Brake Problems

[howlinwolf]

i get fluid from both wheel cylinders

[howlinwolf]

OH DEAR God.  the parking brake cables are seized, causing the parking brake to be partially applied, which in turn the parking brake strut was keeping everything loose, and throwing off my adjustment. i have 2 new ones ordered and will have them by 2:30p!

[Irish_Alley]

now crack the rubber line and see what you see then move up to the next one

[bd]

OH DEAR God.  the parking brake cables are seized....

You make it sound like it's time to jack up the horn and replace the truck.   

The combination valve should have a single wire connected to it.  Unplug the wire from the valve and you will see a single male pin sticking up from a switch that screws into the valve.  Use an ohmmeter on the 1K or lower scale to check the resistance between the pin and the case of the valve.  If you get a reading other than infinity or "OL" the valve is tripped.

You said at the beginning of this thread that a rear wheel cylinder was blown out.  This may have caused the valve to trip.

View this prior thread...

http://forum.73-87chevytrucks.com/smforum/index.php/topic,22940.msg190525.html#msg190525

[Irish_Alley]

OH DEAR God.  the parking brake cables are seized, causing the parking brake to be partially applied, which in turn the parking brake strut was keeping everything loose, and throwing off my adjustment. i have 2 new ones ordered and will have them by 2:30p!

so when you thought you adjusted the brakes so they would drag, it was the parking brake that was dragging

[bake74]

OH DEAR God.  the parking brake cables are seized, causing the parking brake to be partially applied, which in turn the parking brake strut was keeping everything loose, and throwing off my adjustment. i have 2 new ones ordered and will have them by 2:30p!

so when you thought you adjusted the brakes so they would drag, it was the parking brake that was dragging

     That would wear out a set of brakes in a hurry.   

[howlinwolf]

Yup new cables installed last night fixed that, but still no pressure. Ah well, now im chasin valves

[ehjorten]

At first I was thinking the combination valve at the front cross-member, but if I remember correctly I don't think it will shut off fluid to the rear brakes, only the front brakes if the system becomes imbalanced.  At either rate there is a button on one side of the combination valve.  It is under a rubber cap.  Pushing it in will reset the Pressure Differential Switch.  However, pushing it in is what moves the valve away from the front brake circuit and opens up flow to the front calipers.  Hmmm???  I think I would start by looking at the rear flexible line.  If it looks worn with any cracks in the outer layer I would just replace it as they are pretty cheap!  Your combination valve could be froze.

The combination valve has three jobs: 1) Proportioning Valve - It limits the amount of pressure to the rear brakes.  Because your rear drum brakes have a better mechanical advantage over the front disc brakes, they need less pressure than the front. 2) Pressure Differential Switch - This is what makes the Brake Light come on in the dash should the system become imbalanced from front to rear.  Make sure you have power to the bulb, that the bulb is not burned-out and that the wire is connected to the combination valve.  If this light comes on when you step on the pedal then you have a pressure problem in either the front or the rear brake circuits. 3) Metering Valve - This applies brake pressure first to the rear brakes, a fraction of a second earlier than the front brakes for stability.  Applying the rear brakes first make for a more stable braking condition.

[howlinwolf]

the switch is giving infinity.at least im not getting any kind of reading. i do get a continuity reading however. i did note some small wet spots along the rear hard brake line, but considering the pounding, i dont think they are leaking enough to stop the pressure from getting to the back.

[howlinwolf]

fine. dang truck everything else is new on the back brakes, im going to run a whole new hard line from the combination valve all the way back.  then ill replace the flexible line as well. while doing this ill try and work on the combination valve and check that as well.

any tips going in?

[Irish_Alley]

At first I was thinking the combination valve at the front cross-member, but if I remember correctly I don't think it will shut off fluid to the rear brakes, only the front brakes if the system becomes imbalanced.  At either rate there is a button on one side of the combination valve.  It is under a rubber cap.  Pushing it in will reset the Pressure Differential Switch.  However, pushing it in is what moves the valve away from the front brake circuit and opens up flow to the front calipers.  Hmmm???  I think I would start by looking at the rear flexible line.  If it looks worn with any cracks in the outer layer I would just replace it as they are pretty cheap!  Your combination valve could be froze.

The combination valve has three jobs: 1) Proportioning Valve - It limits the amount of pressure to the rear brakes.  Because your rear drum brakes have a better mechanical advantage over the front disc brakes, they need less pressure than the front. 2) Pressure Differential Switch - This is what makes the Brake Light come on in the dash should the system become imbalanced from front to rear.  Make sure you have power to the bulb, that the bulb is not burned-out and that the wire is connected to the combination valve.  If this light comes on when you step on the pedal then you have a pressure problem in either the front or the rear brake circuits. 3) Metering Valve - This applies brake pressure first to the rear brakes, a fraction of a second earlier than the front brakes for stability.  Applying the rear brakes first make for a more stable braking condition.

que?
the reason there is a proportioning valve is to limit the rear brakes so they wont lock up, this is especially true in trucks. As you panic brake you shift the weight off the rear more than normal braking so the valve limits rear pressure so you don’t lock the rear tires up as easy. This was abs before abs granted not as good but kind of the same ideal. it wont limit your front pressure

[ehjorten]

The switch at the combination valve is normally open.  If an imbalance is sensed the plunger inside moves to the side with the imbalance and make contact with the switch.  This grounds the circuit for the brake warning light and causes the lamp to light.  If it is working properly and the lamp remains unlit, then both circuit are pressurized  and you have a blockage somewhere.  I wouldn't bother replacing the hard line unless you found it kinked somewhere along its path.

It has to be something simple.  It usually is!  You can start at the master and just crack the fittings, press the pedal and see if you get fluid flowing.  Keep moving back until you don't and then your problem is isolated to the last section you checked.

[ehjorten]

que?
the reason there is a proportioning valve is to limit the rear brakes so they wont lock up, this is especially true in trucks. As you panic brake you shift the weight off the rear more than normal braking so the valve limits rear pressure so you don’t lock the rear tires up as easy. This was abs before abs granted not as good but kind of the same ideal. it wont limit your front pressure

Yes this is true, but it isn't the only reason.  Drum brakes are mechanically more efficient than disc brakes!  They take less pressure to achieve the same amount of braking force.  The reason why we don't all still have drum brakes is because they stop REALLY REALLY well, but only once!!  As drum brakes heat-up the drum expands away from the brake-shoes causing brake-fade.  They also are not self cleaning like disc brakes.

The point I was trying to make, but must have not come across is that when you have to push the button on the combo valve to reset the pressure differential switch is only if you lose pressure in the front circuit.  If you lose pressure in the rear circuit there is a spring that pushes the valve back into position.  Pushing the button only pushes the valve away from the front circuit.

The OP is having problems with the rear circuit and hence he should not have to mess with the reset button, unless there is something froze up inside the combo valve.

[howlinwolf]

well ill mess with the switch later, possibly today. there is no loosening the rear flex line from the hard line. its way too rusted. its even seized in the holder for the flex line. Also the hard line has very very small leaks along it. no dripping fluid, but there are wet spots in multiple locations.

[howlinwolf]

the combination switch is being a pain to get out. a 9/16 flare nut wont fit, its loose and the 1/2 wont work either, its too small