Very little pressure to rear brakes.

[bd]

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?

[greenmonster]

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.

[bake74]

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.

[VileZambonie]

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

[greenmonster]

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.

[bd]

A BIG pat on the back!!  Well done!   

[bake74]

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.

[VileZambonie]

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.

[bake74]

     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 ?

[VileZambonie]

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.

[bake74]

     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.

[VileZambonie]

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

[bd]

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!   

[greenmonster]

Here are some pics I found.

[greenmonster]

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