Grabby brakes

[Barry Wolk]

I got in a heated discussion with the person that is working on my Treadle-vac. It appears to have two problems. It has corrosion around the rubber seal, allowing it to leak into the vacuum chamber. That will be sleeved as a permanent repair.

He also discovered the vacuum leak that would make the engine shudder when in gear at a light. It is the valve that activates vacuum being sent to the chamber. The device only moves a quarter inch, but makes a huge vacuum leak. He was insistent that the vacuum portion was the wrong unit for my car. He was insistent that the unit on my car was from a '53 Lincoln and that a sleeve type was never used on the Mark II.

I have read in several publications that the early Mark IIs were fitted with an earlier Treadle-vac model. He said his books don't lie. So I asked the Mark II guru and he told me that his early production car was also fitted with a sleeve unit.

Upon further research the mechanic discovered that the '53 Lincoln unit was discontinued in '55, which is when Mark II production began, and my car was built. Now, Mad Scientist's Mark II, also an early one, had a poppet-type but was marked as an unused older unit. I'm sure it was a replacement.

Anybody have any info on the use of sleeve-type Treadle-Vacs on early Mark IIs?

My words with the mechanic surrounded him wanting to rebuild the poppet type unit I have and have me use the "better" one, which people said made the brakes "grab", or coming up with a fix for mine. I've been driving the car for 12,000 miles, I can't imagine changing the way the brake feel and work.

He thinks he can machine a groove for an o-ring to seal the leak. Anybody have a drawing of the sleeve-type cutaway? I'd like to know how it works.

Anybody have "grabby" brakes?

[Mad Scientist]

I believe that my old unit was the original one that came with the car as I don't recall it ever being replaced but I certainly would not swear to it.

As this design change took place right around the time the MKII was getting into production it would make sense that they would have used any units that were available.


I don't have any problem with grabby brakes but then I did make a few changes.

[Don Henschel]

I have the treadle vac unit off of C5681074 when I picked up the windsheild. The owner parting it out didnt get a bid on it on Ebay. How far will I have to disassemble it to determine what style it is? Grabby brakes I wish mine were a bit more agressive but like Barry I have gotten used to the way mine are.

[Shelly Harris]

Grabby brakes ? Yes, they do grab but only after the car has set awhile either in storage for a week or overnight. Then the first push and second on the brakes and it's a grab. After that, they're as good as can be. So I make sure when I start out in the morning to do an initial slow steady even stop. What's causing it?

[Barry Wolk]

I think you are voicing the same complaint I read about in the Ford files.

The sleeve type relies on a proper fit between machined surfaces to contain a proper vacuum value while the poppet type has a means of positively sealing off the vacuum chamber. This is likely what you're feeling in your first couple of pushes on the pedal.

This brings up the other issue as to the proper operation of the check valve on the Treadle-vac. On my car, the hoses were hooked up improperly, negating any value of the stored vacuum in the chamber in the frame.

The manifold vacuum line needs to be hooked to the fitting that is perpendicular to the unit while the fitting that branches downward goes to the storage tank.

In theory, the check valve is opened when there is engine vacuum present. Under normal circumstances the valve is held open, giving vacuum to the booster and to the storage tank. In the event of the engine stalling the check valve spring pushes the plunger closed, allowing the booster to draw vacuum off of the reserve. The tank is supposed to be good for two applications of the brakes before it runs out of vacuum. This can be checked by shutting the engine off and applying the brakes and listening for the sounds of vacuum.

You should also check your large vacuum lines as they are often replaced with fuel line which can easily be crushed by the 19 inches of vacuum the engine provides. You can easily check this with the engine running. Simply run your hand along the length of the hoses to make sure they are round. If they are oval shape they can collapse completely once the engine compartment heats up.

The last item to check is the bullet shaped stopper in the check valve itself. Since these lines can get contaminants such as engine fumes and brake fluid vapors the rubber stopper can deteriorate and change shape or make it stick in place. You can sometimes reshape the stopper, removing the ruined surface, or, I've heard of people getting the plastic that you dip tool handles into to provide a new surface. I would recommend that you let that material cure for several days before reinstallation. The spring that keep the plunger on it's seat can be slightly expanded and the springs do collapse over time.