'76 brakes won't bleed: [FIXED]

[nuclearbeef]

Here's the situation:
Front disks, rear drums.
Went through the rear drums when I bought the pig, improving the braking dramatically, but...
Since then, sticking your foot to the floor gets you about 60% brakes.
Let it up and the pedal is firm about 1/2 way down and stays there. Great brakes.
I just lived with it. pumped the brakes if I needed to stop quick.

So I just replaced both front calipers because I found a stuck pot on the right side.
Vacuum bled the fronts until nice clean fluid exited the bleeder. Still soft pedal on first pump.
Bled multiple times by pumping up the brakes. No improvement.

It acts like there is air in the lines. 2 separate fluid reservoirs, so even if the rear system was disconnected, I should get firm pedal and good front brakes.

Could it be bad rubber section of the lines???
They are old, but no leaks or bulges or outward signs of failure.

I'm stumped.

JW

 

[bobm]

They could be expanding when you hit the brakes. Rubber lines. Or Vaccum leak.

 

[J Mack]

Bled multiple times by pumping up the brakes.

Can you go into more detail on the process you used to bleed the brakes?

 

[nuclearbeef]

Vacuum leak would lead to a hard pedal, no?

I did not bleed them bottom up. Didn't have the vacuum pump fitting for the reservoir.
put vacuum line on the bleed valve, filled reservoir, then opened bleed valve. Kept the reservoir topped off and pulled brake fluid through until it was nice and clear. Old stuff was brown.
Bled passenger side first, then driver side.
No change, so had my 11 year old pump brakes up, then hold them as I opened the bleed. Engine off. No vacuum. PS then DS.
No change, so had my wife repeat process with engine running.
No change.

Gotta be rubber lines, right? Only thing I can think of. Maybe collapsed inside?

Who makes up stainless braided brake lines? NAPA?

 

[wngrog]

Man. It can be lots of things.

If you don’t have your rear brakes bragging a bit it can be effecting your pedal.

Firm the drums up.

Get a clear line. No vacuum and a lot of fluid. Start farthest away and work to closest.

That usually is DR then PR then PF the DF.

 

[J Mack]

Gotta be rubber lines, right?

I doubt it’s the lines.

Have you adjusted your rear brakes to have a slight drag? If the answer is no then start there.

If the rear brakes are adjusted correctly you need to check for a faulty check valve, drum brakes will bleed fluid back to the master due to the springs holding the shoes compressing the slave cylinders.



1976 should have a brake proportioning valve Part Number: 47150-14010, I can’t find an exploded view to tell you if it’s in there or built into your master but if you follow your rear line it should be a brass block of some sort.

 

[nuclearbeef]

FIXED!

Right rear was way out of adjustment. Adjusted, and now I have perfect brakes.

I discounted the rear brakes as a cause of my problems because I (incorrectly, it seems) thought that I would have good front brakes regardless of the condition of the rear brakes.
I thought that the whole reason for separate fluid supplies, master cylinders, lines, etc... was that: if you ripped a rear brake line off on a stump, then you would still have brakes because of the independent systems.

I thought that the brake proportioning valve was only installed on trucks with single master cylinders. Disc brakes requiring a larger volume of fluid to operate than drums makes the valve necessary. I thought it was not required on dual master cylinder set-ups because they just put in different sized master cylinders for the different volume requirement. Hence the different sized reservoirs for front discs and rear drums.

Thanks everybody! Brakes fixed. More knowledge acquired. Incorrect assumptions dispelled.

I'm sure everyone knows what happens when you make an assumption.
It makes an a$$ out of you and umption.

 

[wngrog]

Man. I learned so much about brakes when I did my 4 wheel drums. That took me weeks.

Weeks.

F brakes.

When are you headed out?

 

[nuclearbeef]

Pretty much whenever I want.
Was thinking late morning for an early afternoon arrival.
Like to get there before dark, anyway.
Your ETA?

 

[wngrog]

Pretty much whenever I want.
Was thinking late morning for an early afternoon arrival.
Like to get there before dark, anyway.
Your ETA?

After lunch. See you then.