NO REAR BRAKES

[Batman13]

I have followed along some other threads about brake issues icluding proportion valve problems.  Let's see if you guys can solve this one:

I replaced the proportioning valve on my 78' due to a leak in the old one.  I bought the new one from Carolina Classic Trucks, and is a direct fit.  I also purchased a valve bleeding tool to keep the valve from tripping and going into safety mode.  I bled the system and the rear brakes do not work. The rear drums were rebuilt with new components throughout.  Everything is new.  The entire brake system is new for that matter; new stainless lines, front brakes, master cylinder (bench bled).  I have tried bleeding with and without the valve bleeding tool.  The valve will not reset, unless I take the valve out of the truck and push the piston myself.  I am using a Motive pressure bleeder.  I have also used a mityvac.  The only thing I can think of is either Carolina Classics sent me the wrong valve, or it was bad out of the box.  The valves are simple mechanisms so I wouldnt think it would be bad out of the box.

This is my project truck.  I just finished a 12 year (YEP!!),  resto-mod and this is the only major issue I have had.  I'm anxious to get it on the road.  ANY help would be appreciated!!                 

[Captkaos]

have you call them?  It sounds like something is wrong.

[Batman13]

I did contact Carolina Classics, (a top notch organization btw).  The guy I spoke with said it was possible the valve was bad.  I explained a situation where the internal piston sheared the tip off of my bleeder tool, (plastic tool to prevent the valve from tripping) and he said it sounded like there was a spring missing or was weak.  There should not be that much pressure in the valve.  They are sending me a new one, so we will see.   

[VileZambonie]

valve bleeding tool to keep the valve from tripping and going into safety mode

What do you mean by this? What exactly is safety mode? You have disc drum setup with a combination valve?

If you bench bled the master, bled the combination valve and got all of the air out of the system you should have good pressure and volume at the rear cylinders. Are the shoes installed and adjusted correctly? Do you have good volume and no air when manually bleeding the brakes?

[Batman13]

If the combination valve senses a leak, (lack of pressure) the internal piston will move to one side or the other enabling either the front or rear system to operate.  So if you blew a rear brake hose, the piston would would shut down the rear brakes and allow  the front brakes to work.  The piston will also move during brake bleeding because it senses a loss of pressure.  The tool I use keeps the piston from moving so fluid can flow to front and rear systems.  This is called centering the valve. 

So I installed the new valve on the truck and aggressively pushed the brake pedal a good 15-20 times making sure there was no vacuum in the booster.  My wife was the pedal pusher, (she is used to it) for bleeding the system and just like that I have rear brakes.

I had a bad valve, and this time, I made sure I did have enough volume in the system by eliminateing all of the vacuum for maual bleeding.  Do not count on a pressure bleeder like my Motive bleeder to do this for you.  Pump the brake pedal aggressively first, then bleed with whatever method you choose. 

I'm back on the road!!   

   

[Captkaos]

Glad you got it worked out.

[VileZambonie]

If the combination valve senses a leak, (lack of pressure) the internal piston will move to one side or the other enabling either the front or rear system to operate.  So if you blew a rear brake hose, the piston would would shut down the rear brakes and allow  the front brakes to work.  The piston will also move during brake bleeding because it senses a loss of pressure.  The tool I use keeps the piston from moving so fluid can flow to front and rear systems.  This is called centering the valve. 

So I installed the new valve on the truck and aggressively pushed the brake pedal a good 15-20 times making sure there was no vacuum in the booster.  My wife was the pedal pusher, (she is used to it) for bleeding the system and just like that I have rear brakes.

I had a bad valve, and this time, I made sure I did have enough volume in the system by eliminateing all of the vacuum for maual bleeding.  Do not count on a pressure bleeder like my Motive bleeder to do this for you.  Pump the brake pedal aggressively first, then bleed with whatever method you choose. 

I'm back on the road!!   

   

The pressure differential valve will only trip the warning lamp it's not a safety mechanism. The Master Cylinder provides your dual circuit failsafe. More info here:

http://forum.73-87chevytrucks.com/smforum/index.php/topic,16967.msg133296.html#msg133296