The Stovebolt Site Forums The Shop Area Engine Shop Brake Problem - solution desperately needed

Problem:
After about 20 minutes of driving the LEFT REAR rim is very hot and the wheel is beginning to lock up due to dragging brake shoes, the master cylinder is hot, and the pressure point on the brake pedal when initial pressure is first felt has gone from midway down to almost no movement downward whatsoever. This problem began after I did the following work.

FRONT WHEELS - did complete brake job, i.e. new shoes, new wheel cylinders, drums turned, new brake return springs

RIGHT REAR WHEEL - new shoes

LEFT REAR WHEEL - new shoes

BRAKE LINES/HOSES - installed new stainless brake lines and rubber hoses

Master Cylinder - installed a new unit

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To date, I have (1) checked the linkage at the master cylinder, which will allow about a inch downward travel on the brake pedal before the rod in the MC starts to engaged, (2) readjusted the brakes and watched for brake drum drag on the shoes, (3) replaced the brake fluid, bench bled the MC and bled the brake lines, (4) replaced the LEFT REAR wheel cylinder, (5) turned the LEFT REAR drum, (6) inspected the other wheels for potential problems (7) installed a temporary helper spring on the LEFT REAR wheel in case the current brake return spring had lost some of its strength.

I have run out of things to try, and I could really use some good advice on this troublesome problem. There has to be something I am missing. The brake line running to the master cylinder and the master cylinder itself is close to the exhaust pipe, but the new stainless steel line to the master cylinder takes the same route as the old steel line.
Therefore, I doubt that this could present a problem. Anyway, I am open to and will appreciate all suggestions on ways to cure my brake problem.

How close to your exhaust pipe is the brake line? It sounds to me like the fluid is boiling in the line.

edit: Perhaps I should rephrase. I had to get creative with the plumbing when I swapped out the stock master for a dual-resevoir power unit (roughly 4" offset to fit the booster under the floor). The stock exhaust was suddenly dangerously close to the lines, I managed to reroute them but I was already fabricating a heatshield in my head...

On my truck the tailpipe comes out just behind the left rear wheel. Is yours still hanging where it should be?

Seems like it has to be either the return spring, the brake shoe, or wheel cylinder on the left rear is hanging up?

If you can get the drum on to start, then obviously it's retracting initially, but then is not retracting after that.

Break it down again and see if you have a burr or some obstruction/snag on the shoe itself. Put the old shoes back on to see if the problem goes away which wuold confirm that the shoe is the problem, not the wheel cylinder or the spring. It is clear that something is preventing that LR from retracting properly.

Don't drive it any more until this is resolved or you will be adding new drums to your parts list.

The line to the MC is attached to and follows the bellhousing crossmember in the stock location, and it is 2.5 inches from the exhaust pipe. The MC is just off center of the exhaust pipe and is 5 inches from the pipe. I have bought some "header wrap" to wrap the exhaust pipe and will see if this solves the problem. I will also slightly bevel the sides of the brake shoes to prevent possible binding. The old shoes were traded in on the new shoes.

PS: The truck is a '50 1/2 ton AD!, and thanks for
your posts.

It sounds to me like your master cylinder push rod is adjusted too long, not coming back far enough (inside the master cylinder) when you release pedal pressure. If there is is an adjusting nut, I would try shortening the rod a bit.
Frets

In addition to beveling the brake shoes, I added a thin washer to the pin that secures the brake shoes to the backing plate. This took up some play in this attachment point, which decreased the wobble in the brake shoes. In '50 there was no spring and pin in the middle of each brake shoe that secured the shoes to the backing plate. A test drive revealed the brake problem was still there. My next step is to wrap the exhaust pipe, check the adjustment of the MC rod again and see if there is any binding of the brake pedal.

Are these Huck or Bendix Style brakes ??

Next test please try this

When it locks try to open the bleed screw at the locked wheel....if it spits you have either a blockage in the master,wheel cyl or line

If you were really boiling the fluid the brakes would go to the floor !First time! You'd have to pump them

I've seen masters with the wrong plunger that blocked 'bleed back' and retained pressure.

If the brakes are adjusted and bleed correctly it should lock everything the same rate

One more thing ...Jack the truck up and see if the other rear wheel stops when you step on the brake
Try this when all wheels are cold and free
If you feel a difference in the brake action left or right that will give us a clue

Is there a return spring on the brake pedal?

Is the pedal binding in the floor board?

Do you, by any chance, have a stuck parking brake cable or grease on the shoes?

I haven't tried the header wrap yet. I rechecked the linkage to the MC, and all seems ok. When the pedal is pushed there is about an inch of travel between the firewall and the brake pedal arm (firewall side) before the MC rod has started to become engaged. The pedal return spring seems strong, and the pedal will slap back against the firewall when the pedal is released with no indication of binding.

With the drum removed, the WC on the LEFT REAR seems to work properly when the brake pedal pushed, but not pushed so as to cause the ends of the WC to be expanded too far.

other stuff:

- Huck brakes
- after returning from a test run, the front wheels will also be tight, but not to the extent of the LR wheel
- after the brake system has cooled down and the LR wheel can be turned again, its wheel cylinder is still expanded somewhat and it will spit fluid when the bleeder screw is opened
- never have to pump the brakes to get pressure
- the RR wheel will stop ok when the brake is applied
-the parking brake cables have been removed, as has the arm that attaches to one of the brake shoes in both rear wheels

If the header wrap doesn't solve the problem, I might install a MC rebuild kit on the assumption that I have a bad valve in the MC.

Something is not right in the MC.

I'm with Oly....something is preventing fluid from returning to the MC....

Either the tiny hole in the bottom of the reservoir is blocked, or the push rod is adjusted so that it is a tad too long and not allowing the piston to return to its internal stop in the MC, thus uncovering the return ports in the MC.

The brake pedal slapping back against the toe board is not really much of a guide- it'll do that with no MC at all.....disconnect the clevis yoke at the brake pedal linkage- if the MC spring pushes the piston & clevis yoke past the brake pedal arm hole, then the push rod is adjusted too long.

Recently had this problem with my '64 Valiant after a complete system re-build....turned-out that the pedal-arm mounted stoplight switch was preventing the pedal from returning all the way to "rest", and preventing the fluid from returning to the reservoir....same symptoms: hot wheels, no play in the brake pedal when warm.
A quick adjustment to the stoplight switch took care of this in my case....but the SLS shouldn't be part of your equation.

Also, you might have your shoes adjusted a bit too tight....check your MC linkage first...if the hot wheel/ dragging problem persists, then back off the adjusters a few notches at each wheel.

Good Luck!

I tried the header wrap, and it was a waste of $. It did not cut down on the heat. The compensating port in the MC is not blocked, since I can see the alum. piston move past the port when I push on the brake pedal. Since I can't locate the problem, I am going to Plan B a few months early, i.e. the installation of an open drive rear and a 4 speed, 3.11 1st gear, Saginaw with a Patrick's shifter.

I would try replacing not rebuilding again the wheel cylinder in question and possibly the return springs. If it was the master cylinder all four wheels would lock up at the same time. Jeff

Denny,

There's another port right in front of the large (1/8") one, a tiny one(1/16 or 1/32") that's often blocked with rust. If it's blocked, fluid will have trouble returning to the reservoir.
(If you disassemble the MC, clean it, and look down the bore with a MAGlite, you'll see the two ports on the top of the bore.)

The large port is for fluid to enter the cylinder/piston area(like if you have to pump the brakes); the small port is for fluid return when the brake pedal is released.

Try another MC, or poke around the bottom of this one and make sure BOTH ports are open.

If you're not convinced, drive it and get things hot so that the rear wheel binds-up, then get over to the shoulder and get underneath and crack a bleeder fitting anywhere in the system: if fluid squirts out (not dribbles) and your brakes suddenly "free-up", then "something " is keeping pressure on the system - either push-rod adj or blocked return port.

Have driven several cars and trucks with Huck brakes for years and have not had problems with them.

Good Luck!

I had installed a new MC, and I also took it apart to see if all of the parts looked ok and if all of the passages were open. Also, backing off a bleeder valve when the system is hot will product a stream of brake fluid. However, as they say on "Friends" it is a moo point now. The differential and transmission are now out of the truck to be replaced with more modern units in the coming days. When the truck is up and running again,I will report back here if the same brake problem persists.

Thanks so much for all of the suggested fixes!

Denny

As you know the stream of brake fluid means that there is a port in the MC blocked and you are building pressure

If you are still using the same style master and you can't shake the rod up down and around you will have the same PROBLEM!!!

Unless your putting disk brakes on the back and drums on the front Then you got other problems

Someone asked me once "Why is the exhaust valve was smaller??"

I've only seen it happen once, but a defective brake hose with a frayed piece inside the hose caused similar problems. Applying brakes would force the frayed material through the fitting, but when released it would act like a check valve and prevent fluid from returning to the resevoir. Also if you have a hydrovac, if the spring is installed on the wrong side of the small diaphram (don't ask) or crud inside is interfering with it you will get the same effect.

Denny,

I'm with Proffit & Oly on this one it is your master cyl. or a blocked (kinked?) line somewhere. I had the exact same problem on my '51 when I first got it. A new (not rebuilt) M.C. took care of it.

I also had the same problem with my drivers side rear wheel locking up. My problem was caused by a sticking emergency brake line. Once we took it off, the brake never stuck again.

After installing a 12 bolt Chevy rear and a 4 speed Saginaw transmission, I still had the same brake problem. So I decided to take another look at the new master cylinder. This paid off for I finally discovered why my brake drums were locking up! There is a washer type part on the end of the MC, which the brake pushrod goes through and which is held in place by a clip spring. Well, I discovered that this washer was backwards. This meant that it took up extra space within the MC and caused the primary piston cup to be moved towards the center of the MC. In this position the primary cup now blocked the compensating port in the MC. This meant that when the MC is activated, brake fluid trying to return to the MC reservoir, would now be prevented from doing so. Just turning this washer around cured my brake drag problem.

After I purchased the new MC, I took it apart in order to paint the case with POR 15. I guess I put the washer in backwards when I reassembled the unit. This error has caused me much grief, so I hope that this post will help prevent others from making the same mistake.

Mine, and another guy on this board, had the brake pedal linkage hang up on the floor, causing the brake to come on after some driving. Look at it and see if the linkage hits the floor when released. If it does, that might be it. A simple check, a simple fix, if it is it. - Scott

Scott:
I am sure the backwards washer caused the problem. When the washer is in backwards, I can see that the primary piston cup is blocking the compensating port, i.e. it is directly under the hole. When the washer is reversed, I can see that the compensating port hole is clear. I made these observations while the MC was locked in a vice.

Thanks again to everyone who offered their good suggetions.