1972 K2500 no rear brakes

[Fbks_Cruiser]

Hey guys. Been lurking here for a year or so since I got my truck. The forum is so useful that I can almost always find what info I need and rarely post. Thanks for all the great information. I got the truck about a year ago and have been repairing/performing much needed maintenance as time as money allows.

Sorry for the long winded post. I just want to give as much info as I can:

I recently discovered that the right rear brake wheel cylinder started leaking. So I replaced the brake hose between frame and rear axle, both left and right wheel cylinders, new shoes and hardware.

Next challenge was bleeding. I bled the heck out of the lines (by having a helper pump the pedal, open bleeder at the top of the pedal and close before pedal reaches bottom, making sure master never went dry) and kept getting air with the brake fluid. I ran 2 quarts of brake fluid through the system. Got frustrated and quit for the night. Today I got a couple of speed bleeders cuz I thought air was entering along the threads of the bleeders. Also used a mityvac and pulled a bunch more fluid and air through. Gave up on the mityvac and went back to the helper pumping the pedal with the speed bleeders in place. Finally got to where we got 15 pedal pumps through each side with no air.

But, now I still get no movement of the brake shoes when the pedal is pressed. Parking brake works fine and shoes are adjusted up to where they drag on the drums. Front brakes work fine. Dash "brake" light is on. Yes I did have the button on the front of the propotioning valve depressed using a zip tie.

Grrrrr. Took it out in the drive and tried stomping on the pedal but light didn't go out and still no rear brakes. I can't really pump up the pedal and have it keep firm. With pressure it creeps to the floor in a couple of seconds but I can't find any leaks?

Aside from the leaking wheel cylinder, the brakes worked fine before.

Suggestions?

[special-K]

It doesn't matter if the pedal bottoms out when bleeding as long as your help doesn't lift before you tighten bleeder. Try it again having your helper pump the pedal slowly a few times each bleed. Rapid pumping makes air bubbles.

[LockDoc]

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I've never heard of anyone bleeding the brakes that way. Where did you get the information to bleed them in that sequence? To bleed them have your helper pump the pedal slowly 3 times and hold it down, then open the bleeder. When the fluid stops coming out close the bleeder, then have your helper let up on the pedal and pump it slowly three times again and hold it down while you open the bleeder. Keep doing that until the air is expelled.

You will not have rear brakes until you get the piston in the combination valve re-centered.

LockDoc

[Bigdav160]

Sounds like a restriction to me.

I was going to suggest the rear brake hose but you said you already replaced it.

I would start at the master cylinder and proceed to bleed every connection all the way to the rear. If there is a restriction you'll find it.

[BILT4ME]

Also, always bleed in this order:
RR
LR
RF
LF

Also, what you describe could also be a bad master cylinder. No fluid leaks,, but it keeps going to the floor. Sometimes it will bleed into the power brake booster or it will suck air through the front of the MC.

[jocko]

Originally Posted by BILT4ME

Also, always bleed in this order:
RR
LR
RF
LF

Also, what you describe could also be a bad master cylinder. No fluid leaks,, but it keeps going to the floor. Sometimes it will bleed into the power brake booster or it will suck air through the front of the MC.

This brings up a question - not an important one, but a question - the mantra has always been bleed furthest to nearest, however, I always thought that meant the longest brake line distance, not just wheel location relative to the mc. Since our brake lines run down the passenger side, kinda leads one down the path of thinking the left rear is "furthest" from the mc (as the brake line flies). Anyhoo - wouldn't the actual "furthest" wheel cylinder be the one that has the longest length of brake line/hose combo leading to it from the rear axle line T? (And off the top of my head, I don't recall if that's the left or the right). Just a thought.

[terry b]

Is the bleeder at the top of the wheel cylinder? If you changed them both could you have right on left and left on right? Air will only bleed out if the bleeder is on top. Don't know just asken.

[webfoot]

I am familiar with this problem, you bleed and bleed and think it stops but it stays squishy. I started gravity bleeding, take a jar and pour a little old fluid in the bottom if you have it, put a clear hose over it and run it down into the fluid in the jar and crack the bleeder on the one farthest from the master first. Just let them flow out itself. I have the "pleasure" of never having help around and this method works great. Just keep a close eye on the master and add new fluid as needed.

[Bigdav160]

I've bled hundreds of brake systems and my experience is it doesn't matter where you start or where you end up except for some ABS systems that have specific manufacture bleeding sequences to purge the ABS controller.

The method the OP used, even if LockDoc hadn't heard of it, is the preferred way to move fluid in an empty system. Many clutch hydraulics have this procedure in the service manual.

[Fbks_Cruiser]

Thanks guys. I probably wont get back to it for a couple of days. To answer a couple questions:

1) no way to mix up the wheel cylinders. Brake line comes into it at an angle. If I had them swapped the brake line couldn't screw on.

2) Because the brake lines cross over to the right side on the front frame cross member, then runs down the right frame rail and over to the rear differential then out to the wheels, the longest run of brake line is actually the left rear. (Although right rear is only shorter by a few inches.)

I don't think I have a restriction as I am getting plenty of fluid out of each bleeder. I am fairly certain I now have all of the air out as we were able to get many strokes of the pedal pumped through with no air bubbles.

How do I get the piston in the combination valve re-centered? I tried the "stomp on the brakes" method. I have heard of removing the pressure switch for the brake warning light and manually resetting the piston. I have a spare combination valve and pulled the switch from it just to see what was there and didn't really see anything I could move. Also, with all the bleeders closed, if I pump up the pedal and hold it, the pedal drifts to the floor in a few seconds. Is this a symptom of a bad master cylinder that just happened to show up now?

[BILT4ME]

Originally Posted by Fbks_Cruiser

Thanks guys. I probably wont get back to it for a couple of days. To answer a couple questions:

1) no way to mix up the wheel cylinders. Brake line comes into it at an angle. If I had them swapped the brake line couldn't screw on.

2) Because the brake lines cross over to the right side on the front frame cross member, then runs down the right frame rail and over to the rear differential then out to the wheels, the longest run of brake line is actually the left rear. (Although right rear is only shorter by a few inches.)

I don't think I have a restriction as I am getting plenty of fluid out of each bleeder. I am fairly certain I now have all of the air out as we were able to get many strokes of the pedal pumped through with no air bubbles.

How do I get the piston in the combination valve re-centered? I tried the "stomp on the brakes" method. I have heard of removing the pressure switch for the brake warning light and manually resetting the piston. I have a spare combination valve and pulled the switch from it just to see what was there and didn't really see anything I could move. Also, with all the bleeders closed, if I pump up the pedal and hold it, the pedal drifts to the floor in a few seconds. Is this a symptom of a bad master cylinder that just happened to show up now?

As I posted in post #5, yes.

[randy500]

The master cylinder should never bleed down.

The light should reset with differential pressure, meaning more pressure on one side than the other, problem is I don't think there is a way to tell if it shifted from one side to the other. You could remove the switch to see which way its set and try to move it to center with a tool. I also believe when set one way or the other the fluid flow is restricted by the valve. You need to get it set to center.

Manual bleeding with 2 people is a lot of work, do the gravity method after replacing the master cylinder.

For the amount of time and effort you have spent so far you would be wise to buy a pressure bleeder to do it without a helper, get the master cylinder refill kit too and then it will make your work a lot easier.

[jjzepplin]

If your brake light is on, your proportioning valve is most likely stuck.

[special-K]

Both rear brake cylinders are the same distance from the master cylinder in these trucks because both sides come off the block in the center. As mentioned, it really doesn't matter anyway.

There was actually a problem with the bleed procedure in the first post, at least the way it was described. There is no need to be concerned with closing the bleeder before the pedal bottoms. Let 'er hit the floor, push all the air you can with each bleed. Just don't lift till closed.

[GASoline71]

I do the 4 beer method...

Fill master cylinder with fluid. Crack a bleeder and then crack a beer. Sit in lawn chair and listen to some tunes. After you finish beer, check bleeder to see if fluid is coming out of bleeder. If it is, close bleeder. (If not, it might require another beer... lol!) Check master cylinder and top off if needed.

Repeat 3 more times.

To help with cleanup, some strategically placed rags under bleeders help. Never let master cylinder run dry.

Gary

[El Dorado Jim]

Originally Posted by GASoline71

I do the 4 beer method...

Fill master cylinder with fluid. Crack a bleeder and then crack a beer. Sit in lawn chair and listen to some tunes. After you finish beer, check bleeder to see if fluid is coming out of bleeder. If it is, close bleeder. (If not, it might require another beer... lol!) Check master cylinder and top off if needed.

Repeat 3 more times.

To help with cleanup, some strategically placed rags under bleeders help. Never let master cylinder run dry.

Gary

Ha Ha,love this method, i use it every afternoon in my shop...no matter what i'm working on..

[Fbks_Cruiser]

Success! I went back to square one. Pulled the master cylinder to verify it was not leaking out the back. And while it was out of the truck I bench bled it. Yup it had air in it. I also made a proper tool to hold the pin at the front of the combination valve. Checked all connections to make sure they were tight and then bled all 4 corners. Brakes work great and no red light!

Had so much time left today that I replaced the drag link and tie rods ends. Still had the original one piece drag link.

Now it stops well, steering is tight and it tracks nice and straight.

A good day!

[jjzepplin]

OK- That's just awesome. Gonna try it even if I don't need to bleed my brakes.