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73-87 Chevy _ GMC Trucks => Brakes, Frame, Steering & Suspension => Brakes and Braking Systems => Topic started by: greenmonster on January 31, 2012, 01:25:13 pm
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My 75 k10 rear brakes have very little pressure.I put the rear on jack stands and when I press the brake as hard as i can it just barely stops the wheels at idle.I tried to bleed the brakes and no air came out and plenty of fluid came out.I was reading about proportioning valves and found out about a button on them.It was sticking out and I reset it but it didn't help.I'm at a lose on this one.Any ideas?
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did it just suddenly happen? what do the brake shoes look like? maybe needs adjusting?? not an expert just thought i would ask some questions that came into my mind. im sure someone else here can help you a little more.
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I don't know when it started.The back brakes have a lot of shoe left and I adjusted the brakes to drag alot just to see if that would help.Could the m/c do this.I have fluid flow to the back,just not very much pressure.
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Could be a bad wheel cylinder. Need more info though.
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The wheel cylinders don't leak.With the drum off I can hit the brakes and the shoes open but I can hold the shoes from opening with my hands.That is why I say it has low pressure.
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So, in neutral with the wheels mounted to the axle and raised off the ground, if someone steps on the brake pedal, can you spin the wheels by hand?
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Yes.The drivers side is real hard to turn but the passenger side is alittle easier.
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I know this isn't what you want to hear, but the way you're describing it, it really sounds like adjustment and air in the system.
It could also be the design differential front/rear pressure ratio of the proportioning valve. My '87 was built with JB5 brakes. Braking was so poor that you would pray you were going to stop in time with the pedal nearly mashed through the floorboard! Very scary! I'm surprised there was never a recall. The brakes stayed that way until I reconfigured them. Now, it stops on a dime! Many GM trucks see three or four-to-one wear ratio, front-to-rear. :o
Even so, consider some of these basics:
* Your brakes could be out of adjustment, or less likely not correctly arced to the drums making for poor contact.
* Your rear drums could be worn past their useful service life (effectively, making the rear brakes out of adjustment).
* You may still have air in the system.
- Try bleeding the lines at the master cylinder, then bleed them again at the rear wheels.
* The master cylinder could be bypassing internally.
- When you 'lightly' step on the brake pedal and hold it, is the pedal solid or does it gradually fall toward the floor?
* The proportioning valve internal piston may be tripped or sticking.
- Does the brake warning lamp work in the dash when you set the parkbrake with the key switched ON?
- With the Ign ON and parkbrake released, does the brake warning lamp light up when you step hard on the brake pedal?
* You could have a leaking brake line leading to the rear axle.
- Have you had to add fluid? How do the lines look?
* The rubber brake hose between the body and back axle could be damaged internally (a flapper).
I expect some others will chime in with some good ideas. :D
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All good ideas.I have tried alot of them.Tomorrow I am going to put a new master cylinder on (because they are cheap) and flush the system.
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Best of luck in solving your problem. Let us know what you find. :)
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I'll let you know tomorrow or ask for more ideas.Thanks for your help.
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The only other way to test without throwing parts at it is, (you need a gauge that checks line pressure though), start at back on one side and work your way up to the master cylinder, checking each side one by one. This method is APITA. But that is the way to determine what exactly is wrong.
I would think that if it was both sides doing the same, it would be either what bd said about brake wearing out of tolerance (both sides), or the proportioning valve and master cylinder could have gone bad.
I would start by checking out the brakes in back first thoroughly though.
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This probably doesn't apply to your truck, but didn't some of these trucks have a load sensing valve that would limit pressure to the rear wheels unless there was a load in the bed?
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Replaced master cylinder, wheel cylinders,adjusted brakes,and there is more than 1/4 inch of shoes left.It still can't stop the rear at idle.I can stop the rear with the parking brake.
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have you flushed the lines yet? i think you said something about trying that. i know we had a truck come into the shop and it had some kind of thick sludge (looked like lucas) and it had similar symptoms
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It could also be the design differential front/rear pressure ratio of the proportioning valve. My '87 was built with JB5 brakes. Braking was so poor that you would pray you were going to stop in time with the pedal nearly mashed through the floorboard! Very scary! I'm surprised there was never a recall. The brakes stayed that way until I reconfigured them. Now, it stops on a dime!
This probably doesn't apply to your truck, but didn't some of these trucks have a load sensing valve that would limit pressure to the rear wheels unless there was a load in the bed?
Is your truck equipped with this?
One other thing that I have seen is a piece of the foil off a container of brake fluid getting into the system.
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No load valve.I did flush the system when I put the master cylinder on.I don't think foil or anything is in the system because a lot of fluid flows when i bleed them.
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hmm... these trucks dont have rubber hosing going up to the rear brakes off the axel do they? if they do it could possibly be swelling under load...
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They do have a rubber hose.I had my son hit the brakes while I held the line and it didn't swell
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thats crazy... im really curious as to what is causing your problem...
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The only other way to test without throwing parts at it is, (you need a gauge that checks line pressure though), start at back on one side and work your way up to the master cylinder, checking each side one by one. This method is APITA. But that is the way to determine what exactly is wrong....
Bake made a valid suggestion. Or, you could bypass the proportioning valve and temporarily plumb the master cylinder directly to the brakes. You'll have some time and small expense invested in materials (along with bloody knuckles), but will find out immediately if the proportioning valve is the problem. Try PM'ing Vile and Captkaos to see what they think. I'm sure they've run into this problem more than once.
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I'll see if I can get my hands on a pressure gauge,if not I will bypass the proportioning valve and see what happens.
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Does anyone know an easy way to test a residual pressure valve.
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I assume you're referring to the valve in the M/C that maintains a few PSI of pressure in the rear brake circuit. I doubt that you can test it objectively w/o a pressure gauge. :(
But subjectively... ;D you could apply 1-2 PSI of air pressure (about what you could create with your mouth) through a rubber hose (cheapo M/C bleeder kit) into the outlet port of the M/C and see if air bubbles back into the reservoir. :o
I'm really not serious that you try this "distasteful" test. :P Not even sure if it would work. Rather, if you already replaced the M/C with no improvement, the residual check valve was replaced as part of it. What's to gain?
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Replaced master cylinder, wheel cylinders,adjusted brakes,and there is more than 1/4 inch of shoes left.It still can't stop the rear at idle.I can stop the rear with the parking brake.
Big question, did you bench bleed the master cylinder before putting it on ? If not you might not get brakes, or sometimes they will work and sometimes not.
When you flushed your system, did you make sure that all lines were clear and you were getting the same amount of fluid out of all 4 lines ?
1 other thought, the hard rubber lines for the back could have broken down inside causing a restriction or blockage (that is why the question of all lines flushed and the same).
They do have a rubber hose.I had my son hit the brakes while I held the line and it didn't swell
They should be reinforced rubber, should not swell enough for you to feel unless they are weak.
1 last question, ( please see #1 on my signature), do you have the right brakes for your truck, and if you did anything with them, are the back on the right way ?
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Yes I did bench bleed the master and flushed all the lines until i got clean fluid.With the new master the brakes feel the same as the old master.I think I have the right brakes,11 11/32 x 2 3/4
with wheel cylinder bore of 15/16.I hooked up a small pressure gauge (15 psi) to master and there was no residual pressure.I was reading online that a lot of aftermarket masters don't have the valve in them.Could that be the problem.
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If the residual valve were missing, pumping the brakes 3-4 times would build pressure. The more you tell us, the more I question the proportioning valve. At this point, I'd check the pressure out of the M/C, then out of the proportioning valve (both to front and rear), using a fixed application pressure on the brake pedal, and compare the readings.
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I'll try that,but I'm still trying to find a pressure gauge.
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http://www.summitracing.com/parts/SSB-A1704/ (http://www.summitracing.com/parts/SSB-A1704/)
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I took the proportioning valve off to clean it out and figured out that the switch for the brake light was gummed up.After cleaning it and putting it back on the light is on.So that as something to do with the problem.How do I reset that,I tried hitting the brakes hard and it didn't work.
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One of the functions of the Prop Vlv is to warn the driver (via the cluster brake light) when there is a major failure of the brakes, such as a broken hydraulic line, blown wheel cylinder, severe misadjustment, or other mechanical failure. The piston trips when there's a major pressure imbalance between the front and rear systems. When the piston trips, it also limits fluid flow to the offending system. So at some point there was a major failure in the rear brakes, OR someone stepped on the brakes with the drums removed, etc, etc.
GM Prop Vlvs are "supposed" to be self-centering, but I've seen many stick. Usually the solution is exactly what you tried - hammer the brake pedal a bunch of times with your foot. I've been successful at centering the pistons using a small screwdriver in the V of the piston, but you have to be careful not to scratch the piston or bore. Alternating brake clean and compressed air into the valve ports might get it to free up.
Once you get the piston centered you can keep it centered while working on it by anchoring it with a handy little tool you can make. Make the tool by filing a 1/4" long 1/8" diameter protrusion at the end of a 3/4"-long 3/8" NF bolt. When you're done it will pretty much look like the switch you removed from the Prop Vlv. With the piston centered, gently thread the bolt into the switch hole so the protrusion seats in the V of the piston. This way you can bleed and test the system to your hearts content w/o tripping the valve again. But, be sure to remove the tool and reinstall the switch when you're done!
Do you have any idea how long the brakes have been this way and what might have caused it?
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I have owned the truck for almost 5 years and the brakes have always felt the same.I'm not sure if they ever worked good,I always thought they worked because if it was humid out they would lock up very ease the first couple times I hit the brakes.
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I took the proportioning valve off to clean it out and figured out that the switch for the brake light was gummed up.After cleaning it and putting it back on the light is on.So that as something to do with the problem.How do I reset that,I tried hitting the brakes hard and it didn't work.
I am not sure if you can "reset" the switch.
My guess is with bd and you have a problem with the proportioning valve or a combo of that and the brake switch.
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Post a picture of your brakes and if you haven't measured the pressure how do you know it's a low pressure problem? The proportioning valve should hold off at about 600PSI
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Just to let everyone know,I GOT IT FIX!!! It was the combination valve doing all of this.I took it off today and I had to order one, so I tore the one a have apart.There was a lot of gunk in it.I cleaned it out,reassembled it, and put it back on the truck.Between that and all the new parts I put on trying to fix it,the brakes are awesome now.Thanks for all the help everyone.That was the weirdest brake problem I've ever had.
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A BIG pat on the back!! Well done! :)
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Just to let everyone know,I GOT IT FIX!!! It was the combination valve doing all of this.I took it off today and I had to order one, so I tore the one a have apart.There was a lot of gunk in it.I cleaned it out,reassembled it, and put it back on the truck.Between that and all the new parts I put on trying to fix it,the brakes are awesome now.Thanks for all the help everyone.That was the weirdest brake problem I've ever had.
Did you happen to take pics, it would be a great teaching tool for everyone on here ? I am not sure what the "combination" valve is, could you be a little more specific.
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This is the combination valve. It's a multifunction valve that has 3 major functions.
1)Differentail Pressure Valve which illuminates the brake warning lamp when there is a hydraulic failure
2)Metering valve which allows the disc drum combination to have equal braking force. The shoes need to overcome return spring pressure before they start to work so essentially it delays the application of the front brakes until the pressure rises in the rear braking circuit.
3)Proportioning valve which limits the pressure to the rear brakes to help prevent lock up.
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Thanks Vile, so excuse me for my non existence knowledge ( I really do like to learn new things ), is the combination valve and proportioning valve the same ?
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The combination valve encompasses the proportioning valve with the exception of some trucks which have the load sensing proportioning valve in series. Vehicles with disc drum typically use a combination valve. 4 wheel disc does not need a metering valve so will utilize a proportioning valve (or proportioning valves) or an electronic hydraulic control unit for anti-lock and traction control.
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Again thanks so much, it is very hard for me to accept things as fact from people, you are one that I have come to believe that does not give miss information. I will bookmark this thread for later reference.
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Bake,
There's actually quite a few threads that have better explanations, pics & procedures for servicing if you search "combination valve" Take a look
http://forum.73-87chevytrucks.com/smforum/index.php/topic,16967.msg133296.html#msg133296
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Thanks from me too, Vile! You've helped clarify more than one question that I had almost puzzled all the way through. Your a wealth of information! :D
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Here are some pics I found.
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...
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See, this is why I bookmark some threads, just so much useful information here.
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New To Me ~ 1990 Suburban 3/4 ton 4wd with a similar combination valve stuck / poor brakes / brake light on.
(http://i9.photobucket.com/albums/a53/whitfimb/Blazer/8a5fc44e-84b4-4b9a-8e0f-da01ff985845.jpg)
No visiable brake fluid leaks. *** At some point a Dana 60 Full FLoater rear axle was swapped in, but it is a clean swap and has been in place 10+yrs (talking to a friend who was po's state inspector and a fellow chev truck enthusiast). Truck has 8" of lift and 36" Swamper TSL Radials.
Bled brakes / gravity bled / Reverse bleed from the rear with Phoenix injector pump ~ have flow but low pressure, not able to correct stuck combination switch.
Also reverse bleeding was spraying a squirt gun stream of brake fluid out of the top of the MC ~ (Cover off). This is not normal ~ been using this pump for 5+ yrs. I will try to snag a used combination valve Monday and also try the aforementioned above stomping the pedal 10x quick, along with disassembly and recentering if it comes down to that.
Pic of my funny looking combo valve and I believe that is Generation 1 ABS just behind it. My lines come in and out of the combo valve in a H pattern the lines right and left are what I believe to be rear ABS. Does ABS complicate things?
(http://i9.photobucket.com/albums/a53/whitfimb/Blazer/8fe2a65d-2208-459e-ac9a-a533eb1deae0.jpg)