedo does work but could not say if it is accurate in either high or low. Can you feed me a little bit more detail on this? Thanks

See if these factory drawings answer your questions...detail "B" shows the speedometer adaptor.

Mike B