Midland Ross Hissing

Is it normal to hear the brake booster hissing inside the car when you're pressing the brake pedal? I was convinced that the seals in the unit would have been old, brittle and torn for their age but they're actually quite pliable and look practically new. The only thing I didn't do was disassemble the hub assembly. Could this potentially be where that problem is coming from or is it normal? To me it hisses abnormally loud so it seems like there's something going on inside the unit. There are rebuild kits readily available and I have no problem at all with digging into it but I just want to be sure before I do. I know there are folks that rebuild these things but like I said I have no problem doing it myself. Just curious if anyone's had the same experience.

Had the same thing happen in my 68 about 5 years ago. Called the Ram Man and said it definitely needed a rebuild. Works like new again.

A lot of people recommend Booster Dewey to do the job too! He did mine. Good Luck

A loud constant hiss would not be right. An audible hiss when the engine is off and you first apply the brakes would be. There is a valve where the booster pushrod enters the booster.
This allows atmospheric pressure in to the back of the booster to allow the vacuum to suck the diaphragm forward when you apply the brakes.
There are several tutorials on the internet.

If I lightly push the pedal on the Bird I can hear it hiss, push it further and it quits. Brakes work fine so I ignore it.. for now!

Ok. I didn't think it was normal. And just to be clear, this is happening when the engine is running. Constant hiss with the pedal depressed. I have the booster completely disassembled, outside of the valve, and everything else looks great, so it must be the seals inside the valve. I'm going to order a reseal kit and I'll report back when complete. Thanks!

You can test the booster on the bench before you put it in. I mean apply vacuum not operate it. You could operate it but you would need to make a test rig.
When you reinstall there should be a bit of slop in the booster pushrod to allow the atmospheric valve to seat. If your car is factory power it is probably OK but after market it will need to be checked.
That valve does not often give trouble from my experience the diaphragm usually goes and the engine runs funny when you apply the brakes or the booster is dead.

steve340 said:

You can test the booster on the bench before you put it in. I mean apply vacuum not operate it. You could operate it but you would need to make a test rig.
When you reinstall there should be a bit of slop in the booster pushrod to allow the atmospheric valve to seat. If your car is factory power it is probably OK but after market it will need to be checked.
That valve does not often give trouble from my experience the diaphragm usually goes and the engine runs funny when you apply the brakes or the booster is dead.

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Correct. Not the valve/hub assembly entirely, just the seals inside. That is the only component I have not disassembled. And good point as I should be able to bench test this fairly easily.

Here is the hub assembly rebuild kit on eBay, fyi. This is about the only company I can find that makes it. I will probably disassemble it first before I order just to be sure I get everything I need.

https://www.ebay.com/itm/Midland-Br...070157?hash=item27cde5218d:g:zZ8AAOSwaEZcdWA8

Looks like a great kit. Good luck with the rebuild.

How loud is the hissing? Bought a new 90 Dakota for our furniture store and it had a very faint hiss even when not on the pedal and thought it maybe it was a hose coming in for the blend doors leaking but it was the PB. Took it to the dealer and at first no one could hear it in the shop so took it out back where things were quiet and then a tech heard it. He said it was pretty minor and it may never get worse.....so, drove it for 110k miles.

Cranky said:

How loud is the hissing? Bought a new 90 Dakota for our furniture store and it had a very faint hiss even when not on the pedal and thought it maybe it was a hose coming in for the blend doors leaking but it was the PB. Took it to the dealer and at first no one could hear it in the shop so took it out back where things were quiet and then a tech heard it. He said it was pretty minor and it may never get worse.....so, drove it for 110k miles.

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Loud enough to raise an eyebrow ;)

I just wanted to report back that I did end up rebuilding the booster and I am happy to say that the hissing is gone and the booster is performing as it should. I ordered this rebuild kit HARMON CLASSIC BRAKES (parts123.com). They were kind enough to sell me the full kit less the large diaphragm (priced accordingly) since mine was in good shape and I didn't feel like dealing with a core.

I will say that I was pretty shocked in how good the condition all of the internals were. I was expecting to find a ruptured seal or something obvious but everything seemed to be in really good shape, especially considering the age. The only thing that I can say that was indeed bad was the vacuum check valve. Blowing through the valve in the direction that should allow airflow was rather difficult, whereas the new valve was free flowing. For this reason, I used the new valve (although not exactly the same as the original) but I am curious if this was perhaps the whole issue?? Once things warm up around here, I will throw the old valve in and test. That would be pretty crazy if that 's all it was.

Either way, I am still glad I went through the rebuild process. The job overall was not all that bad. I think most of my time was spent cleaning up and painting the housing. Getting the valve apart was a bit tricky as well. I found a link from a Ford forum where the guy did a pretty comprehensive walkthrough. I will see if I can find it and post it here as it was rather helpful. I did shoot some of my own video as well.

If you are fortunate to have a Midland and are in need of a rebuild, don't be afraid to DIY ;)

Here is the link to the walkthrough I mentioned

https://www.fordforumsonline.com/threads/1966-ltd-resto-mod-thread.14264/post-115273