1966 F-350 Midland brake booster (firewall mount) failure?

[douglloyd]

Hello,

I have a stumper and am hoping someone knows the answer.

This Midland vacuum booster is on my 1966 F350.

gallery/image.php?album_id=381&image_id=13052
gallery/image.php?album_id=381&image_id=13051

Brakes worked fine for years with regular fluid changes. Today the pedal went to the floor. I was in the driveway, so no stained pants.

I've checked things over. The M/C has plenty of DOT3 in it, and there are no visible leaks at any of the drums. Brake lines and rubber hoses look fine too.

Here's the strange part. I can pump up partial pressure in the M/C using the pedal, but as soon as I start the engine, the pedal drops and all the pressure disappears.

I pulled the vacuum hose off and discovered my 300 inline six doesn't like running very well without this hose plugged. So I stopped it up for now.

Without vacuum, aren't power brakes still supposed to work, just be somewhat harder to push? Here the pedal just sinks down. What should I check for?

thanks,
Doug
Kingston, TN

[douglloyd]

Update - I pulled the M/C forward and removed the vacuum booster. A snap of the parts tag #C4026 is here:

gallery/image.php?album_id=381&image_id=13053

There was a little - not much, but some - dark brake fluid inside the rubber boot. So evidently the M/C is leaking enough for the vacuum to suck fluid into the booster. Not good for the diaphram or other seals.

From what I could find, these Midland-Ross boosters are hard to find. When you do, they are pretty proud of them. Unlike the Bendix types, these are clamped together with a center band and can be rebuilt. Good thing, because they have a reputation for short service life, like 10 years at best.

I just like this truck - it drives fine, works hard and is fun. Has anybody converted theirs to a non-power M/C? Or is there a less expensive dual M/C setup that would fit? Something from a later model that is more common?

thanks,
Doug

[FarmMotorSports]

Internal leak in the master cylinder. ... One or more seals has failed in the master cylinder, time for a replacement, is my knee jerk reaction.



My take on the problem is, if the MC were good and the booster had failed / to a self applied state, the brakes would be locked up tight.

[douglloyd]

Thanks for the quick reply. Yes, after I saw the used fluid in the booster housing, I guessed a new M/C or at least a rebuild kit is necessary. That's not a big deal as far as cost. But those boosters - ouch!

I had noticed an air 'whoosh' once in a while from under the dash while driving, but thought that was normal. Pedal travel was normal and pretty firm, which I always thought was a good thing with hydraulics - no spongy feel at all. With this truck, we're normally not going all that fast anyhow. I downshift during traffic stops so as to not have to rely completely on brakes alone.

I think I understand what you mean about the booster. Normally a check valve senses brake pedal pressure and starts allowing air in behind the diaphram. The vacuum is already built up in front of the diaphram from the manifold suction. If the M/C was bad, but the booster still functioning, it could create what I saw yesterday - the pedal dumps to the floorboard, what with the full power from the booster and the little resistance from a leaking M/C. Power off, I could get some pressure built up on the M/C, but not much.

The way my 300 inline six ran so badly with the vacuum hose off the booster probably suggests the booster diaphram is still OK. Now that this is apart, I'm going to do a more thorough clean out of the booster. Have not messed with gadgets like this before, but apparently it can be bench tested. I have the 1966 shop manual for reference. There seems to be a bunch of YouTube videos on boosters - both for and against owners doing booster rebuilds themselves.

Thanks again for the post - I'll update this later when I get the parts.

Doug