I don't know if it was original or dealer installed, but there was a 56 400 with standard shift and dual quads that was advertised on eBay around 3-4 years ago. The car was painted all white and was located somewhere in Colorado. A friend was interested in the car and called the owner who wanted something like $16k for the car that was a bit out of his price range. I think the car ended up at the Packard museum in Dayton and was eventually sold to someone in Tenessee.
I thought it was an interesting vehicle but I was interested in finding a Caribbean and knew I would not be able to restore two cars.
I have not run across another since.

I spoke to the CP rep today. Long story short- it is not going to work. His initial assessment and sizing were quickly revised when I directed him to the Forum and diagrams. His unit is 16- 16.5".... Without the rod which would cause interference with the frame. He was a bit taken back by the diagram and the 1-1 arrangement. He didn't think the pump would work...and the master cylinder has to be level ....etc, etc.
Pretty much a waste of time.
I will keep looking.
By the way- the unit does not afford several pumps before putting you into the ditch. You would be trying to stop with a conventional brake attached to the original pedal.
Sorry if I got anyone's hopes up...but this is a bust.

Howard- Thanks for the diagrams. The email received from CP sounds promising. The unit appears to be apx the same size as the BTV. Overall length with master cylinder is supposed to be 16-16.5". The width is 4.5" with a 2" accumulator making overall width 6.5" with widest point being at 2 and 7PM positions.. The unit can be mounted at any angle- even upside down for clearance.
The email recommends a 1 1/8" master cylinder bore rather than the 1" I suggested since the smaller bore may make the unit too touchy since the boost is purportedly 100-200% greater than the BTV.
There was a unit listed on eBay today for $498.75 and a Wilwood master cylinder w 1 1/8 bore and remote reservoir was listed at $64.99.
The email asserts that the 1-1 ratio is not a concern due to the accumulator.
I will check on the pressure needed and whether the current Power steering unit can be adapted to operate both. I will ask specific info on stopping ability w 1-1 ratio if engine or hydraulic pressure loss occurs.
In a way I assume we have a similar problem with the BTV coupled with the inherent concern with a single line master cylinder.
A bit tied up this week but I will try to get answers and keep all posted.
Thanks to all for the support info.
Stay well
RJR

I received an email from Classic Performance that affords some clarification. HydroBoost is a copyrighted name controlled by Bosch. This is probably the unit employed by GM.
The unit to which I have referred is a hydraulic booster produced by Classic Performance CP 3100R.
The initial good news is that it is smaller in dimension than the BTV and can be mounted on an angle. The pedal ratio of 1-1 is not an issue either since the unit generates boost over 100 times the boost of the original. There is a reserve unit to provide braking in the event of stalling or hydraulic loss. The master cylinder is activated by the unit and can be a dual system with recommendation of a larger diameter piston of at least 11/4. Still need to get details on plumbing and ability to utilize existing power steering pump.
I have a number for their engineer and he believes the system will work . I anticipate that my reply will be delayed since they will be at the SEMA show this week.
I am not an engineer and frankly not that great a mechanic but I would like to assemble enough accurate information to discuss the application intelligently.
If there is a good diagram of the BTV showing dimensions and line and pedal connections, please post or direct me to the info. Someone on this post set forth the pressure output of the two power steering pumps which I hope is accurate. I believe I have a good diagram of the pump and hose assemblies for the Bendix and Monroe systems.
I would like to assemble this info with some photos and send it to CP to see if it is a viable replacement.
If preliminary findings bear out, it may afford a dual master cylinder with remote reservoir that affords safe braking with reserve for engine stall but essentially invisible.
If anyone has time to forward diagrams to me directly, the email address is rjrussell13@atlanticbb.net.
I will forward anything I gather.
Thanks
RJR

Well maybe I can blame the day on Halloween. I received a purported NOS check valve from Merritt and planned to follow the suggestions posted on the thread. Turns out that the valve afforded no vacuum and probably was not NOS. I am sure they will send another when I return this one.
I reinstalled the old valve with Teflon tape shot starting fluid around the carb bases and intake to assure no leaks and checked idle vacuum at the carb, t- fitting to the unit and at tank and got a consistent 18 inches of vacuum at idle.
The pedal remained hard on testing with foot on pedal,and eBrake,set when the car was started. I was able to get some pedal relief by bleeding the BTV but as soon as the car was started, the pedal was again firm.
My guess is that the BTV is the problem but I will pull all four drums,to see if there is any obstruction.
I hate the thought of pulling the carpeting, brake and gas pedal to remove this BTV and send it back for refund.
I will probably talk to the guy in Florida that everyone recommended to see what he wants for a rebuild.
In the long run I would prefer to find a replacement system that affords the dual master cylinder.
As soon as I get a reply from the tech guys at HydroBoost about whether their unit with a Wilwood master cylinder with remote reservoir will work if connected to the original pedal on the 55, I will post the info.
I checked the HydroBoost on a Chevy pickup today and it runs 3 lines from the power steering pump rather than the two coming off the pump on the Packard
. Have to see if the current pump can be used.
It was a frustrating afternoon.

I sent a tech question about the HydroBoost as a retrofit for the Packard to Summit They had no answer and referred me to the manufacturer. I will do that next week.
The units with master cylinders are posted on eBay for around $650 or less. The lines are listed at around $250. I think the units could be picked up at salvage yards for much less.
The unit seems to be designed similar to the original BTV with the brake pedal functioning to activate the unit and not the master cylinder. The new units seem compact to fit in the limited space.
I am uncertain whether the units share boost with the power steering or require a separate pump....and also unsure if the old pump has enough boost to handle both.
I appreciate your concern about reserve capacity if the engine shut off or there was electrical failure with the ElectroBoost. To some degree the ElectroBoost site asserts that there is a reserve of up to 20 pumps if the electricity failed and manual brakes would still work.
I have not heard whether the ElectoBoost will work with the 12 v positive ground system.
I would prefer a dual master cylinder to mitigate the risk of brake failure from more common issues of line or wheel cylinder failure, but the recent episode with the Cadillac HydroVac sucking the brake fluid into the engine and sudden brake failure has prompted my inquiries.
If anyone seeing the posts has installed either system and maintained the drum brakes and original pedal, I would hope they,would respond.

I was trying to check out Howard's comment on use of an ElectoBoost system as a replacement for the Bendix Tredlevac. I was interested in determining whether the positive ground of the 55 would be a problem. I got a note from a friend suggesting that I look at a HydroBoost unit and sent a web site for Altech motor sports for a Classic Performance CP3100R unit. The system looks like it would bolt to the floor plate that holds the Tredlevac and connect directly to the existing brake pedal. The new master cylinder connects to the end of the unit- similar to the stock unit. The system connects to the power steering hoses for the boost. I called the toll free number( 800 512-1040) and was advised that the unit will accept most master cylinders but that a Wildwood unit with a remote reservoir would probably work best. The unit is supposedly around $500 retail through Summit.
I will follow up with Summit to see what total,price including master cylinder and required hoses will cost, but I think it will be less,than the $1200+ that I was seeing for the Electroboost.
Just curious as to whether anyone else,has tried this solution.

Howard- You mention the Electrovac system as a possible solution with price being a negative factor. Are you aware of anyone who has installed one of these units? I also wondered whether the positive ground system on the 55 would eliminate this option. Money is always an issue, but with the rebuilt Tredlevac units that I have already purchased at nearly $400 plus shipping and core charge......how much more expensive could a safe system cost?
I take it from your comment that the limited space and pedal placement are the two factors that make the conversion difficult. I agree. A remote power brake unit is not uncommon- my Caddillac and Lincoln both employ remote units. I would prefer a system that eliminates the possibility that the vacuum from the booster can siphon the fluid from the master cylinder and surprise you with a pedal that goes to the floor. Having a dual master cylinder that will continue to offer stopping- with more effort- would be clearly preferable to complete failure from even a broken line or blown wheel cylinder.
I will do some digging and calling, but hopefully something will come up that does not require removing the air duct or a cobbled pedal.
Thanks again
RJR

Thanks for the clarification and the photos. I was looking at a possible cable replacement at Hershey because the washer was loose on the cable. Interestingly several vendors offered cables with no washer and another offered one with the same loose washer as the one on my car. Glad I waited to get an answer before spending more money.
I must admit that the service manuals for the 55 Packard are not as clear as someone with no prior exposure could want. The Forum does help fill the information void.
I may get the wipers working before getting back to the Tredlevac . Might be nice to have something work correctly at the end of the day.
I am going to check some old car magazines to see if I can find an article about a power brake conversion done but Troy of Rad Rides by Troy on a 61 Chevy ( " Bisquik"). Pretty sure he used a unit from a Geo metro that was compact and stopped a heavy car with a 600 ci engine. I would like to have a dual master cylinder, but the limited space and the pedal arrangement will require some thinking. I just had total brake failure on a rebuilt Hydrovac unit on my 55 Cadillac- sucked all the fluid into the engine. I was able to secure a modern replacement from Karp brake in California that seems to solve the problem of using old parts to refurbish the units.
The Packard will be more difficult since I do not wish to destroy the appearance.
Thanks again
RJR

I have read this post and looked at the manual and photos attached. My question is whether the disc at the end of the Bowden cable is affixed or slides on the cable. On one part car, the end does not move, but the unit on my car slides freely.
When the wiper switch is turned on the cable is extended to its maximum length. When inserted into the designated slot it could pull the plastic mechanism but not push anything.
Seems that it should push and pull to have a functional switch but this is on the to figure out lost....after I get the brakes figured out.
Anybody know?