Drum brake problem!

[dmw56]

I'm to the point of bleeding the brakes and seeing what I've got. So far the front discs are working but the back drums aren't.

There is a small amount of fluid coming out of the wheel cylinders but not enough to move the cylinders. I've bled them several times and I'm not getting any air.

I've checked all connections for leaks and fixed them. The vacuum is not hooked up engine isn't running yet. That shouldn't be an issue but I could be wrong. Wrong master and valve?? I'm stumped any suggestions please. Thanks guys!

This is what I have. GM disc/drum master cylinder with 7" dual power assist. Front brakes hooked up to the front reservoir and rear drums to rear. Hope that's right?



GM proportioning valve hopefully it's a disc/drum.


9 1/2" GM brakes new wheel cylinders and shoes

 

[sgtpontiac]

Even if the lines were reversed, which I don't think they are, I'd expect you would still get pressure to the rear brakes...
When you bled them how much pressure did you get back there?
wondering if maybe that proportioning valve might be shifted over to the block off the rears and isn't centering again.... ? if that isn't it then I'm thinking that there is something wrong with the master cylinder... maybe an internal bleed back?

 

[Dr Crankenstein]

HUGE PICTURES.



Are you getting fluid consistently and steady at the rear?

A common problem is lack of free play at the MC pushrod... if the piston and rod can't fully return with adequate free play, the piston never exposes the "return" orifice... if that's the case, the cylinder cannot refill, nor add fluid to the circuit...

.

 

[Toad]

Did you bench bleed the MC? [S

Toad

 

[sgtpontiac]

HUGE PICTURES.



I didn't want to mention the HUGE PICTURES but since you did yes they are HUGE!!

 

[smallfoot]

I'm thinking the front reservoir goes to the rear drum brakes and rear reservoir goes to the front discs...

 

[Old Iron]

If the reservoirs are two different sizes then the large one is for the disc brakes and the small one is for the drum brakes.
If the reservoirs are the same size then, it doesn't matter.
The reason for the large bowl is because the calipers hold more fluid as the pads wear. Drum wheel cylinders always return to their original position pushing the fluid back into the reservoir.

 

[smallfoot]

Can't see pix...the large reservoir is in the rear on mine and several others in my barn...don't know what MC dmw56 has here...

 

[dmw56]

Sorry about the huge pictures! Don't know how that happened. Both reservoirs are the same size do I figured it didn't matter.
I did a little more digging and found a combination valve bleeder that is screwed in the warning light fitting. I guess to keep the valve centered. Anyone know anything about that. I've never bled a new combination valve.

 

[dmw56]

Did you bench bleed the MC? [S

Toad

Yes I did. I was getting good flow too. I'm thinking something up with the valve.

 

[Toad]

Yes I did. I was getting good flow too. I'm thinking something up with the valve.

I'd call whoever you bought it from and ask what's up, maybe it's something simple. If it's defective they should replace it!

Toad

 

[dutch]

how old is the flex hose on the diff.

 

[soltz]

My 2 Cents

I had the same problem with the 71 GMC Sprint and found the rear shoes were not adjusting the way they should, why[S There were no adjusting holes on the backing plate, so I made some, adjusted with drum on and all work well. [cl
Good luck[P

 

[Blue Eyed Devil]

how old is the flex hose on the diff.

This can be a unknown source of severe frustration when dealing with rear brakes! Even a hose that looks OK on the outside can be deteriorated internally, and allow severe fluctuations in pressure. If there is any question whatsoever, replace it. Cheap insurance...

 

[dmw56]

Flex hose is new, good end play, drums are off to see any movement.
? ? ? ?

I'm going to start removing some lines and try to track this problem down.

 

[05snopro440]

The first thing I'd check is the combo valve. I've heard more than one being the problem.