Desperately need help with Brakes

[Short68Chevy]

Good evening.
I am completely stumped. I have replaced about everything in the Brake system. I have completely new rear brakes and springs, four new wheel cylinders, new rubber hoses, a new Master Cylinder, new distribution block and a new line kit. I have everything in and tightened up. The master cylinder was properly bench bleed. I now have two issues. First no matter how long we pump I can not get any pressure at the rear wheels. I can get fluid out but it is just a trickle. I have looked at every connection and I do not see/feel any spots that are leaking. I don't know what I am doing wrong. The other issue is that I am able to pump the brakes and get pressure in the front. But when I try to bleed them I have fluid coming out for two tries then nothing and I loose the pedal. If I switch wheels or wait a few minutes I am able to build up pressure aging but it goes away when I try and bleed again. I need any suggestions you all can give me. This is the last thing I need to do before I can get my truck on the road again.
Thanks in advance for you help.
Jeff

[toolboxchev]

Hey Jeff,

Could be possible you have incorrect parts in the system, like the distribution block.

I could not count on my hands and toes how many auto parts stores will give you what they have on the shelf instead of what is correct.

Have you tried bleeding the rears at the junction on the axle?

[Short68Chevy]

Thanks for the reply.
The distribution block is from inline tube and it is an exact match to the original one. As for bleeding at the back block i have not done that, i will look into doing it. Not sure if it will work because i have fluid but no pressure.

[sduckworth13]

Did you switch from a manual master cylinder to a power brake booster? I only ask because if you did you'll need to adjust your push rod so you can bleed the brakes properly.

[Short68Chevy]

It was power and I bought a stock booster MC replacement.

[rsgt]

Loosen the two nuts holding the master cyl to the booster and pull the master cyl out may be 1/8 inch or more from the booster. See if you get fluid to the rear. Sometimes the push rod is a little long and is holding the piston in some. On the front make sure there is no leak at the hose to caliper washers.

[sduckworth13]

Originally Posted by rsgt

Loosen the two nuts holding the master cyl to the booster and pull the master cyl out may be 1/8 inch or more from the booster. See if you get fluid to the rear. Sometimes the push rod is a little long and is holding the piston in some. On the front make sure there is no leak at the hose to caliper washers.

What he said .

[toolboxchev]

Proper bleeding starts at the longest distance from the MC. I believe that would be at the back passenger side.

1/2 or 3/4 ton? Assuming 1/2 ton? You may be sucking air somewhere in there. Or another possibility the connection from MC to Booster is not adjusted properly.

Use a glass jar with a clear hose from the bleed knut to keep from reintroducing air back into the system.

[swamp rat]

Yep, the rod that came with my booster was too long, i ended up using my stock one, it gave me about .050 clearance from the rod to the piston in the M/C.

[mike16]

down below the master cylinder there is a distribution block that both lines go into and come out of. centered between the front and rear hydraulic systems, inside that distribution block is an electrical switch that completes a circuit when one system failes and the other system pushes the switch to the closed position. another feature of this design is that the switch, actually in the form of a piston, when it is pushed over to the failed side of the system , it blocks any additional fluid from escaping from that failed side of the syste and stops any further loss of fluid from the reservoir,

when you bleed your brakes, and you have this system, you essentialy create a leak when you blead the brakes. this causes the hydraulic system to push the piston to one side , it completes the circuit( the idiot lite for failed brakes). but its also blocking flow from the master cylinder reservoir to the "leaking or bleaded" side of the system.

its all explained in the service manual, but I dont know if you have that idiot lite switch distribution block component.

the fix is real easy. the switch/ combination block has a wire going to it. remove the wire and you notice the connection on the distribution block is got a hex head on it, wrench that off, look down inside and you see the little piston that moves back and forth and completes the electrical circuit for the idiot lite. reach in there with a stout piece of wire and recenter that switch, then assemble....in reverse order. GM has a special tool to center and hold it centered while brakes are being blead. but wire will do.

not sayin you got that system but its common and the piston thing stumps alot of people. Its all in the service manual...

[cdowns]

do you have a factory service manual?? they have great directions how to setup a brake system and the proper way to bleed them as it is not as easy as one might think read the book to get the answers

[67/Heaven]

Originally Posted by mike16

down below the master cylinder there is a distribution block that both lines go into and come out of. centered between the front and rear hydraulic systems, inside that distribution block is an electrical switch that completes a circuit when one system failes and the other system pushes the switch to the closed position. another feature of this design is that the switch, actually in the form of a piston, when it is pushed over to the failed side of the system , it blocks any additional fluid from escaping from that failed side of the syste and stops any further loss of fluid from the reservoir,

when you bleed your brakes, and you have this system, you essentialy create a leak when you blead the brakes. this causes the hydraulic system to push the piston to one side , it completes the circuit( the idiot lite for failed brakes). but its also blocking flow from the master cylinder reservoir to the "leaking or bleaded" side of the system.

its all explained in the service manual, but I dont know if you have that idiot lite switch distribution block component.

the fix is real easy. the switch/ combination block has a wire going to it. remove the wire and you notice the connection on the distribution block is got a hex head on it, wrench that off, look down inside and you see the little piston that moves back and forth and completes the electrical circuit for the idiot lite. reach in there with a stout piece of wire and recenter that switch, then assemble....in reverse order. GM has a special tool to center and hold it centered while brakes are being blead. but wire will do.

not sayin you got that system but its common and the piston thing stumps alot of people. Its all in the service manual...

I had a very similar issue when I upgraded to front power disc brakes, however mine was due to the proportioning valve shifting and shutting off the rear brake circuit. For this there is a dummy plug that is placed where the electrical switch is mounted on the proportioning valve to keep the inner hydraulic spool in place as the bleed process takes place. Very similar to what I was experiencing, don't know what your setup is or if you have a proportioning valve. Just my 2 cents.

[71swb4x4]

Originally Posted by mike16

down below the master cylinder there is a distribution block that both lines go into and come out of. centered between the front and rear hydraulic systems, inside that distribution block is an electrical switch that completes a circuit when one system failes and the other system pushes the switch to the closed position. another feature of this design is that the switch, actually in the form of a piston, when it is pushed over to the failed side of the system , it blocks any additional fluid from escaping from that failed side of the syste and stops any further loss of fluid from the reservoir,

when you bleed your brakes, and you have this system, you essentialy create a leak when you blead the brakes. this causes the hydraulic system to push the piston to one side , it completes the circuit( the idiot lite for failed brakes). but its also blocking flow from the master cylinder reservoir to the "leaking or bleaded" side of the system.

its all explained in the service manual, but I dont know if you have that idiot lite switch distribution block component.

the fix is real easy. the switch/ combination block has a wire going to it. remove the wire and you notice the connection on the distribution block is got a hex head on it, wrench that off, look down inside and you see the little piston that moves back and forth and completes the electrical circuit for the idiot lite. reach in there with a stout piece of wire and recenter that switch, then assemble....in reverse order. GM has a special tool to center and hold it centered while brakes are being blead. but wire will do.

not sayin you got that system but its common and the piston thing stumps alot of people. Its all in the service manual...

I don't believe that this is correct on a drum/drum distribution block. It is correct on a disc/drum proportioning valve, but the drum/drum distribution block does not cut the fluid off.

[old Chevy guy]

If all that fails to fix it don't discount the obvious. I've seen brand new master cylinders be bad straight out of the box.

[mike16]

if there's a wire comming out of the distributor or as you call the perportioning valve. Its probably what I explained. Gotta get the manuals to do it right and understand the system. Got to!

The aftermarket brake stuff is all ove theplace. I stick with the factory stuff because spares are around and the manuals work.

[71swb4x4]

From what I understand a distribution block is what is used on a drum/drum system, it does not have the check valve. It does have the wire coming off to tell you that you have a problem with your fluid. The proportioning valve is what is used on a disc/drum system, it has the check valve in it, and also has the wire coming off to tell you that you have a problem with your fluid.
Maybe a distribution block and a proportioning valve are the same thing, I can't answer that. But I am almost certain that the drum/drum system doesn't have the check valve, or whatever that piece is called that stops the flow to the rear. Now I really sound like I know what I am talking about, don't I?
Let me see if I can find some more info.

Edit:
here are a couple of related threads. I read a bunch, and am sharing these three. The common theme It seems that there is nothing in the drum/drum block to hinder your progress, but that most people are finding it to be a bleeding issue. Most people are saying the same thing you are, they bled the heck out of it, and still nothing. And then they bleed some more and finally get things going.
http://67-72chevytrucks.com/vboard/s...d.php?t=454290
http://67-72chevytrucks.com/vboard/s...d.php?t=467025
http://67-72chevytrucks.com/vboard/s...d.php?t=522033

[snipescastle2]

Did you replace all the brake lines? The reason I ask is, I had a similar problem on mine, turns out I really had to tighten the crap out of the flare nuts on those new brake lines in order to get them to seal. Once I started to get fluid to the flare connections, I was finding leaks,safe to say it was sucking air at the same time.maybe double check your fittings to see if that is giving you trouble.
Ben

[snipescastle2]

Drum brakes had residual pressure valves in the master cylinders or wheel cylinders that would keep a small amount of pressure on the wheel cylinder to keep the shoes out farther for faster braking. Disc brake calipers use a
"square" seal that flexes while the piston comes out under pressure, when you let off the brake pedal, the pressure falls off, then the square seal flexes back, allowing the piston to retract, moving the brake pads away from the rotor. Hope that helps.
Ben

[Short68Chevy]

I wanted to say thank you to everybody who responded with suggestion and advice. I also wanted to put some closure to this issue for future posters. It came down to the the new master cylinder I bought was defective. I finally had time to go to Oreilys and make the exchange. After spending the time to properly bleeding the system my truck now has a firm pedal and stops great. Again thanks to everyone.
Jeff.

[Chevy Fan Always]

Just a footnote concerning bleeding. I was always taught to bleed the wheel furthest away from the MC. While reading the service manual I was surprised to read that its exactly the opposite according to Chevrolet. And don't forget to have the button on the end of the proportioning valve depressed. That was news to me also. Page 5-4 in the Manual.

[old Chevy guy]

I hate when that happens. You buy new stuff figuring that you eliminated a problem and its still a problem. I've had new parts be defective more than a few times, that's why I threw that out there. Glad you got it fixed.