My '67 Chevelle has brake problems. The car came with the brake problems when I bought it. Disc brakes front, drum brakes rear, with power brakes.

The brake pedal nearly goes to the floor. Pumping the brakes doesn't do much for it. The front calipers do grab the discs and stop them from turning. But the rear shoes do not expand at all when the brakes are applied. I checked with the drums off.

All brake lines are recently replaced. No leaks are evident at the rear wheel cylinders or the front calipers. There does seem to be a small drip between the master cylinder and booster. But the brake fluid does not seem to drop when the car is parked.

When bleeding the brakes, a powerful squirt of fluid comes out at the front wheels. Much less comes out at the back wheels - more like a dribble than a strong squirt.

Even after repeatedly bleeding the lines at all four wheels, there are no brakes in the back and the pedal remains very spongy.

I thought of replacing the master cylinder, but would rather not just start replacing parts without knowing what is causing the problem.

Any help will be appreciated!!

Sounds like the wrong master cylinder. If you are getting good flow at all the wheels and the rears are adjusted properly than it stands to reason you simply are running out of pedal travel before you build enough line pressure to activate the hydraulics. Lots of parts swapping has already gone on, no way of telling what parts were swapped or from where. There are several different configurations available at the master cylinder, different push rods for manual or power brakes, etc...

Good luck, I would start from scratch at the master cylinder/booster and see what happens.

Do you have a large iron distribution block below the master cylinder which the lines all pass through before routing to the wheels? A faulty distribution block can cause problems like this too.

another problem that happens is the pushrod will sometimes not release the plunger far enough for the hole at the bottom of the master cylinder to open up and receive more fluid
shortening the pushrod may be part of the fix
sometimes there is an adjustable pin between the power booster and the MC
if they have been used before and nothing has been changed there
then the problem would more than likely be the rod between the pedal and the power booster
also if this is a recent swap did you change the proportioning valve that came with the disc system?

Thanks for the ideas!

I plan to change the MC. According to the parts store, the one I bought today is for both disc and drum brake systems, and for '67 and '70 Chevelles. The parts store told me it was a different part number for '68 and '69.

I will keep the push rod suggestions in mind. My pedal only has one hole on it, so I can't move the pushrod to another hole. But maybe I can adjust the length??

The proportioning valve that is on the car is a brass looking rectangular unit (about three inches long by 3/4 inch wide and 3/4 inch high) fastened to the frame of the car. It has two lines going in from the MC, one line out to the rear, and two lines out to the front wheels. In the middle on top is a black rubber type projection. I have no idea if this is right for disc brakes or not. Any ideas??

Thanks for your help.

the pushrod length is what needs to be adjusted
not putting it in another hole
and this afternoon I will post pictures for you of the two different proportioning valves

Tom -- I look forward to seeing the pictures.

In the meantime, I've checked a few more things.

Both my old and new master cylinder (which is not yet installed) do not seem to have a built in residual check valve -- a small wire goes deep into both outlet holes.

Should a disc/drum proportioning valve have a residual check valve built in? Or do some have this and others not? How can I check if mine has valve?

I compared the external appearance of my proportioning valve to a few on other cars. A '67 Skylark parts car with drum/drum brakes has a proportioning valve that is wider than the one on my Chevelle. The Chevelle's is about 3/4 inch wide and more than an inch high, while the Skylark's is more than an inch wide and a similar height. The length appears roughly the same. They have the same number of in and out lines connected. Might this size difference mean that I already have the disc/drum proportioning valve?

I also looked at a '78 Impala parts car parked out back. It has a factory disc/drum set up and a proportioning valve with the same number and size of lines going in and out as the '67 Chevelle. But the proportioning valve on the '78 is big and clunky. I'm wondering if it would it work on the '67?

I have a feeling that we are narrowing in on the problem but I still need more advice. Thanks again!

http://img146.exs.cx/img146/7479/p31100421zo.jpg (http://www.imageshack.us)
this is the 4 drum prop valve for a 67

http://img146.exs.cx/img146/5186/p31100452ha.jpg (http://www.imageshack.us)
this is the prop valve that goes with the system that does not have the valve that is next to the mc

http://img146.exs.cx/img146/2374/p31100444fb.jpg (http://www.imageshack.us)
and this is a picture of that prop valve that is next to the mc which uses a 4 drum prop valve mounted on the frame

Tom,
Nice pictures.

Mine looks like the one in your first picture, drum/drum. But I don't have the unit by the MC as shown in your third picture.

Your second picture - is that of a '67 prop valve with disc front/drum rear?

And can you or someone else explain what the operational difference is between the disc/drum and drum/drum prop valve? Is it that one diverts more pressure to the back than the other one does, etc??

are your front disks the four piston style that would be correct for that year
or are they the single piston floating caliper style that looks like all the newer stlyle cars
69 on that is
the prop valve for the discs on the 69's and on can be like the one that is like the 2nd pic or the 3rd pic with a regular prop valve
alot of times you will find the newer disc setup on the older cars since they are easier to find
and fit basically the same
unless they came stock with the car
you can swap the newer style to the older cars and lots of times thats what people will do
if you do have the four piston setup I am pretty sure they came with the prop valve in the 3rd pick
as for the exact function I am not completely sure and don't want to give you wrong info so hopefully someone else will chime in there

The discs are the single piston type, while the prop valve is like the one in the first picture.

So it looks as if I have to change the prop valve. Neither the parts stores here nor the dealer can get them. That leaves looking on parts cars.

Does anyone have any information whether the one from a '78 Chevrolet full size wagon (disc/drum) would work well? Or would it divert too much or too little pressure to the back? I have one of those out back. Otherwise, I'll have to look in a junk yard.

as for not being sure the exact way these things work I am not positive
but
a friend of mine had one that leaked on his monte carlo
he replaced it with one from a pickup truck
it stops good for him and he has had it on there for about 4yrs now and his brakes wear fine and the car stops great under hard braking too

I just went through the very same problem. I took of my master cylinder and saw some fluid near the booster. Not much but just a little. I put pressure on the cylinder and watched the fluid shoot out and could see that the front had good pressure and the rear about 2/3 less, at least that's the way it appeared with fluid shooting out, so I just went ahead and replaced the master.
Solved my problem. I now have pedal and also my brake light that had been disconnected by the previous owner, seems to work. I had the 2nd type of comb. valve pictured and was worried that it might have failed but everything now seems to be in order. I think you need a new master.

This is an update on the brake problems and efforts to resolve them. I replaced the master cylinder with a rebuilt unit, a proper unit for disc/drum brakes.

Then I installed a used proportioning valve from a 1978 Chevrolet with disc/drum brakes.

To help a greater volume of brake fluid reach the rear wheels, I increased the line size from the proportioning valve to the rear flex line at the axle from 3/16 to 1/4 inch. (Brake shoes also replaced, drums in good condition.)

Now the rear brakes work!!! And the pedal has good response. It is no longer anywhere near the floor. Once the snow goes, a road test will tell if the front/rear balance works well. But for now things look good. Thanks for your suggestions and help!!

Tom's 68 conv,
I'm having similar issuses with my conversion.
The Prop valve in your 3rd pic, is that needed?
It came with my setup off a 69 396 car. Unfortunately I did not get the valve off the frame.

I also would lean toward bad or incorrect master. edit: (sorry didnt see page 2. good job!!)

As far as exact functions...
The first pic shows a warning switch/distribution block. That's all it does. It was the same for disc and drum cars, though there were 2 or 3 different part#s and shapes. 67-72 drum and 67-70 disc used this.

The 2nd pic shows a 71+ COMBINATION valve. Depending on design and vehicle, this valve can contain a front delay (metering) valve, a rear proportioning valve, and sometimes a residual valve, as well as the warning switch. If it has more than 1 function, it's a combination valve.

The third pic shows a master cylinder and metering valve used on pre 71 disc/drum cars.

The metering valve delays pressure to the front brakes and is used on almost all disc/drum systems, either separately or integral. (it METERS flow to the front brakes). It's needed to allow the rear drums time to overcome their return springs.

A PROPORTIONING valve reduces pressure to the rear brakes in PROPORTION to the front during hard stops. On pre 71 cars, this was a seperate unit, and not all cars had them. (there was a post recently with a picture of the 1970 prop valve inline with the rear brakes)

After 71, the COMBINATION valve still didn't necessarily have any proportioning on all models. Some cars still got full pressure to the rear. Most disc/drum cars still all got a metering valve inside the COMBI. valve.

I'm sure that's clear as mud.