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Does anyone have a scrap Bendix Treadle-Vac master cylinder that i can cut up for operational inspection ???? It can be a BARE casting or anything u have laying around. I already have the comp port and ram to put in it afte4r i cut it up.

It can be broken or cracked or severely corroded as long as the rear section of the casting from the front of the compensator port to the rear seal bore is intact. It does not have to hold fluid.

I want to cut one apart and view the mechanical action of the compesator port as the ram trips it open and closed.

I'll pay for the shipping parcel post. Will submit pics of my findings about the compensator port.

OR, someone else can cut one up for testing.

I do NOT need the power unit. I need only the master cylinder in any condition.

i have several, in various conditions. how many you want?

Great idea, thanks for coming forth with it; maybe we can finally get some facts. But in light of a comment that the mode of failure is the loss of brake fluid into the vacuum portion, why not expand your inquiry to the whole unit?

I see there is a TV rebuild kit for sale on ebay. It DOES have a compensator valve, BUT no spring!

I'll go out on a limb and say there are 2 modes of failure possible in an otherwise undamaged unit--(hydraulic portion only). Hopefully PV8 will be able to confirm my theory.

One is the compensator valve. It is a fairly large piece of rubber molded onto a stem. The actual seal and contact area is approx 1/16 in wide around the top circumference--maybe 3/8 of an inch diameter. As soon as the piston moves, the valve spring has to pull and keep the valve squarely upright and hold it until fluid pressure builds and reinforces. If spring has broken or weakened and unable to pull upright quickly, fluid is pushed past valve and either all goes to reservoir or maybe a little stays but not enough for wheels. If there is any type of debris such as old crystallized fluid bits or dirt etc that is hard and large enough to land on the circumference and keep the rubber from sealing completely also same result. Symptom there is the dreaded pedal to the floor as you glide thru the stop light.

The other mode of failure is the seal between the piston and vacuum portion. While there are some scraper rings and assorted alignment washers, there is only a single lip seal. If the fluid has been neglected to the point piston has corroded or otherwise been damaged at the exact area where the seal sits, fluid will be sucked into booster--same with the seal if it has hardened. Symptoms here would be the obvious no fluid (assuming lines and wheel cylinders intact) and resulting brake loss. If refilled periodically while losing fluid, there could be a condition of hydraulic lock where enough fluid had accumulated in booster and wouldn't allow pedal to depress far enough. I think it would take a lot of fluid loss before it could be sucked into engine due to position of components.

Can't comment on failure modes, never had one and you may well be on target, but I wonder how many owners of vintage collectible cars with hydraulic brakes actually flush and change brake fluid every few years? I'll bet not more than a handful in a hundred.

Yes HH56. Your explanation is correct. BUT there is another issue.

IF the compensator port fails for ANY reason then ABSOLUTELY NO brakeing effect whatsoever will obtain. NOT even the slightest amount that would be equivelent to dragging ones foot outside of the door.

That is to say there is ABSOLUTELY NO warning that there is a problem.

With a Conventional CUP SEAL type master cylinder THER IS WARNING. IF the cups become weak or slightly worn then the pedal will begin to feel spongey or exhibit other UNusual symptoms. Also, PUMPING or slamming the pedal real hard with the conventional cup seal type master cylinder will produce some reasonable braking effect. THis is because FAST HEAVY SUDDEN pressure put on a worn cup will force the lips out to seal better. SUCH WARNING WITH SOME reasonable amount of braking effect would prompt a reasonable person to immediately take action to service the problem!!! That is to say that the driver would have a second chance to actualy STOP the car.

BUT NO SUCH WARNING WOULD BE EXHIBITED wit ha BTV type cylinder. WITH BTV it is AON!!! A conventional cup type cylinder offers the possibility a second chance.

with hydraulic brakes actually flush and change brake fluid every few years?

Very true but most brakes don't have the same mechanics. In most conventional master cylinders, the piston and cup physically move past the compensating port so there is no possible way for the fluid to leak back into reservoir. Hence the reason for pedal free play being so important. When relaxed, the piston has to be behind the port so fluid can return or be replenished. With BTV having no piston as such there is nothing to move past, so port is exposed all the time and relies on the valve.

I can't help but wonder about the layout of the BTV in the cars that don't have many issues. Do they all sit with the fairly steep angle as the Packard does?

VISUAL INSPECTION is the watchword for fluids and greases in vehicles of KNOWN ownership and operational environment history.

Based on my mileage and maintenance track record over the last, say 35+ years and maybe a total of something close to a million miles or more of driving:

WHY HAVE a BTV requireing a $300 rebuild every 10 years when a more modern conventional power unit will run 20-40 years for about $150 ????