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A TEMPORARY test mite be to plug the hole in the master cylinder wall to see if that eleminates fluid entering the power unit. IF plugging the hole eleminates the fluid from entering the power unit then the OUTER seal is the problem!

edit: IF the outer seal is the problem then that mite explain the experts' recommendations to have the system rebuilt every 3 to 5 years. But then again, maybe if someone can find a a better seal then the rebuild time frame can be extended to 8 to 12 years.

Thanks for all the good response, The outer seal was installed the correct way round lip facing forward and had at best only been exposed to partial vacuum during brake bleeding.
I pulled the power piston out again with the BTV still installed which was pretty easy because I wasn't getting proper power braking due to bad poppet valve seals. That was when I realised fluid was still leaking back. The vent hole back to the reservoir is clear and was blown out during overhaul.
I then pulled the whole BTV and disassembled once more. The original outer seal is marked TROSTEL 124 - 42 - 5 and is of a pressed 2 piece construction with the seal sandwiched between which has an internal garter spring as some of this is now exposed. The replacement seal is metal with just a bonded rubber seal lip and a hollow back.
I noticed the BTV repair kits advertised on ebay look to have the TROSTEL type seal rather than this other type.
I spoke to a buddy who works in a machine shop and is using his contacts to try and match the seal with another and I'm getting a new piston from Kanter as the old one is worn which I'm suspecting to be the main problem.
I also opened up the power piston and replaced the rubber seals on the poppet valves with ones I made from soft rubber so hopefully that will be good to go when I get everything else.
Maybe I will get to use this car sometime this year... guess that's the fun of owning old cars.
Cheers,
Dave.

What is the number on the National seal in the first pic above?????

Also beware that any replacement seal needs to be compatable with Dot 3 (alcohol) and/or Dot 5. A regular seal for Petrol lubricants will not hold up to Dot3 nor visa-versa.

I strongly believe that an upgrade modification is needed for the BTV units. That is a vacuum seal in the Master Cylinder END of the POWER unit. Based on my analysis of the BTV i can see absolutely no reason whatsoever that vacuum should ever meet or touch the master cylinder AT any point or any time at ALL.

PackardDave. Be aware that this problem u are facing with the fluid in the power unit has been a chronic complaint by many others over the last 10-12 years that i have followed the BTV. I say that only because i don't know how long u have followed the BTV controversy.

Hi PackardV8
The markings on the National seal which is the one from the kit are 481073 MX.
As you probably know after the outer seal is installed there is a large o-ring which fits in the recess before the cork gasket to stop fluid leaking from around the metal seal. The fluid was definitely coming through the outer seal and not leaking from around the edge.
I have been following several of the threads on this subject and I agree with you regarding the poor design both with this and the possible brake failure due to the compensating valve. I guess power brakes were in their infancy when this was developed and manufacturers learned from their mistakes.
I would ideally like to get rid of this BTV junk once and for all and am watching closely for the pictures of the electro boost installation from another poster. If someone made an off the shelf kit I would buy it tomorrow as I suspect would many others. The main stumbling blocks seems to be pedal ratio and restricted access. If it were mounted up on the firewall it would probably be a matter of drilling new holes to suit whatever. Meantime I'm just trying to repair what I have to get some use out of the car this year.
I'm busy fitting an alternator now but making it so it can be swapped back easily for originality if required, but thats another subject...
Cheers,
Dave.

Just out of curiosity how fast does the power unit draw the fluid from the mc????

I guess power brakes were in their infancy when this was developed and manufacturers learned from their mistakes.

I think that's only partially true, power brake systems like the Bragg-Kliesrath were common on the heavy Classics including Packard from at least 1932, though these were fitted to mechanical brake systems. Then when Packard converted the Senior cars to hydraulic brakes in 1937, the Twelve continued to have power brakes, but rather than an intergrated unit, a separate vacuum power cylinder acted on the pedal linkage. I think the more accurate statement would be that what was in it's infancy was the combining of the hydraulic and vacuum booster portions into a single, intergrated unit.

Perhaps the most unique power brake system of the prewar era was used by Pierce-Arrow, a concept they borrowed (licensed perhaps) from one of the European makers (EDIT - it was Hispano-Suiza) and was sourced from Stewart-Warner. It was used in conjunction with mechanical brakes and utilized a large disc-brake type affair on the driveshaft which was not the brake itself but the source of the power boost. When the brakes were applied, the "caliper" on the flywheel came into contact with the disc and attempted to rotate with it, but was restricted from doing so and the resultant attempt to rotate with the driveshaft was translated into a mechanical force acting on the pedal linkage. The really nifty thing about this system was that the faster the vehicle was going, the greater the amount of boost. Not all Pierce-Arrows used this system but those that do are easy to spot, the brake pedal has a hinged lower pivot with an upper pushrod and is shaped like the accelerator pedal. Wish I had a photo of it, it's incredibly clever!

Text from the 1934 P-A specifications.

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I'm not sure how fast the fluid is passing, the car hasn't been driven since the BTV overhaul as I knew I still had power boost problems. I bled the brakes and filled up the M/C then I had to leave it for a month to go to work. I pulled the power piston this week and noticed the fluid which must have been migrating through while I was bleeding the system, I actually replaced all the fluid so pumped quite a bit through.

Owen, thanks you are a wealth of knowledge...the car is older than me and certainly the oldest brakes I have ever worked on. The BTV seems to be common on a lot of US cars from early 50's. I had a 59 Pontiac previously which had the Moraine booster/MC this was still primitive but an improvement and easier to work on and replace. The fact that vacuum can act on the outer seal on the BTV is surely a recipe for failure.
Back to my alternator now... hell the sun has even came out.

"I would ideally like to get rid of this BTV..."

See post #31 in the following thread:

https://packardinfo.com/xoops/html/modules/newbb/viewtopic.php?topic_id=2686&forum=12&post_id=27924#forumpost27924

This conversion is from COMMONLY AND WIDELY available current production/reproduction parts. Cost about $200 or less. About 20 hours worth of liesurely time to install. WILL NOT WORK on cars with FACTORY air.

It works just fine for me and Erric. Noone else seems to like it tho. Probably because it doesn't cost enuf money and the parts will most likely be available for the next 20+ years from any local auto parts store. Or maybe noone likes it because about the only special tools required is a drill motor and some lite arc welding with a cheap sears buzz box. NO butchering to OEM Packard platform either.

That Pierce system does sound interesting. Did it work when in reverse? Principle sounds a little like the Borg mechanical power steering that Studebaker tried in early 50's. In that one the engine drove a clutch pack that the more or faster you turned the harder the clutches grabbed so the more assist. It apparently had issues since it was short lived and soon replaced with hydraulic.