Technical An interesting observation, no rear brakes

I made an interesting observation yesterday. I had to pull my 40 Ford out of the garage onto my snow and ice covered driveway in order to put my daughters Ranger in for some work. When I backed the 40 out, the tires were on snow and ice. I noticed that even though the truck was stopped, the speedo was still reading about 5 MPH. I pushed the brake pedal even harder but no change. My daughter who was in the driveway said the rear wheels were indeed turning.

The truck has a 7" dual diaphragm booster and a "corvette" style" dual master cylinder with an attached proportioning valve. Truck hasn't been driven in a couple months due to weather. When It was being driven, it stopped with normal pedal pressure and seemed fine.

The pedal feels fine, very high and hard with the engine off and then it drops when you start the engine with your foot on the brake, but still perfectly firm.

I plan to put the rear end up on stands this weekend and observe the problem. I find it hard to believe that there is basically no pressure to the rear brakes.

Have any of you guys ever seen anything like this before?
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Seen it before many times but doesn't seem to be one answer to the problem.
Check how far your pushing the piston in the M/C bore, that last 1/4" is very important.
Bet if you jack the rear up and put in drive applying the brakes won't stop the tires from turning.
Might want to pinch off the rear rubber brake line to both sides to see if there is any diff. Make sure your shoes are adjust up. Maybe take a drum off one side have someone push on brakes and watch the Wheel cyl. movement.

 

If you have an automatic trans, and the engine was running at a higher (cold) idle, overcoming the rear brakes can easily happen while on ice or snow. Applying more pedal force will normally stop the wheels, although higher effort may be required with your small 7" booster.

 

I doubt there's actually a problem.

The truck must have an auto trans otherwise there'd be no power to the axle when stationary. There's minimal brake force being applied to hold the car against the creep of an auto trans and when there's a sound surface under the tires the car will hold. In this case we've got just about the slickest of surfaces under the tires so the creep effect will overpower the minimal braking effort and finding no resistance at the contact with the ground will make the wheels turn.

Throw in too low a stall and the choke operating on the cold motor and the recipe just gets worse.

Chris

 

Some good points made here. My simple brain says to check the rear brake shoe adjustment before making any changes.

 

All good points gentlemen! The truck was on high idle as it was cold. The brake adjustment is ok and like I said the truck drives and stops perfectly. Although I never did adjust the pushrod when I installed the booster and MC.
Hopefully I will get to look at it this weekend

 

If pressing harder couldn't overcome a high idling auto trans car, I'd say there's definitely a problem. Strangely enough, this problem may have been there for a long time so this could be a blessing in disguise. Could be maladjusted prop valve, among other things.

 

High idle, auto trans or not the brakes under pressure should have stopped the rear wheels,what may be the problem is a collapsed hose to the rear axle,the booster sounds like its working as the pedal drops when the engine is started.

 

I had that happen on my roadster. The pistons in the rear wheel cylinders were stuck and later found the rear brake hose had collapsed internally. I am glad I found it in the driveway and not on the road.

 

What he said.

 

I had something similar happen with my car. Totally new brake system, Lincoln 12" drums up front, Fairlane 10" drum in rear, Corvette style master with dual booster from Speedway, all new lines. Turned out one of the pre- fab lines I bought had a slight defect in the flare allowing a slow leak. I had checked the lines for leaks several times before I started driving it but missed that one somehow. Half the master got low on fluid, resulting in no rear brakes, only fronts. Wasn't really noticeable, those big Lincoln brakes stopped the car just fine by themselves! Only when sitting at a light did the pedal seem a little low, that's what made me start looking. Replaced that short line and the T, bled them out, now again have full brakes.

 

The rear rubber line is new maybe a 2 yeas ago. I was bleeding the brakes and there was a lot of "swelling" in the line as the pedal was pumped. It didn't look right to me so I replaced the line and bled the brakes again, like I said that was a couple years ago. All of the brake system was re plumbed when I built the truck.
I understand the forces acting on the rear end when the transmission is in gear and the engine is idling fast but you can't get me to believe that my foot stuffed through the firewall can't overcome that force. If that was normal, I'd be grinding the parking pawl in the transmission every time I stopped my DD on ice or snow.

 

Pull the drums only way to find out for sure.

Posted using the Full Custom H.A.M.B. App!

 

Had a minute to pop the top of the MC this morning before work. Front half is low. Quite low, probably low enough to expose the port during braking. I need to figure out where it went. It could be a very small leak. Tomorrow it goes up in stands, I'll inspect every inch of the tubing and if there's no signs, I'll pull the wheels and start looking at wheel cylinders.

 

Also,, check for a leak at the crimp on the "new" flex line,, too much furrin junk out there, despite the name on the box.

 

Just did some more snooping. The front half and the rear half of the MC are the same size and the metering/proportioning valve is plumbed in correctly according to diagrams I have found online. The front half of the MC feeds the front brakes and the rear half feeds the rear brakes. I wanted to double check this because some of the MC's have unequal reservoirs and the smaller one feeds the rear brakes. So now it looks like there's 2 things to look at: the front reservoir is low and the rear brakes aren't working

 

I drove a 1969 Corolla from BC to California and back in the '90s, and the brakes were barely enough to slow it down on the mountainous secondary highways we drove on. It had drum brakes all around, of course not power either.

When I looked at the brakes on returning home, I discovered one of the rear cylinders was seized up, and from the looks of the corrosion, had been seized for years. After freeing it up the brakes worked somewhat better. Oddly, it never seemed to pull to one side when braking with one rear brake completely inoperative.

 

Just encountered that myself, one wheel cyl seized on my Chrysler, first time for me. It can happen.

 

There is a little shuttle valve in the proportioning valve, it could be shifted and be blocking off the rear brakes. This is supposed to happen if you burst a line, but sometimes they shuttle at the wrong time. In normal usage your rear brakes do so little that you might not even notice the problem until a panic stop, or when you can't stop your rear tire from spinning on snow.
On some of these shuttle valves they are detented so pressure has to be applied backwards on them to make them re-center

 

I have seen were a light layer of rust on the drums would allow the shoes not to grab after it cleaned off ( a few miles of driving and several applications the brakes worked normally) , but still you should have clamping action ,
as for the low front chamber if its discs its the pads wearing and the fluid is doing take up on the pad wear so this might been going on for a while as the front brakes are more than 70% of the braking power . and if you never haul a heavy load in the vehicle you will not notice it as much as you would if it was drum brakes as most are equal sized all around .

check the lines then the rear shoes to see if they are moving , if the backing plates are old and have wear points the shoes can get stuck in the notches or worn a hole and are stuck in place , the shoes should be able to move around the pivot fulcrum easily on the backing plate ,

 

This would be my guess of it has that shuttle valve

 

I thought a Corvette MC was for Disc Disc and you have Disc Drum brakes ... not sure if a Disc Disc MC can be used for a Disc Drum. I'm sure someone will chime in on this.

 

I've done brakes for 40 years and never heard of a shuttle valve to block pressure if a system has a leak. What people call a proportioning valve is really a combination valve that has 3 valves in one. One is a proportioning valve to limit pressure to rear drum brakes. The other is a hold off valve or metering valve to allow rear drum brakes to begin react before the front discs. The other is a differential switch that turns the brake light on if there is a loss in pressure of some part of the system. Modern differential switches are self centering since the seventies. Do a compete brake inspection and pull all 4 wheels to find out where the fluid is going. Sometimes it will leak into the booster. I've also seen fluid leak inside the master cylinder from one resevoir to the other. One chamber will be low and the other overfilled. The fluid level on the disc side of the master will go down as the pads wear.

 

The shuttle valve he's referring to is the differential valve portion. Doing brakes for 40 years I'm sure you've had a leak, fixed it, and had the diff valve stick closing off pressure to one circuit.

Probably a moot point if he doesn't have one of those valves anyways.

 

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1969 Corolla on dry road.

RWD or FWD with open diff or posi will slow both wheels with one brake as long as both tires have traction.

While both tires have traction the axle acts kind of like this -
http://www.formula1-dictionary.net/Big/kart_rear_axle_big.jpg

 

The differential pressure switch operation is a common and misunderstood topic. I, along with BobMcD, have worked with and on brakes for (almost) 40 years, and never came across a dps that closed off any circuits. They simply turn on a dash warning light. Some early designs do require careful bleeding to "center" the switch and turn off the light after activation.
The op stated "corvette style dual master cylinder with an attached proportioning valve" which is probably an aftermarket combination valve that many suppliers sell, and many think they need. More info and/or pictures would help clarification.

 

Didn't Rambler have a differential valve that would shut off the bad end? Seems I remember that being a marketing feature; i was a kid so maybe it was just a light..

 

I'm assuming it is the attached chunk of brass that most call a "factory" proportioning valve
There is a little piece (I call it a shuttle valve) that holds the switch open, when it moves to the side because of too much flow to one end or the other it blocks off that side as it lets the switch close, turning on the dash light and plugging the leak. (at least temporarily)

 

The differential pressure switch is one part of a combination valve, the "brass chunk", which can also include metering and proportioning. The (shuttle) valve moves in the direction of the lower (+/- 400 psi) master cylinder output and triggers the light, but does not shut off or plug that circuit, at least in all I have ever come in contact with. I've even seen a written vehicle OEM explanation of the valve stating the incorrect "block off" theory of a dp valve that the company I worked for manufactured, and tested!
What good reason would there be to block off any brake fluid that still may be able to provide some braking?

 

Maybe in more recent years my knowledge is wrong, but the ones from the 60s-70s definitely plugged the low pressure side in a case of a major leak, rusted off line, blown cylinder seal, etc. Where there would be no braking available on that wheel/ axle, the original idea was to preserve as much fluid in the master cylinder as possible to feed the other half of the system. I know for certain that an old Toyota I had, worked this way, because I had no brakes on the rear axle, just like the OP and I had a full master cylinder. (both sides) when I crawled under the truck the rubber hose to the rear axle was broken off and hanging free and there was no wetness anywhere to be seen, This problem had gone unnoticed for some time until the high idle pushed me into an intersection with my foot hard on the brake pedal. (front wheels sliding on ice)
Maybe the Corvette set-up isn't supposed to work this way, but if I were the OP I would check this brass chunk that is called several different names!