Packard Forum Index » V-8 (1955-56)

Believe you hit upon the reason BTV can get away with 1:1. It does not have a piston in the conventional sense. Actual dimensions are on the forum somewhere, but the "cylinder" is a chamber filled with fluid around an inch in diameter. One end has the "piston" which is a rod around 3/4" diameter but does not touch the cylinder walls. As soon as the pedal is moved, the compensator valve closes trapping the fluid. The only place fluid can go is to the wheels when the rod is pressed further in to displace it. How far pedal moves depends on wheel cylinder/shoe adjustment, but since it is a narrow rod with a long travel, that provides the advantage and volume.

Conventional masters have all the fluid pushed ahead of the piston and since there is a greater surface diameter, more volume with a shorter stroke is obtained at the expense of mechanical advantage due to that same large surface area having to push all the fluid instead of just displace or squeeze it out. Hence the need for a higher mechanical advantage at the pedal to achieve the output pressure.

The 7" booster is larger than the BTV and will provide more assist, all other factors equal. Those with math skills can work out the actual boost but the disadvantage again comes due to the short travel and large piston vs small piston and long travel in the BTV.

There are other products similar to the inline boost you mention but again, they just provide a slight addition to what is there to start with.

The BTV piston (ram or rod or whatever anyone wants to call it) measures 0.655 inches in diameter. Call it 5/8" diameter close enuf. This IS why the BTV can use the 1:1 pedal ratio. The catch is that it has to travel alot further than the more conventional and widely used 1.000" piston of nearly all other MC's to displace the same amount of fluid/pressure.

THE CONFUSION comes in with the service manual spec. The service manual indicates a 1" MC for the BTV. It is strictly an error. There is nothing even close to 1" about the BTV at all. It is a 5/8 cylinder. Therefore the 1:1 pedal ratio of the BTV and the rather excessive over all length of the entire BTV unit as compared to nearly all other units..

Hi, I had measured the dia and realised it was not 1'' as stated in the manual and understand that it is in effect a plunger not a piston which displaces fluid as per it's volume.
Would a .625 or .750 manual master with the remote 1.9 ratio booster work, or would the pedal travel be too great and require a 1'' master with greater pedal ratio... opinions from you learned packard guys please...
I am seeing BTV's in my sleep now...

The small diameter has been brought up on occasion and there are threads here with some possibilities. Don't know if anyone has tried one but the theory is good. I think the bigger problem will be volume. With 4 large wheel cylinders having to move, shoe adjustment would be critical. I would be reluctant to depend on much reserve capacity out of a single small master since it again will be limited to a fairly short stroke. If it did turn out that the ratio issue wouldn't be a problem with the small bore, then wonder what would happen if there were a way to gang two small cylinders each feeding a pair of wheels.

I tried to find a conical compression spring with the dimensions of the BTV conical spring.
The dimensions I searched for were:
Thickness .016
Length - 1 inch
diameter top 1/4 inch
diameter bottom 1/2 inch.
I could not find one, but anyone who has wound small springs should be able to duplicate this critical part that is not in any of the advertised kits.

The closest spring I ever found was from Reid Supply, their item CX-14.http://www.reidsupply.com/Detail.aspx?itm=CX-14 While not exact with your dimensions, except for length so close I think it might work. Comparing with the original, if the free loops are the same (3) it might be able to stretch -- there is another with slightly larger diameter wire also a possibility.

Agreed, the spring is critical but I am not sure that is the entire problem. From the very few pictures of other units I've seen so far, Packard seems to be the only one who mounted low and at such an angle. Am not totally convinced that the angle doesn't have something to do with issue. To my mind it positions the valve port in the perfect location to have any debris or chunks of congealed fluid float right to it and lodge.

What's the material of construction of the original spring? It's a corrosion-resistant material, so I'd think beryllium copper or some such alloy and I'd be reluctant to trust my brakes to a "make-do" spring. Thus my belief that you're best served by having the unit rebuilt by someone who has the proper springs manufactured.

Spring composition is a good point. The spring from Reid is 302 stainless and I believe but can't say for sure the original is also stainless.

In Ross Miller's excellent how-to article on rebuilding the BTV master cylinder (https://packardinfo.com/xoops/html/modules/article/view.article.php?404), I believe he mentions that the compensator port valve spring is made of stainless wire, but I can't fully agree with that - at least, not for the OE part. Also, the one shown in the corresponding illustration in his article appears to be a little longer than what I recall - as if it were some substitute.

In the past, I have removed springs from the unrestored BTV units in my cars that showed some corrosion in spots. One was a a bit PITTED, but another had BROKEN IN TWO from corrosion. Clearly, these spring weren't made of plain steel wire, but they aren't highly corrosion-resistant in my opinion. Yet, as I've often said, I doubt if any engineer ever envisioned that any automotive component they designed would still be in use 50+ years after production.

If the Easamatic unit from one's car merely needs an overhaul of the hydraulic section and is in good working order, otherwise, it doesn't make sense to pay hundreds of dollars for a rebuild because of one little spring - however, critical that part may be.

IMHO, the hydraulic section of the BTV is easier to overhaul than the average carburetor (and I've heard plenty of stories of some pretty crappy rebuilt carburetors sold on an exchange basis from highly visible vendors). The vacuum section is a little more involved, but can be done, with some due diligence. Frankly, I'd rather rebuild most anything myself, but due to the highly litigious nature of our society, I'd never wanna hang out a shingle for that kind of work.

Yet, I digress...

Given the recreational use that most of these vehicles now experience and the hygroscopic nature of DOT3/4, it only seems natural to look for a stainless replacement spring.

I began looking for sources, online, last winter. I found conical compression springs at Raymond Springs (a division of Bowman?) made of stainless steel, but their pricing for small quantities is ridiculous. Reid Supply has better pricing in that regard. However, due to continued problem with my folks' health and property (and no idea when things will lighten up), I've not had the time get any of my Packards out of long-term storage to remove the power brake unit and measure that spring.

Kudos to Randy Berger for picking up the ball with measurements. I'll be watching to see what develops.

I'd have to agree with Owen on the spring composition/application. I'm no expert on springs but one has to ask the question about how many cycles the SS spring is good for vs the CuBe spring that Owen mentions. I'm guessing that the SS spring has a lower cycle life.

On the otherhand, even if the SS spring has shorter cycle life, it is ez enuf to replace on-car if required at shorter intervals than replacement of the CuBe spring. SO maybe go ahead and use the SS spring and just replace it every 3 to 5 years or 10K miles. Whereas the CuBe spring is probably good for 8 to 10 years or 60K miles.