Brake question and advise please

[engineer_gregh]

Sorry for the lengthly description but looking for advise. When I stored the truck (71 C-10 front disk and rear drums with power brakes) a year ago the brakes worked, but once stopped the pedal would creep down until it bottomed out. I assumed the master cylinder was bad. Now its time to get it back on the road; the left rear wheel was locking up when applying the brakes when I backed the truck out and took it for a run around the block. Replaced the master cylinder, both rear wheel cylinders, rear shoes, spring kits on the rear, two hard lines at rear axle, two hard lines from porportioning valve to master cylinder, and two hard lines from porportioning valve to distribution block and juction at left front lower control arm. Bleed the master cylinder and all four corners. Good clear fluid at all four corners, emergency brake works great, and truck stops evenly. Problem is pedal goes almost all the way to the floor and brake light is on. Sorry for the lengthly description but looking for advise.

[Dagr8npwrfloz]

Did you bench bleed the master cylinder? You might get fluid at the four corners but still have air trapped in the master.... Causes symptoms like you describe...

[engineer_gregh]

No, I bleed the mc once on the truck rather than doing it on the bench. After several rounds there was no air at the mc. Is is ok to bleed it at the lines on the mc after installation?

[Dagr8npwrfloz]

My pops was always adament about bench bleeding it before installation. Can't say I've ever done it in the truck... I'm sure someone else will chime in... Sounds like air SOMEWHERE in the system though... Only cause I can think of in a 71...

[Dagr8npwrfloz]

Did you adjust out the rear brakes after you put the drums on? ( turn the little gear on the self adjuster till the shoes just barely start to grab) I doubt it would cause " pedal to the floor" but maybe if they're really cranked down.... Driving in reverse riding the brake will adjust them out.....shot in the dark...lol

[engineer_gregh]

The back brakes seem to be adjusted properly. I did quite a bit of backing and hitting the pedal to make sure. The emergency brake works great as well.

[knomadd]

Do you have a vacuum booster? I had a booster fail on me that did this to the pedal. It started off fine, but would eventually let all of the vacuum out and dropped the pedal to the floor. If you don't have a booster, then it has to be air somewhere. I bet it's in the master cylinder. Manufacturers always recommend bleeding the MC on the bench. Something about the angle of the master and the travel of the brake pedal can't always get all the air out.

[engineer_gregh]

I think that I'll pull the mc on Friday and bench bleed it. Angle and pedal travel makes sense. Thanks Duane.

[GRX]

Always bench bleed and carefully observe the fluid. Only takes a small bubble to interfere with hydraulic pressure.

[midniteblues]

When bench bleeding I like too tap the master all around.
I think it helps to dislodge the air bubbles.

I also believe a bumpy ride after doing the bleeding process helps dislodge bubbles.

[engineer_gregh]

Just a quick update. I've bench bleed the master cylinder and all four corners numerous times. Same problem, pedal goes to the floor. I'm going to try another master cylinder as I've never had this big of a problem with brakes.

[knomadd]

Have you checked your vacuum booster? Like I said in my previous post, I had a similar problem, where everything seemed good, but if held on long enough the pedal hit the floor and the brakes would release. It turned out the booster was bad.

To check it, make sure the truck has run within the last couple days and go pull the vacuum hose off the booster. If it's easy to get off, there's no vacuum and probably a leak in the booster. If it's a PITA or you hear air suction when you move the valve, your booster may be good.

If that's not it, maybe you have air in the proportioning valve. Not sure how you bleed that, but it's still a possible culprit. Or, like you're thinking, maybe you have a bad master cylinder.

[midniteblues]

I recently had a similar symptom with the c30.turns out it was the master.after removing it it was slightly wet in the rear at the piston.i didn't think it was enough to make a difference. But the new master proved me wrong. Instance pressure and a great pedal with very little bleeding.

After a short bumpy ride I noticed the fluid level dropped ever so slightly. But the pedal was still great i figured it worked a bubble out.

Good thing is most brake parts are fairly cheap.

[engineer_gregh]

It was a bad master cylinder, new out of the box. Thats what I get for buying non GM parts at Advanced Auto Parts.

[jocko]

thanks for the follow-up, I was betting it was vac booster... how did you determine it was the mc? Just by swapping and seeing if that fixed it?

[engineer_gregh]

Yes sir, had the parts house swap it out. Frustrating, the parts houses here typically stock one of each part and they had to order the replacement from the warehouse. For the younger folks or folks like me that don't do this to often, 1. bench bleed the mc 2. bleed the mc on the truck 3. bleed the right rear 4. bleed the left rear 5. bleed the right front 6 bleed the left front.