Why cant I bleed my rear brakes?

[slow-1]

Had to replace a brake line that I tried to move away from the exhaust. Tried to bleed brakes after line put in, unable to do it using the pump the pedal method.

Went and bought a vacum brake bleeder, did front in just a couple minutes, sucked out all the old stuff until nice new clean fluid came through.

Tried to do the back, vacum pumps up to 25 inches on gauge, no fluid flows.

Took rubber hose off thinking it could be clogged, fine. Hooked vacum to line, pulled new fluid from res.

Put vacum back on bleeder valve on rear brake, same thing...25 inches of vacum, no fluid flow.

Undid line into rear brake, old fluid drips out, vacum still wont pull through the brake even with line loose from back.

Tried other side thinking cylinder went bad on this side, same thing.


What am I doing wrong?
Posted via Mobile Device

[Husker]

Are the shoes adjusted to the drum?
Ron

[LONGHAIR]

Disc front, drum rear?
Sounds like the proportioning valve is popped. It is a safety system that "cuts off" one end or the other if a leak develops. It "balances" between the front and rear until one end loses pressure.
Bleeding the front first, especially with the problem in the rear, made it worse.
You need to get it re-centered first, then the rear will bleed.

[50mercfan]

assuming every thing was working fine before you replaced the line, here's what i would do.

always start at the wheel farthest from the master cylinder and work your way to it, in the order, right (passenger side) rear, left rear, right front, left front. tighten all bleeders and make sure the master cylinder is full.

have some one push the brake pedal, don't pump it up and down, just push it to the floor, open bleeder, close bleeder. do that several times until all fluid and no air bubble come out. do this at each wheel. make sure you don't run out of fluid in the master cylinder or you'll have to start over.

another thing you can do to get it started is gravity bleeding. take the cap off of the master cylinder and open all bleeders at once. watch until fluid starts coming out of each bleeder and then close them. this method can take a long time.

[50mercfan]

also, when using the vacume bleeder, take the cap off of the master cylinder. it'll never pull any fluid if the system is closed.

[slow-1]

Thanks for the replies.

It is disc/drum.

I can pull fluid to the line that goes to the backing plate with the vacum.

Fluid nor air will move through the brake itself...did both wheel cylinders freeze up? Shouldn't I be able to at least pull air through them?

I am at a loss...I am no mechanic, but a pretty good parts changer and this I have never run in to.

Keep the ideas coming!!!!!!!

[slow-1]

Originally Posted by LONGHAIR

Disc front, drum rear?
Sounds like the proportioning valve is popped. It is a safety system that "cuts off" one end or the other if a leak develops. It "balances" between the front and rear until one end loses pressure.
Bleeding the front first, especially with the problem in the rear, made it worse.
You need to get it re-centered first, then the rear will bleed.

How do you "recenter" the prop valve?

[slow-1]

Originally Posted by LONGHAIR

Disc front, drum rear?
Sounds like the proportioning valve is popped. It is a safety system that "cuts off" one end or the other if a leak develops. It "balances" between the front and rear until one end loses pressure.
Bleeding the front first, especially with the problem in the rear, made it worse.
You need to get it re-centered first, then the rear will bleed.

How do you "recenter" the prop valve?

[Daddy Brim]

Originally Posted by slow-1

How do you "recenter" the prop valve?

Stomp on the brake peddle like your going 100 mph and need to stop now. You may need to do this several times. You may also hear a pop or a click when it resets.

[slow-1]

So it looks like there ar LOTS of reasons I cant bleed my rear brakes.

1 - Gummed up bleeder passages

2 - Prop valve tripped (more chicken dinners due).

3 - Another brake line rusted through enough to suck in air when I use my handy vacuum bleeder.

4 - Who knows

I also poked a hole in my radiator while it was out of the truck during my engine rebuild with the cherry picker, then the replacement I bought doesn't fit...this truck is really challenging my sanity

[LONGHAIR]

Originally Posted by slow-1

How do you "recenter" the prop valve?

Daddy brim's method works most times. It is not as much about pressure as it is speed. Stomp it quickly, it may take a few times.
If that does not get it, (you are going to hate this) open one of the front bleeders and push the pedal too the floor. When it gets close, you will feel some resistance, push past that and you will feel/hear the click. Tighten the bleeder. Now start the bleeding process again, from the right rear, working toward the closest.

[slow-1]

Originally Posted by LONGHAIR

Daddy brim's method works most times. It is not as much about pressure as it is speed. Stomp it quickly, it may take a few times.
If that does not get it, (you are going to hate this) open one of the front bleeders and push the pedal too the floor. When it gets close, you will feel some resistance, push past that and you will feel/hear the click. Tighten the bleeder. Now start the bleeding process again, from the right rear, working toward the closest.

I can tell you that this method does NOT work if you still have a leaking brake line.

I am learning more about 40 year old brakes than I ever wanted to know.

I did take the light switch out of the prop valve and was able to easily center the internal safety valve using an ice pick. Of course it tripped immediatly when I pushed the pedal because of the yet undiscovered leak.

[slow-1]

Originally Posted by Husker

Are the shoes adjusted to the drum?
Ron

I assume they are...I drove the truck to where it sits, only change is the brake line.

[68gmsee]

Many years I had a similar problem.

With assistant's stepping on the brake I loosened the line at the master cylinder going to rear brakes to make sure fluid was coming out of there. I followed the brake line all the way back and any where there was a fitting I would loosen it up and again check to make sure fluid came out. When I loosened the rubber line above the rear axle, it oozed out. It was swollen inside.

In your case, since you know that one is okay it may be the master cylinder piston or clogged somewhere else along the brake lines.

[88Ironduke]

If its both left and right wheel cylinders I would start cracking lines at the proportioning valve out put to rear brakes and then work backwards. Where ever fluid flow stops is your problem. A frozen wheel cylinder will only stop working, the little cylinders have nothing to do with flow thru the body of cylinder for bleeding purposes.

88Ironduke

[slow-1]

Originally Posted by 88Ironduke

If its both left and right wheel cylinders I would start cracking lines at the proportioning valve out put to rear brakes and then work backwards. Where ever fluid flow stops is your problem. A frozen wheel cylinder will only stop working, the little cylinders have nothing to do with flow thru the body of cylinder for bleeding purposes.

88Ironduke

Yep, I know the problem is between where the brake line goes into the backing plate and the bleeder.

[special-K]

Sounds like proportioning valve issues,as others have said.

[ol_skool_chevy]

Replace the rubber hose in your brake line at the rear end..........it will look perfectly fine but it can be collapsed inside and you wont know it. I have had it happen many times.

[71swb4x4]

I agree with the others check the proportioning valve.
Posted via Mobile Device
Posted via Mobile Device

[da-burb]

Replace the rubber hose in your brake line at the rear end..........it will look perfectly fine but it can be collapsed inside and you wont know it. I have had it happen many times.

I agree with ol skool, hose has collapsed. I have seen them work like a check valve, you can pump the brakes and the brakes lock up and never release because the hose lining has closed off.

[slow-1]

Thanks for the continued replies!

I took the hose off, it is not clogged (gonna replace anyway while its off since its 40 years old).

I can pull fluid with vacum easily up to the backing plate so I dont think its the prop valve.

I'm hoping its the bleeding passage clogged, will check tonight.
Posted via Mobile Device

[Daner]

I would rebuild the wheel cylinders while you have that stuff tore apart. If there is enough stuff in there to clog the bleeder I don't think it would hurt anything.

[Sandemonium]

Sounds like rebuilt/new wheel cylinders are in your future.

Rebuild them yourself for about $4 each or new for about $10 each.
Posted via Mobile Device

[cdowns]

sooo many threads about this.. doesnt anybody read a service manual ever?? the CORRECT procedure is in there in detail

clamp the pin on the combination valve

[JointTech]

wheel cylinders are what $40? get new ones and be done with it.