Packard Forum Index » Post-War (1946-54)

Hi Folks,

I found this cap, available from many different parts stores that supply trailer brakes, fits (1.25-18 UNEF thread) in the Treadle-Vac master cylinder (I tried it), and I believe also in the manual master cylinders.

It has an integral bladder, so your master cylinder is sealed from the elements without inducing any suction. the total volume of the bladder I haven't measured, but it isn't enormous, so perhaps I would put the regular cap back on for a long trip.

Still - at least while the car is "resting at home", should help minimize water absorption versus leaving the system vented-to-the-atmosphere all the time.

Here is one source:

trailerparts.com/tie-down-reg-master-cyl ... fill-cap-with-internal-bladder.html

Here's another:

etrailer.com/Accessories-and-Parts/Titan/T4480701.html

Comments/Reactions welcome.

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  MasterCylinder_Cap2.jpg (174.35 KB)


  MasterCylinder_Cap3.jpg (154.47 KB)

Since it has a bladder, why couldn't it be used all the time?? John

I think it could, in general. But I don't have enough experience with the normal variation in level due to thermal expansion/normal operation to know yet.

I assume the cap would fit a regular manual brake master cylinder?? I do like the remote reservoir you found. Would make things a lot easier to fill.

From what I understand the 1.25-18 UNEF thread is the same as used in the Packard manual brake cylinders (as well as the top of the BTV), but as I don't have one I cannot confirm.

Some earlier models, possibly prewar, used a smaller thread (maybe 1" dia) but most of the later ones used the same 1-1/4-18. Even the massive master on my 1951 Henney-Packard has the same.