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.