Packard Forum Index » V-8 (1955-56)

Enjoyed the video by Ross and really liked his process for adjusting the hand brake. Found the mention about primary spring being weaker than the secondary.

Just finished measuring mine and the appear to be the same diameter.

I had a similar problem with my 55 Caribbean brakes.
Frankly, the brakes never felt right and I was always wary of whether they would stop in an emergency.
A friend who has been a professional mechanic volunteered to do an evaluation but opted not to deal with the BTV. Ross suggested someone near my home who had worked for him to do the rebuild( Ross was booked or I would have had him do the work).
It turns out that rebuild kit from Kanters has mismatched wheel cylinders for front wheels and front springs were not the lighter tensioned springs used by the factory. We replaced both, front shoes and all vacuum hoses…..still not right. Andy drove down from State College and went through the BTV and master cylinder….better but still not right.
The brakes worked fine if the vacuum tank was bypassed but did not give stopping power after two pumps.
Ross agreed to squeeze the car in to a busy schedule and determined the problem was high idle, carb and linkage adjustment ( he also adjusted the shift linkage for the trans. My car was one referenced in his note on shift linkage mistakes impacting durability).
The brakes are fine now…I drove the car from Parkton on the Interstate and Turnpike and it felt like a new car.
The people who helped me put my car together originally and since are capable mechanics….much better than me….but they do not have experience with Packards. There are fewer and fewer people who know the cars.
New parts will not solve the problem if adjustments are not correct. Ross’s video is great but it does not address the idle issue or the adjustmemts.Cudos to Ross for his efforts and willingness to help but in hindsight his recent advise that if you want a car for dependable transport….avoid the 55 Caribbean
They are pretty cars but as the Forum posts document…there are too many problems arising from the new V 8: engine, torsion suspension, transmission and other quirks that Packard never got the chance to correct.

I checked the front brakes after driving the car about 15 miles before the fade and grabbing/noise occurred. There was shoe drag occurring. Pulled the drums on the front measuring the shoe lengths. The primary is 10.0" vs. 11.5" in the specs with the secondary 13.0" vs. 13.0" in the specs The springs on both primary and secondary appear the be the same with a diameter of 0.110" (according to Ross the front shold be weaker and the diameter about 0.01" smaller) which causes the primary to grab first and pushing the secondary against the anchor pin.

I see that in the spec sheet the material for the primary is Marshall 4112 and secondary Marshall 9051. I'm guessing that there is no way today to know what the differences are between the two.

The grabbing and the noise feel like the brakes are grabbing and releasing with a pulsing situation very fast causing the noise. I'm wondering if the cause might be with shorter length and stronger spring on the primary prevents the secondary shoe from seating against the anchor pin when the primary heats up a bit.

In my experiences a pulsating sensation often is an indication of warped drums.

Marty;

I think you’re on the right path of identifying the differences in the primary v. secondary springs, and the secondary leaving the anchor at the beginning of the process. I’m at a loss what would sustain that activity to produce a pulsation. If both wheels suffer the same condition, and become out of phase, I suspect some pedal pulsation would be felt. I’ve never had a set of drum brakes cause a pulsating pedal, but when a set of disk brake rotors warped the pulsation was quite severe. I would think putting the drums on a lathe would prove or disprove whether the drums are warped.

My vote is the primary spring is too stiff, or the initial preload is excessive. Perhaps the difference in the two springs is quite subtle, and if the wire diameter is the same, as you reported, the difference is embraced in number of active coils, coil diameter, or perhaps the length of the hook ends.

Without knowing the coefficient of friction, I don’t know what to say about the length of the primary shoe friction surface. You would think with 15% more surface area the initial spring force should be reduced. Once the available wire diameters are known, and the winding mandrels have been made, I would think one of the few variables left is the length of the primary shoe. That’s the last ‘adjusting screw’ that’s turned.

The two part numbers for the friction material may be nothing more that the geometry of the shoe (thickness, width, arc diameter, and location & diameter of rivet holes), and given the known difference in shoe length having two different part numbers makes sense.
The sensation of fading could be the loss of a portion of the ‘self-energizing’ character of the brake . . . they just won’t stop as well, and the pedal force is increased.

Warning: I’m going to go full geek on this part of the posting:

The equation for the spring constant (k - force per unit deflection) for a coil spring is given by the relationship
K = Gd**4 / (8nD**3) where d**4 = d to the fourth power and D**3 = D to the third power
G is known as the Modulus of Rigidity, which is a property of the material, steel in this case, which would be the same for both retraction springs in a drum brake
d - wire diameter
n - number of active coils.
D – mean coil diameter which would be the outer diameter of the coils minus the wire diameter
A weaker spring will have more coils, all else being equal
A weaker spring will have a smaller wire diameter, all else being equal
A weaker spring will have a large coil diameter, all else being equal.

The shape, length, and configuration of both ends of the spring will not affect the spring constant, but will affect the initial hydraulic force required to initiate motion. That alone can also affect the initial motion of the primary shoe. It is my understanding the design intent is for the hydraulic end of the primary shoe to move first, which infers the static force of the primary shoe is less than the secondary shoe (with single diameter wheel cylinders). The initial static force is the combination of the spring constant and how far the spring is stretched.

With the wire diameter being raised to the fourth power small changes can become significant. In this case a 10% change in wire diameter suggested by Ross’s estimate of primary/secondary spring wire diameter difference will result in approximately a 46% change in spring constant (all else being equal). With the mean diameter being raised to the third power small changes can become significant. If the same 46% change is used the outer diameter of the coil would need to grow by 13%.

All of the variables can be changed simultaneously, and I’ve intentionally not included initial coil binding, which is another contributor to initial spring force to be overcome by hydraulic force.

dp

Pulsation will almost certainly be caused by out of round drums. Check your diameters; if they are as much as 12.06" throw them away. Other wise you may have them turned true. Thin drums have lost their heat storage capacity and will fade quickly.

The secondary shoe will press against the anchor pin regardless--otherwise the shoes would just spin with the drum. If the weaker spring is on the secondary, that shoe will come out first, but then be thrown violently back onto the anchor pin as soon as the lining touches the drum. If the shoes are adjusted nice and tight that would be hard to notice as the motion would be slight, but as the adjustment wears, that abrupt movement can cause grabbing.

From time to time I get cars in with the idle set high to overcome a variety of old age issues. When I drive back from Wally's store with afternoon coffee in hand these cars careen down the hill like something from a seventies movie chase scene. The brakes can not handle the addition of all that extra energy. 500 rpm max please and if your car won't idle that low find out why.

I checked the idle yesterday and in neutral the car idled a shade over 600 maybe 610. Reviewing the information in the Rochester Carburetors MODLE 4GC 1955-56 PACKARD bulletin on page 5 step 7 I saw it called out for an idle 400 in "Dr." so I set it there. This provided an idle of 430 in neutral.

I drove the car about 5 miles in city traffic. There was not a lot of stop and go. When I applied the brakes real hard, the noise came back, which is a real loud groaning noise. They don't pulsate per say, but they don't feel right. I would think that a pulsating from out of round drums would happen all the time when trying to stop.