Brake Stumper

[johnfarm]

I recently took my '55 Super to my mechanic because one or both of the front brakes wasn't fully releasing and I could smell burning brakes. The brakes worked fine other than that. After he put in new front wheel cylinders and bled the lines, I found that from time to time when I applied the brakes they would work as usual, but suddenly the pedal dropped a couple inches. There was still enough room left to work the brakes, but it certainly was disconcerting. The car is back with the mechanic, who's been working on old cars for the past thirty years or more. He's made sure there isn't air in the lines, adjusted all four brakes, but the pedal issue is still there and he doesn't know what to make of it. Is it the power brake booster? The master cylinder?

[lancemb]

Could be a deteriorated or damaged cup seal in the master cylinder...

[Guest NikeAjax]

I agree it sounds like a sealing issue: perhaps there's some schmootz in the reservoir that's getting on the lip/edge allowing it to bleed past instead of building pressure?

Jaybird

[johnfarm]

thanks Jaybird and lancemb, I'll pass your thoughts on and let you know the outcome.

[JohnD1956]

John, did the mechanic also replace the three flexible lines? One at each front wheel and one on the rear axle assembly?

[trp3141592]

Hi,

I'm with JohnD1956 on this: I'd be very suspicious of the rubber flex lines. They deteriorate and swell internally and block fluid flow---especially return flow which explains your stuck brakes. Flex lines do not last forever.

You can find really cheap Argentine-made ones on EBay if you are willing to do the job twice. Or you can contact "goodbrakes" EBay store (Brake Hoses Unlimited) for a freshly-made DOT-compliant set. The three hoses should cost you about $80.00.

--Tom

[johnfarm]

John D and Tom, the problem at the moment is not that the brakes are sticking, it's that the brakes start to apply as normal and then the brake pedal suddenly drops to near the floor.

My mechanic does not feel that the flex lines would cause this problem. We're on hiatus through the weekend; he says he has a couple ideas to try Monday. I have relayed the ideas in posts 2 and 3 to him as well, and will keep you posted. Thanks for your efforts.

[JohnD1956]

John, my thoughts are the rear hose has become blocked, and it is allowing minimal pressure through. Since most of the braking is accomplished by the front end, you may feel like the brakes are working normally when in fact the fronts have stopped the car and the rears are not yet fully applied. This is just a guess...

[trp3141592]

Hi,

John, my thoughts are the rear hose has become blocked.

Again, I stand with JohnD1956. Assuming that the rear brake flex hose has become clogged from internal swelling and deterioration, it will restrict or even not allow fluid flow to the rear brakes until there is enough pedal pressure to "pop" the fluid through.

If you find that the flex hoses are interior-swollen and deterorating, you will find chunks of rubber-crud in the wheel cylinders. After replacing the bad flex hoses you will need to remove the bleeders completely and do a thorough flush of the brake system to get that debris out of the cylinders.

--Tom

[packick]

JohnFarm:

Your situation is similar to the one I have on my on my ’37 Packard 120. (Its brake system is virtually identical to the one on my ’36 and ’55 Buicks.) The lining was OK but I put in new wheel cylinder kits, new rubber brake hoses (2 front, one rear), rebuilt the master cylinder, and flushed the entire system. The usual brake job.

While I was redoing the brake system I discovered that the wheel cylinders on both front wheels were only 1” bore and should be 1 1/16” bore. So I ordered brand new wheel cylinders from a common and much-used source, put them on the car and now my brake pedal goes half-way down before I get braking. And the brake is solid. Before the brake job it went about ¼ way down before braking.

A friend of mine (who has literally done hundreds of brake jobs) and I did the following to try to remedy the problem:

• Pulled off the master cylinder again and made sure the rubber washer was of the correct thickness, and that the piston was the correct length (we compared the new ones to the old ones I removed). They were OK.
  
• Readjusted all four brakes . . . again.
  
• Bled the brakes . . . again
  
• Played with the rod coming out of the master cylinder to make sure it was properly set.
  

No change. We are stumped and can’t believe that going to the larger, and correct wheel cylinder bore would cause this change. There are no leaks in the system.

I still question the quality of the new brake kits we buy today compared to those we used to get. But then, I am now grabbing at straws for any reason.

[Bhigdog]

If you increased the amount of fluid needed to fill the wheel cylinders by increasing the bore size and did not increase the bore size of the master cylinder that's filling those wheel cylinders the MC must move more (lower) to provide the needed fluid. I would say what you are seeing is both normal and logical. If you are of a mathimatical bent you can easily calculate the added volume of fluid needed to fill the cylinders and compare it to the volume displaced by the additional movement of the MC. My guess it will exactly match................Bob

[packick]

Bob, that makes sense. So now I am wondering if the master cylinder I have, which has been on the car for years and years, is the incorrect one? They may have put in a different master cylinder when they installed the non-correct bore wheel cylinders.

Thanks for the input Bob.

Edited July 29, 2013 by packick (see edit history)

[trp3141592]

Hi packick,

I just noticed that Bob responded to your inquiry even as I was typing out my response. Well, here it is anyway.

I discovered that the wheel cylinders on both front wheels were only 1” bore and should be 1 1/16” bore. So I ordered brand new wheel cylinders from a common and much-used source, put them on the car and now my brake pedal goes half-way down before I get braking.

I found your brake pedal problem quite interesting, so I looked up some parts information. Assuming that your master cylinder free pedal play is adjusted correctly, please consider this:

When you increased the size of the bore of the front cylinders, you also increased the volume of brake fluid required to move each end of each front cylinder to its pressure point by about 13%. {(.53125 x 53125 )/(.5 x .5) x 100 = 112.89, means a 12.89 percent increase in area of the surface area of each rubber disk.} Since the diameter of the master cylinder remained constant at 1 inch, the master cylinder piston has to move further to push more fluid to pressurize the system.

I doubted that Packard designed brakes that didn't "come up" solid until 1/2 way down to the floor. In looking up parts for your car at Hagen's I found that the 1937 Packard 120C and 120CD chassis are both listed as having 1-1/16" front cylinders and 15/16" rears. But the 1937 Packard 120CA Commercial chassis parts list shows all four wheel cylinders are 1". In all cases, the master cylinder is 1". All three models apparently used the same master cylinder.

You didn't say whether your Packard 120 is a commercial chassis or not. According to the parts lists, if it is not a commercial chassis, your cylinders should be 1-1/16" in front and 15/16" in the rears. If it is a commercial chassis, it should be 1" all around. If you have 1-1/16 on the front and 1" on the rears then the excess pedal stroke makes sense.

Cord had a cylinder-diameter occurence with their Lockheed/Wagner brakes. When it was found that the original Lockheed/Wagner brakes were woefully and dangerously inadequate, the decision was made mid-year (1936) to change the ID of the front half of each front stepped cylinder from 1-1/4" to 1-3/8". Apparently no change was made in the master cylinder though, so I expect that when I have brakes completed in a few weeks on my project Cord (a 1937 that was built as a 1936 with the larger front cylinders) I'll find that the pedal does not come up as high as I would like. The improvement left the Lockheed/Wagner brakes merely awful, and Cord went to Bendix brakes in 1937. One way to tell if a given Cord has Lockheed/Wagner brakes or Bendix brakes is to get it moving and step on the brakes for all you're worth. If something actually happens, you've got Bendix brakes.

A most interesting and intriguing question, indeed!

--Tom

Edited July 29, 2013 by trp3141592 (see edit history)

[packick]

Tom, thanks. I guess the mystery continues because all of the diameters you stated (wheel cylinders and master cylinder) are what is installed in my 120. And it is NOT a commercial model, just a plain old 120. We are still stumped.

[trp3141592]

Hi packirk,

Your brake phenomenon is a fascinating issue, and I hope viewers will not consider that we have kidnapped this thread. However, since all our vehicles have hydraulic brakes, we all have an interest in finding out what's going on.

I am attaching a drawing of the master cylinder kit that Hagen's says belongs in your 120. It interchanges with beaucoups Packard, Hudson, Nash, and GM master cylinders. Note that is is labelled "2.19 piston length." You can use this to measure your piston and compare.

Beyond this, I am stumped--for now!

--Tom

[packick]

Tom:

You are right, my intent was not to kidnap this thread but I thought that since my ’55 and ’36 Buicks have the same system it would be appropriate and may relate to johnfarm’s original issue with his system.

Regarding some of the possibilities you have brought up, I got to thinking. We compared my old master cylinder piston length to the new one before we installed it and they were the same length. But what if the old piston length was not correct in order to compensate for the incorrect 1” bore wheel cylinders that were installed?

So I will check the length of the old one to see if it is 2.19” and let you know.

[johnfarm]

So, after a week of bleeding lines, adjusting brakes, and head scratching on my mechanic's part, the problem has not reappeared to date. The brakes seem normal, except that occasionally when applying them there is what I would describe as strong pushback in the pedal.

It's anticlamactic, I know, but I wanted bring everyone up to date. Many thanks for the suggestions, and I will report any change in situation going forward. I try to anticipate stops anyway in this car, and will continue to drive carefully.

[trp3141592]

Hi,

Possibly was an air bubble that has since been bled out, or perhaps worked itself upline and burped out through the master cylinder?

Still scratching my head... :confused:

--Tom