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Peter -

Swelling of the plunger seal sounds more plausible, in that it might prevent the compenstor port from opening. However, I believe that the primary purpose of that port is to replenish the hydraulic chamber due to loss of fluid, which can occur as the brake fluid will condense over time.

More likely, the plunger was not returning to the fully release position. Also, perhaps there was some swelling of the rubber cup used in the residual pressure check valve (in the outlet fitting) that had some negative effect.

All these rubber parts are available in an affordably-priced reproduction of the minor overhaul kit as supplied by Bendix, which is widely available from any part vendor that specializes in vehicles equipped with the TreadleVac system.

Both of my 56 master cylinders have "Treadle-Vac" plainly cast into the side of the housing. I don't know what 55 has.

The "check valve" (thats what the manual calls it) at the line fitting is really NOT a valve at all but rather a pressure regulator of sorts. When the pedal is pushed down for braking effect the 'check valve' somewhat restricts or slows the application of the brakes. When the brake pedal is released it allows a quick release of the brake fluid pressure. However, there is NO direct on/off regulation of the brake fluid. Brake fluid passes freely thru the check valve in both directions at all times. Fluid just passes thru on the down stroke of the pedal a little slower.

NOTE that for lubrication of the seal there is a gallery driiled from the reservoir to the seal area. The LID (NOT the cap) of the reservoir MUST be removed to inspect or clean the gallery. The gallery is decieving because it looks like a screw hole for the lid. I've seen 2 such cylinders where the gallery became plugged. THIS gallery is WHY the vaqcuum unit sucks fluid from the cylinder if not correctly sealed during rebuild.

Hi Brian and Keith, I'm pleased to see some good info flowing and it is all appreciated. I concur that the most likely culprit was excessive swelling of either the master cylinder primary cup or a faulty residual check valve. All that I know is that the "failure to release pressure" situation has not re-occurred since I conducted a crude blow -out and replacement of the dot 5 with a dot 4 "guaranteed no seal swell" brake fluid. I did replace the primary cup, residual check valve and compensator valve with NOS items when I put the replacement booster in. The NOS items were however 50 years old so they may have been very shelf life affected.
The 56 overhaul manual Section on Brakes has the heading ESAMATIC POWER BRAKES, so I guess the Bendix family name was TREADLE-VAC.
I am hoping that all goes well with my brake system and thanks for your input, which has increased my understanding of the system. Best regards Peter Toet

Ahem! Well it has developed into a case that the system blocks up with pressure after about 5 - 10 brake appliations, then you have to hydraulically release one of the wheel cylinders. It came to a head (literally) when we were returning on a 200 mile trip, towing about 750kg of motorbikes. Because it occurs at all wheel cylinders I have determined that it is in the Boost Cylinder. I shall remove it and replace the residual check valve with a NOS item and advise if it cures the problem. I shall also comment on any swelling of the parts as I still do not blame silicone brake fluids at this stage. I agree with previous comments that it is unlikely to be the compensating port. best regards Peter toet

"Because it occurs at all wheel cylinders I have determined that it is in the Boost Cylinder. I shall remove it and replace the residual check valve with a NOS ..."

I assume that means that u have to pop ALL FOUR wheel cylinder bleeders to geta release????? If so then u must have BRAKE HOSE problems. With good hoses then popping ANY ONE, EXACTLY ONE, and ONLY ONE bleeder should release ALL of the rest of the cylinders.

I don't care if the hoses are new. Either the hoses are bad or the brakes lines are plugged up. Problem, as u describe is NOT in the master cylinder not the power booster.

Sorry, I was not clear in that I only have to bleed any one of the four wheel cylinders to relieve the pressure. I inserted a tell-tale light under the dash which glows whenever I pressurise the brake system. It normally goes out immediately I release the brake pedal, but lately it has remained on after about 5-10 applications, this is accompanied by a loss of pedal travel as the system does not release. The brakes do not release themselves within a half-hour (the longest that I have left the pressurised situation). It also makes no difference if the vacuum system is operating or not. I am very interested to find out the cause. I have lots of spares so I shall start the replacement program as soon as I get some time.

On top of the vacuum booster there is a VENT with some felt like packing in it as a filter. If the vent becomes plugged then the pedal will not release because the vacuum piston can not return. Also, INSIDEof the booster is a vacuum hose. If it gets kinked or is not installed right then that could cause the problem too.

thanks for the tip on the vent and the hose, I shall check them on stripdown..

Hi BH and Packard V8, Hopefully the end of the story. I pulled the Treadle-Vac out and on inspection, found that the bottom tip of the Compensator valve (below the seal) had broken off completely. Hence no cylinder to reservoir transfer and no temperature compensation, etc and a good reason for system build-up. I find it most interesting that it took so many applications of the pedal to induce the symptoms of system pressurisation. I now have way too much pedal travel and fear that the rear linings are "toast". They certainly smoked as much as 5 minute toast anyway. I am pleased to advise that I am going to revert the system back to Dot 5 Silicone as soon as I have checked on the linings etc, and that I am sorry to have blamed Silicone in the first place. Best regards and thanks for the responses. Peter Toet

Wait a minute!!! In your 9/20 post u indicated:

" I did replace the ... and compensator valve with NOS items when I put the replacement booster in. The NOS items were however 50 years old so they may have been very shelf life affected."

That means there were faulty parts from the factory???? Were the parts in Packard FACTORY boxes or aftermarket?????

It's the details like this that we all need to know when we are shopping for parts.