Brake issues on 72 burb

[Reeds70]

Hey guys, any suggestions will be appreciated. I am about at my wits end chasing my brake issue. I was driving and headed down an old mountain road on the Blue Ridge Parkway and I felt my pedal get spongy, so I pressed it to the floor and it didn;t stop. I was able to gear down and get it stopped. When I got out the front rotors were so hot they were smoking, anyway, I got it home by a tow. I put it on a lift and couldn't find any leaks or broken lines. I double checked that the rear brakes were adjusted, bled the brakes several times. The pedal still seems weak to me, it stops but I don't think its as good as it was. The only other thing I can think of is the plunger for the proportioning valve is worn out. I can move it with my finger and I thought it should have pressure on it so it would be moved so easily? And I think it would be rare that it would fail? Im having trouble finding a replacement that is AC Delco, so if I have to replace it any suggestions on quality aftermarket. Thanks...Also right as I got it stopped the brake light came on in the dash and hasn't went out since.

[highwinger]

I had to replace my master cylinder when my pedal would slowly go to the floor. I had to pump the brakes even at a stop light to stay stopped. A new master cylinder fixed though.

[bowtieguy79]

It does sound like a master cylinder. I upgraded mine to a 72 C30 booster/Master Cylinder combo from Oriellys. Stops really well now!

[Advanced Design]

Sounds like front brakes became over heated and started experiencing brake fade. Same as doing a number of high speed stops and the brakes become less effective. The pads are likely glazed and don't work as well now. If mine, I would replace the pads, look the rotor surface over for pitting / grooves, measure for parallel, bleed to balance the warning switch and evaluate. And may be a bad master too.

Also check the front hoses for collapse. If you weren't using the brakes much and they got hot, a hose internal collapse can keep the caliper applied overheating the brakes.

[LockDoc]

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If the light is staying on that normally means that one of the brake systems is not working, either the front or the rear. Was either of the fluid reservoirs low on fluid when you first took the master cylinder cover off?

LockDoc

[Bigdav160]

I've seen one, in my lifetime working on cars, proportioning valve fail like the OP said.

Very similar symptoms to a failed master cylinder.

[Reeds70]

Thanks guys, I replaced the master cylinder and as I was bleeding the brakes, the front passengers side had a small flow and the pedal didn't go to the floor. So I went ahead and cracked the drivers side (after closing passenger) and on the second try and bunch of junk came out and then really good flow, went back to passengers side and got good flow. So is it possible that the trash in the lines caused the problems? Thanks again

[Bigdav160]

It might be time for new brake hoses. They don't last forever.

[TexasT]

Originally Posted by Reeds70

Thanks guys, I replaced the master cylinder and as I was bleeding the brakes, the front passengers side had a small flow and the pedal didn't go to the floor. So I went ahead and cracked the drivers side (after closing passenger) and on the second try and bunch of junk came out and then really good flow, went back to passengers side and got good flow. So is it possible that the trash in the lines caused the problems? Thanks again

I would bleed until I got good clear fluid coming out(be sure to keep topping off the master cylinder). With trash comes rust and issues rust can cause. Brake fluid attracts moisture. This just adds to the rust problem. Even though you replaced the mc the lines still have old fluid and I would bet more of that crud.

[Reeds70]

Everything is getting ready to be new
..thanks guys