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Strange hydraulic brake issue, 1950 Windsor.


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My 1950 Windsor is almost ready to start driving to cruise-ins and such. A new radiator solved her cooling problems, and I've replaced both rear axle bearings and seals. Rebuilt every wheel and master cylinder, with all new lines, hoses, and linings.

After I drive her for a few miles, shut her off, and walk away, the brakelights come on all by themselves. Have to pull the battery cable to shut them off. Tonight I determined that the problem is a pressure buildup in the system, which I presume has something to do with the master cylinder. New master cylinders are hard to find and pricey, so I'd like to figure out what is wrong with this one and fix it if possible.

Have any of you had any experience with a similar problem? I'd sure appreciate hearing about it if you have.

Many Thanks.

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You must fix this at once, it can be dangerous if you drive the car that way.

Here is what has happened. You must adjust the rod from the brake pedal to the master cylinder. It needs to be shortened slightly. There should be some free play before the rod pushes the piston.

What is happening is this. The piston is not retracting all the way, therefore the brake fluid cannot return to the reservoir. Heat buildup can cause pressure, this is why the brake light comes on.

Of course the more pressure, the harder the brakes apply themselves. The more the brakes are on, the more friction and heat. Which makes more pressure, creating a bad feedback loop where the brakes apply themselves harder and harder until the brakes burn out or you stop the car.

All because the master cannot release the pressure.

It costs NOTHING to fix this and NO new parts are required. You only need to adjust the rod for the correct clearance.

Free play (travel of piston rod before touching master cylinder piston) - 1/8" to 1/4" per Chrysler manual.

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Rusty: If only it were that simple. Some further clarification is in order:

If anything I have too much pedal free play when the car has been sitting, and would need to adjust the rod the OTHER way. What is happening is that pressure builds after I have parked the car for a few minutes, and the free play goes away, which is how I know that pressure in the system is why my lights come on.

Here's the scenario. Went for about a 10 mile drive last night to visit one of my wife's coworkers. When I got there, all was normal: normal amount of free play in the pedal, brakelights off. Visited for 10 minutes or so, and offered to take the husband and son for a drive in the cool old car. While backing out of the driveway, I noticed almost no free play in the brakes. Drove it for a mile, returned, rock hard pedal and slight brake drag; brakelights on. Drove home slowly with the lights on all the way, disconnected the battery (to shut off the brakelights). This morning, there will be lots of free play (no pressure in the system), and the lights will be off.

Sorry I didn't clarify the problem well, and thanks very much for your replies!

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Could it be that I've got my brakes adjusted too tightly, causing a heat buildup?

Perhaps I've got them over-adjusted in an effort to get more pedal, when I really needed to adjust the M/C rod.

Your reply got me thinking, and I may be onto something here. What do you think?

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. . . What do you think?

I think you need to get a copy of the factory service manual.

And I also think that, while your symptoms are a bit strange, it is likely that the relief port in your master cylinder is being blocked. Rusty's response gives the usual reason for this but it could also be some dirt.

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Your master cylinder is messed up. There is supposed to be a spring that pushes the piston all the way out every time you release the pedal. This is not happening. I believe PLY33's theory of the relief port being blocked is correct.

Rebuild the master cylinder properly, and adjust the free play with the piston all the way out. There is a snap ring to stop it falling out, it must come back against the snap ring every time.

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Rusty: I agree that if the port is plugged, the whole thing needs to come out and be cleaned up well, which I intend to do.

Joe: No booster on this very plain jane car (also no clock or radio!), but thank you for the suggestion!

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  • 4 weeks later...

I've got another MC rebuild kit on order with plans to tear into it as soon as that shows up. I rebuilt this MC 2 years ago, and it worked fine for a year and a half before this issue reared its head sometime last winter. My question is, what should I look for when I get the thing apart? I presume that the relief port is the smaller of the two holes that can be seen in the bottom of the reservior with the cap off. It is clean and clear (I can see the aluminum piston through it), but the problem persists. Not enough to lock the brakes, just enough so there's little pedal play and enough pressure in the system for the lights to come on, and only after it has sat for 5-10 minutes after a drive. The pressure dissipates after about an hour of sitting, and I can hook the battery back up.

While I've got everything apart, I'm going to use a heat wrench to brake the pedal adjuster loose (PB blaster hasn't helped) so I can adjust that in the future. I've checked the brake adjustment at all 4 wheels, and none of them are dragging.

If any of you can post ideas about what specifically to look for while the MC is apart, I'd appreciate it. Otherwise, It will get a thorough cleaning, honing, and new parts with fingers crossed. In any case, I'll post back once the job is finished.

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The light comes on because there is pressure in the system. There would not be pressure in the system if the fluid was free to pass through the pressure relief hole.

Why is this happening? I can't see your master cylinder from here. One of us is going to have to look, and you are closer than we are.

You have already been given 2 possibilities, the piston is not retracting all the way or the hole is plugged. You say the hole is not plugged and the piston is visible in the port. This suggests it is not retracting all the way.

Once more, you cannot have a pressure buildup if your foot is off the brake pedal, and the master cylinder is functioning as designed. The rest is up to you.

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  • 4 weeks later...

If the brake pedal return spring is not pulling the pedal all the way back to the stop the result would be the same. Sure sounds like the equalizer port is blocked somehow.

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Update: Pulled the master cylinder, overhauled it, insured the relief port was open. Also replaced the return spring and made sure the pedal is returning completely.

As I suspected (see post #5), the issue I was having before was that the adjustment was too long (no free play at the master cylinder). I've been playing with it, and have learned that if I adjust it long enough to get a good pedal, I'll have the excess pressure after a short drive. If I adjust it short enough to eliminate that problem, I've got a low, spongy pedal. Which means that I've got air in the system still, or excess clearance at the wheels.

As to bleeding: for some reason, this car is tough to get completely bled (more so than other cars I've owned with frame mounted master cylinders). I've tried suction bleeding as well as traditional, and only traditional seems to get me any air out of the system. I'm considering buying some bleeder screws with one way valves in them; have any of you tried those?

As to clearance: I have replaced the shoes, and don't have any extra brake drums to cut up for a clearance tool, so I'm using the back and forth between anchor pins and center stop method to adjust each shoe. I'm reasonably confident that I've got them sufficiently adjusted, but can't of course be sure. I'm willing to listen to input from anyone with experience on this; everything I know about the major adjustment process I read here.

My gut feeling is that I've still got some air in the system. The brakes work, but it isn't right, and I'm not happy with it.

Thanks to everyone that has provided input thus far.

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Some people have made a home made adjusting tool out of wood, with a hole that goes over the spindle and a wire pointer. No I do not have any blueprints, you will have to figure it out yourself or do a search.

Do you have the factory manual to go by? These brakes were the best you could get in 1950 but must be set up correctly to work efficiently.

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I have never had any luck with the vacuum bleeder kits on my old Plymouth. The traditional two person pump the pedal method works. But it only cost me about $20 in parts to get a small garden sprayer from the hardware store and turn it into a pressure bleeder. To avoid getting air in the brake fluid (I use DOT4 which is infamous for that) I don't use the built in pump on the sprayer. I just put a Schrader valve in the top side of the container and then use a tire chuck with a pressure gauge to pressurize it. Makes it an easy one person job.

With respect to the shoes, my experience is that the shoes must be arced to match the drums or you will never get them adjusted adequately. If you have a local (probably truck) brake shop that can arc the shoes then great. I ended up buying the thinnest sticky back roll of sand paper my local hardware supply stocked. Put a piece around the inside of the drum and then rub each shoe for that drum until it matches the arc. You are matching the shoes to the drum here, so do each drum and its corresponding shoes as a set.

To adjust the shoes, the period factory or garage aftermarket tools work the best, but you can eventually get a reasonable adjustment if you take your time with nothing more than some hand tools and patience. For example, see http://www.ply33.com/Repair/brakes

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^^^^^ Yep,

Arc the shoes to fit each drum. Leave the anchor arrows at the stock factory position. Adjust the shoe cams till the shoes just touch the drums.

Drive it 1000 miles... adjust shoe cams if needed. Drive it. The pedal quickly will get very firm and solid and you will have excellent brakes.

Have done these brakes time and time again.

I do have all the miller and ammco tools but don't use them that much as the way I described works. Have a bit of patience for the first 1-3,000 wear in miles.

Easy-done!!:)

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Ply33: I want to make sure I understand your arcing procedure correctly (nobody around here knows what a shoe arcing machine even is, but I remember them from my own youth).

sandpaper glued to inside of drum: (maybe 2/3-3/4 of the drum's diameter?) drum upside down on bench.

1. Take first shoe and rub against the sandpaper until the shoe arc matches the inside of the drum.

2. Repeat process with second shoe.

3. Repeat 1 and 2 for each drum. Keep shoes with the individual drums they're matched to.

Am I understanding this right? It sounds reasonable to me.

Rusty: I do have the manual, which I've used enough to give it the well used greasy look! I agree about how fine these brakes are for the era: Having grown up with Chevrolets and Buicks from the 50's, its amazing how well these things stop this large heavy car by comparison, even when they aren't entirely up to snuff. The more I work on and drive this thing, the more impressed I am with it.

C49er: Those are encouraging words, and now I know not to worry so much while things wear in together.

My thanks to all of you for sharing your experience and knowledge: there's just no substitute for getting advice from someone who really understands the system you are working on.

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Ply33: I want to make sure I understand your arcing procedure correctly (nobody around here knows what a shoe arcing machine even is, but I remember them from my own youth).

sandpaper glued to inside of drum: (maybe 2/3-3/4 of the drum's diameter?) drum upside down on bench.

1. Take first shoe and rub against the sandpaper until the shoe arc matches the inside of the drum.

2. Repeat process with second shoe.

3. Repeat 1 and 2 for each drum. Keep shoes with the individual drums they're matched to.

Am I understanding this right? It sounds reasonable to me.

...

Yes.

I did not really glue the paper to the drum: My local hardware store had a long roll of 2" or 2 1/2" wide sandpaper with a slightly sticky back. Sticky enough that it would stay in place, not so sticky as to be hard to remove and once removed seemed to leave no residue. I just cut lengths of it about the inside circumference of the drums.

I don't know what the intended use of the roll of sandpaper is, I assume for some wood working power tool. I just went out to the garage to see the brand, etc. on the box but apparently I stored it away in a place that seemed obvious at the time and is now no longer obvious.

One thing I did not mention before, but to assure that I'd gotten the shoe properly sanded, I first used a marking pen to make some cross hatches along the entire length of the lining. Once I had all the markings removed by sanding I knew I had a pretty good match on the radius.

Edited by ply33 (see edit history)
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Thanks again, Ply33: the marker idea is a good one, and I'm glad to know how to proceed. It may be awhile before I get around to trying all of this (that pesky day job keeps getting in the way of my automotive fun!), but I'll report back whenever I get done.

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  • 4 weeks later...

Update: Last weekend I re-adjusted all 4 wheels using just the 'minor' cams and went for a drive. Not as much pedal as I'd like, but solid brakes. I think after I've driven it, let them bed in, and readjusted a few times, they'll be fine. I've been driving her regularly, and am having a ball! Thanks to all who've advised.

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