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73-87 Chevy _ GMC Trucks => 73-87 Chevy & GMC Trucks => Topic started by: howlinwolf on May 14, 2012, 01:26:59 pm
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Several years ago, just before i took the truck off the road, the rear brake wheel cylinder failed. that was 2 or 3 years ago. well i have been working on it.
so far i have a *new* driveshaft (straightened, and new yoke and u joints
new front upper and lower balljoints and tie rod ends
serviced front axle, inspected brakes
now i have also replaced the rear shoes and wheelcylinders, and after bleeding there is no brakes on the back. there are no leaks and it has been bled and rebled and readjusted as well. now as long as it sat with no fluid to the rear, could the master cylinder have failed?
its a 78 chevy k10
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whats it doing?
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the back brakes wont engage at all with the pedal. the ebrake works fine, but the back drums wont engage and stop the wheels from turning. there is fluid and everything is bled, but there is no movement on the back brakes with the pedal. the front brakes are fine.
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Not to offend you or your skills but did you adjust the rear brakes
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hey no offense there. everyone makes stupid mistakes sometimes. yes, there is a light drag when you rotate the drum. but i had my wife press the brakes, truck running with a drum off, and nothing moved at all.
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brake light? hows the pedal?
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no light, pedal feels the same as it has since i got it back out. there is a lot of drop before engagement.
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for me it seemed when i bled it there wasnt much pressure when the pedal was pressed
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hate to say i dont know much about brakes. but i think the proportioning valve can go bad(most time under panic braking as its setup to be open until then) also, the rear rubber line (this most time causes the brakes to lock up) i want to lean to the master cylinder being bad. but i dont know how to check it. could the brake line of rusted on the inside and clogged itself? idk i would think the only way to see this is to undo the metal line and see if theres any difference in the amount of fluid that gets pushed out
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You are going to have to pressure check the rear brake system, each side starting at the backing plates of the rear brakes. You need to eliminate as you go forward to brake master cylinder to determine where the problem is or if you might have more than 1 problem.
I would think along the lines of the either brake master cylinder first as Irish said, or second proportioning valve, though these are less likely to fail.
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went ahead and replaced the master cylinder since it was under warrenty, still having problems. but now i have help
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there is what seems to be pressure at the rear when i bleed, but it wont move the wheel cylinder. the one im checking is new, but there is no movement.
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Does the brake light in the dash work with the e-brake set, key on? Your description sounds much like a tripped combination valve.
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Actually no. Where is this valve. Plus the springs are getting replaced tomorrow. Its all droopy looking compared to fresh rebuilds
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the valve is on the cross member in front of the engine, follow the brake lines and it will lead you right to it. but i would crack the other lines from the rear and check the fluid there then move up to the next spot where you can open it
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i get fluid from both wheel cylinders
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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!
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now crack the rubber line and see what you see then move up to the next one
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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. ;D
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 (http://forum.73-87chevytrucks.com/smforum/index.php/topic,22940.msg190525.html#msg190525)
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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
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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. :'(
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Yup new cables installed last night fixed that, but still no pressure. Ah well, now im chasin valves
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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.
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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.
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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?
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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
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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.
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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.
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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.
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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
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If you have wet spots along your brake line that's your problem. It doesn't matter how small they are or if they drip. You'll suck in air from those spots and not be able to develop pressure or bleed the rear brakes--the combination valve will likely stay tripped as well. I think there's a good chance you'll be good after you replace the line to the rear. I recently replaced the combination valve on my truck, but I can't remember for sure what size wrench the rear line took.
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the rear line is just seized.
where can i order the combination valve
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If you're going to replace the line anyway try cutting it off the fitting at the combination valve and using a 6 point socket. If you need a combination valve a pv2 was a direct replacement for my c10. Summit, ebay, or Classic Performance Products on that (assuming it also works for your k10).
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if it works by power going to the bulb and the switch grounding out, it is indeed activated and stuck. i took my multimeter, hooked it to the switch solid, and touched it to the positive on the battery and had a full 12.6 read on it
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You might be able to free up the valve, but it sounds like you may have had some long-term water contamination in the brake fluid, causing internal corrosion and multiple failures (wheel cylinders, perforated lines, etc), in addition to the obvious external corrosion. Conservative approach would be to replace the valve along with the lines, etc, and give it a thorough flushing with fresh DOT 3 or 4 fluid.
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most of the way there already. k. ill work on it.
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I cannot remove any of the lines from the valve. Any way to help free the fittings
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Penetrating oil, wd40, pb blaster. Only use a flare nut/line wrench, not a crescent. On lines you're replacing you can cut them off and use a 6 point socket if you have to.
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Got it loose. One fitting had the hex snap off, and one rounded to the point i just clipped it.
Got it freeed up inside too. The piston and bore look good as well, so as long as i can get the fittings out im gonna reuse it. I dont have the money to replace the valve and the lines and such, so there is my comprimise
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alright guys, exploded view of the combo valve comin up. i remember someone was asking for photos in another thread