1972 K20 Brakes - Need Help

[1972RedNeck]

Over the past couple of weeks, my brakes have been acting up a bit. Every now and then, I would get a hard pedal and the brake light in the dash would come on. If I came to a stop and firmly reapplied the brakes, the light would go out. About this time, the rear brakes started to be over zealous as well trying to lock up with light pedal application. Furthermore, whenever I checked the brake fluid, the front brake reservoir (towards the rear of vehicle) would be completely full and running over and the rear brake reservoir (towards the front of vehicle) would be low. Fill the rear brake reservoir and empty the front and in a week or so, the front brake reservoir was over full and the rear was low.

Yesterday, I stopped at Napa and grabbed a new master cylinder, bench bled, installed, and tried to bleed the brakes. I could not get the rear to bleed out - I keep getting air bubbles. I moved to front and they bled out perfect and then went to the back again. Ran another pint of brake fluid through it trying to bleed the rear and am still getting air. Got it to where I was just getting a bubble or two every other pedal stroke and said the heck with it and took it for a spin.

My brakes are acting EXACTLY the same as before. The rear tires lock up with little pedal effort except for every now and then when I get a hard pedal and the "brake" light comes on. Haven't driven it enough to tell if the reservoirs are still swapping fluid or not.

What gives? Why can't I get the air bled out? Why was my M/C swapping brake fluid between reservoirs in the first place? Why did a new M/C not change anything? Could the proportioning valve be bad? It doesn't leak...

Anyways, I'm out of ideas and could use some help.

[Stocker]

Originally Posted by 1972RedNeck

I could not get the rear to bleed out - I keep getting air bubbles. I moved to front and they bled out perfect and then went to the back again. Ran another pint of brake fluid through it trying to bleed the rear and am still getting air. Got it to where I was just getting a bubble or two every other pedal stroke and said the heck with it and took it for a spin.

Why can't I get the air bled out?

Dunno about your other symptoms but I had the exact problem bleeding the rears on my K20 several months ago. Long story short, the wheel cylinders were bad.... only had a tiny bit of leakage which I didn't notice, but new wheel cylinders fixed it.

[B. W.]

The dash light is coming on because there is a pressure differential between the front &rear brakes. (unsafe condition). Usually caused by a leak.

The reservoir for the rear brakes is low because there is a leak in the rear brake system - probably the wheel cylinders.

The wheel cylinders leaking =grabby brakes (brake fluid on shoes) & low/empty reservoir.
The reservoir being empty = no pressure = light on.

[1972RedNeck]

Originally Posted by B. W.

The dash light is coming on because there is a pressure differential between the front &rear brakes. (unsafe condition). Usually caused by a leak.

The reservoir for the rear brakes is low because there is a leak in the rear brake system - probably the wheel cylinders.

The wheel cylinders leaking =grabby brakes (brake fluid on shoes) & low/empty reservoir.
The reservoir being empty = no pressure = light on.

Agreed with all of the above except:

I never let the reservoir get empty and

As the rear brake reservoir gets lower, the front brake reservoir gets fuller...

Which could have been a bad master cylinder along with possible failing wheel cylinders. There is no visible leak, but I will pull the drums and take a look-see.

[1972RedNeck]

Pulled the drums, the rear brakes look to be almost new. Pulled the rubber caps back on the wheel cylinders and they are dry as a bone.

Pulled the sender out of the proportioning valve and there is no fluid leakage there either.

Still can't get the master cylinder to bleed out.

Thoughts?

[CharlieBrown]

try jacking the truck up to level the master cylinder....sometimes this helps..the master cylinder doesn't sit level on my k20 so I jacked it up to level..then bled the brakes.

[B. W.]

Originally Posted by 1972RedNeck

Pulled the drums, the rear brakes look to be almost new. Pulled the rubber caps back on the wheel cylinders and they are dry as a bone.

Pulled the sender out of the proportioning valve and there is no fluid leakage there either.

Still can't get the master cylinder to bleed out.

Thoughts?

I don't know, one thing you could try is to remove one brake line from the master (the reservoir that is getting filled up), install a short pipe back into the master (just like you're bench bleeding).
Have a helper apply the brakes & look for fluid coming out of the disconnected brake line (this will turn on your brake light) - no fluid should come out, if it does then the combo valve is bad.
I know you pulled the switch & it was dry but, there are only two parts in the system that will allow one reservoir to go low & fill up the other, the master (you replaced) & the combo valve.
One more thought, the new master could be bad. What are the odds it would fail in the same way your old one did? Keep us posted.

[RedRoad]

Collapsed rear soft line?

[HO455]

Originally Posted by 1972RedNeck

Pulled the drums, the rear brakes look to be almost new. Pulled the rubber caps back on the wheel cylinders and they are dry as a bone.

Pulled the sender out of the proportioning valve and there is no fluid leakage there either.

Still can't get the master cylinder to bleed out.

Thoughts?

Went through similar issues on my Javelin I ended up putting three master cylinders on it before I got a good one. I tore the two bad ones apart an found one of them had a deformed cup and the other one the anodizing on the plunger was flaking off and there were dozens of pieces preventing the cups from contacting the walls of the bore. Both were pulling air in from the back. I would pull the master cylinder apart and if you can't find an issue replace the rear hose and try again. Old hoses used to be good for twenty years. New hoses, good luck getting five years out of one.

[1972RedNeck]

After parking it in the weeds for awhile, I drug it back in the shop and put on another new master cylinder. Brakes work perfect now. I guess I should have just rebuilt the old one in the first place...

[dannyboy]

Take it for granted you held the combination valve switch on its front in, they show a tool J-23709 to hold it in for bleeding, but a clamp does the same thing. Without doing this the brakes don't seem to want to bleed correct.
I just learned this the hard way after 2 tries I got my manual out and bingo.
manual page 5-23 figure 4 shows tool figure 41 shows valve for power assist brakes

[1972RedNeck]

Originally Posted by dannyboy

Take it for granted you held the combination valve switch on its front in, they show a tool J-23709 to hold it in for bleeding, but a clamp does the same thing. Without doing this the brakes don't seem to want to bleed correct.
I just learned this the hard way after 2 tries I got my manual out and bingo.
manual page 5-23 figure 4 shows tool figure 41 shows valve for power assist brakes

Yup i did. It was just a bad new part.