Packard Forum Index » V-8 (1955-56)

Our '56 Clipper has the Bendix Treadle-Vac power brakes and mysteriously loses brake fluid. No noticeable leaks anywhere.
Is this a master cylinder issue?
Too many pros and cons on the Treadle-Vac system to get a clear answer.

It sounds like the seal between the master cylinder and the booster has gone out so best to get a kit to rebuilt the master. Not hard to do and when maintained (pressure-flushed annually with fresh fluid) these units are quit dependable.

If the plunger rod in the hydraulic section is pitted, it will work like a cheese grater on the big rubber seal. Then, fluid finds it's way back toward and could end up in the vacuum cylinder. There are reproduction plungers available that are now made of stainless steel.

However, how much time has elapsed between top-offs? Unlike more modern vehicles, the BTV was not sealed to atmosphere, and over time, the fluid will condense due to evaporation - even when the vehicle is not being used for prolonged periods. This ultimately leads to the congealed mess that is often found when trying to revive one of these cars from a long sleep.

In a past thread, someone provided sources for a replacement cap used with trailer brakes that has a diaphragm/bladder that will fit the BTV.

Along with a pitted piston shaft destroying a seal, the seal could be leaking around the outer edges. The bore surface in the pot metal casting where the outer circumference of the seal must fit tightly could have corroded and now looks like Swiss cheese.

It was also found thanks to a study of failed units done a few years ago by Kanter Auto and others that with the later generation BTV units used by Packard starting in 53 -- those having the relief port opening flush with the top of the casting -- the cover gasket being used along with Packards first version cover which is completely flat totally blocked the relief port. The purpose of the port is to allow the occasional drop or so of fluid designed to leak past the seal in order to keep the piston shaft moist and lubricated to be sent back to the reservoir.

As if the gasket issue was not enough, for whatever reason some rebuilders were also found to be filling the relief port with silicone. There is speculation that apparently some changes (after Packards time) were made to the operation, seal design and casting so the port was no longer needed. There were later versions of the Treadlevac with quite different hydraulic components and while the outer look of the casting is identical to the earlier versions, some repro castings sold today do not have the port. That might make a rebuilder think the port was no longer needed and is the reason for the silicone.

With Packard used versions the port is needed and if the hydraulic seal is leaking more than the designed amount and the relief port is blocked, even if the vacuum side seal might have been good, instead of being able to return to the reservoir the excess fluid has no where else to go but be forced under hydraulic pressure into the vacuum side. That could be how the fluid is disappearing. If so it will eventually congeal to the point of the unit sticking or the vacuum side will be filled with enough fluid it will start being sucked into the engine. Either way, a rebuild is more than overdue and should be done soon before there is a catastrophic failure. If you do the rebuild yourself make sure the port is open and cut a small opening in the gasket material around the port opening so any fluid can exit into the reservoir.

Send that master cylinder to Ross Miller of the Speedwell garage he will rebuild it PROPERLY; whatever you do do not send it to White Post restorations!

Unless it is in really poor condition, these are relatively easy to rebuild and I’ve done several of them with a couple more to go in the near future. The one shown here needed a new casting so a reproduction was used with the vent hole that Howard described being added and the top gasket in the kit used already having the opening for the vent. This one is now in my 1954 Patrician which has never stopped so well in the decades that I’ve owned it!

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If you DO rebuild it yourself, note that this dust seal is not included in any of the kits I’ve seen but I had some custom made in case you need it.

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