Egg on face

[lutronjim]

Not long ago I posted about front disk hanging in my 70 C10. I installed a rebuilt booster and problem went away.

Until 3 weeks ago when it reappeared. I jacked up the from wheels so I could check without driving. Stood on the brake as hard as I could and sure enough I couldn't turn front wheels. Since both fronts locked I assumed it was something before calipers. I was planning to loosen line fittings to see if I could free brakes.

At a whim, I popped the proportioning value and the brakes freed. Stood on brakes and locked them again. Popped value and they didn't unlock. Popped it harder and they unlocked. Ran out of time and left it for the night.

Over breakfast I was wondering if the shock could have propagated to the master cylinder via the connecting lines. Decided to lock em up again and pop master cylinder.

Well try as I might, I couldn't make fronts lock this morning.

The problem went away with the booster for better part of a year - booster probably had nothing to do with temp fix. Might have jarred something during install.

Anyone hear of a valve doing this? Thanks

[Old_Gold]

I had not had any experience with a valve doing that, but have found a lot of sticky calipers due to rusty slides and hardware. Have you had the calipers off, checked and/or cleaned up all of the bolts, mating surfaces, etc? It doesn't rule out the valve, just something pretty common up here in the rust belt!

[HO455]

When you say you "popped the valve" what exactly are you doing?

[Getter-Done]

How old are the rubber brake hoses?
The deteriorated rubber inside the hose
has been known to cause trouble.






.

[franken]

Might want to look at the manual or on the internet for bleeding info. The block under the MC has a valve in it that blocks fluid if there's a sudden loss of pressure, like when a line breaks. Check to see if the tan (if I recall) wire is connected to the block. If the valve is in the wrong position, the brake light should be on.
As far as the wheel not turning when the brakes are applied, that's the idea of brakes.

[RichardJ]

I thought I recalled your previous thread, searched and found it.

You stated that you had checked the boosters adjustable rod length before installing that booster. Your problem occurred after driving for some period of time.
You replaced the booster and the problem was solved. You didn't say if you had checked the booster rod length when you pulled that booster off.
I'll assume you checked the booster rod length when you put that new booster on.

On my post #12 of that thread, I explained, in a very long story, that I had the same problem on my Vette. I explained that I pulled the MC away from the booster and checked the rod length. That booster adjustment had somehow changed after a couple thousand miles of use. I readjusted the booster, put it back together and hasn't given a problem since.

The next time it locks up, loosen the MC bolts and see if that releases the brakes. Or simply recheck the booster pin length.

My booster was from NAPA also.

[lutronjim]

Popped - hit it with a big ball peen hammer.
Conversion put all new calipers, rotors, spindles, rubber lines, valve, and MC on truck about 3 years ago.
Booster push rod to mc length is OK.

Took truck out to lunch today and when near the house got on brakes hard and they started the not release all the way trick. Back in garage jacked up front, got on brakes hard, let off petal, and I couldn't turn wheels. This time I hit the mc with a small hammer and they released.

I will try to find time tomorrow to get another mc.

I am hoping that after I bench bleed mc that I can let fluid leak when connecting lines to valve so that I wont have to bleed entire system.

[saxart]

I still stand by my advice in your previous thread. The calipers (for what little they cost) need to be replaced. It's the only way you know what you're working with.

[RichardJ]

>>I am hoping that after I bench bleed mc that I can<<

Bench bleeding a MC at this point is a sure fire way of trying to get air INTO the system. If you somehow had air still trapped in the MC you would have a soft pedal.

Bench bleeding is not a cure all for anything. In the drawing below, air is trapped in the B-B and A-A pockets which happens with a MC mounted at an angle. Prior to the '70s , the MC was always mounted horizontally and bench bleeding was never done or needed. When mounted horizontally, the air simply escapes naturally as you fill the reservoirs. Mounted at an angle and not bench bleeding, means you have to push all that air, all the way through the system when bleeding.

Retrace the steps you just outlined above to lock the brakes. This time just loosen the MC bolts a couple of turns and pull it away from the booster. NO HITTING. NO HAMMERS

[Steeveedee]

I had this issue with the fronts not releasing all the way, with my disc brake upgrade kit. Even though I was very careful to adjust the free pedal (I've done that more times than I can count, with brakes and clutches), once the brakes warmed up, they dragged. Until I get back under the dash to adjust the free pedal again, I'm settling for a couple of 1/16" washers between the master and the booster. It's working fine, that way.

Knocking it with a hammer makes me think that there is something binding that releases with the impact. I don't expect the proportioning valve to act like that- it relies on pressure differences internally.

[bloo]

That does sound like the pushrod. Next time it is stuck, try backing off the nuts holding the master to the booster, maybe a turn or 2, and see if it releases.

[special-K]

I didn't see the other thread. Are the brake hoses new or not? Please answer this question before moving on. Someone else mentioned the hoses with no response from you. This is exactly what happens with collapsed brake hose or hoses.

It seems you are asking advise but stuck on the proportioning valve and now replacing the master cyl. The other suggestion given was the possibility of the push rod adjustment changing. Both would explain why brakes were fine but now not. Those need to considered before throwing parts at it. Maybe brake lines are brand new, but nothing here says so,

[52napco]

The best advice so far is lessening the master up to see if it relieves pressure.if it does you just ruled out all parts south of it..i say loosen the rod a little not gonna hurt anything.
And no calipers aren't bad they don't go bad toghter all of a sudden.and yes I was a tech for 28 years sooo don't start throwing parts at it...figure the problem out

[lutronjim]

Quick update.

Just had a little time this weekend. Got front wheels jacked up and pushed on brakes as hard as could - motor not running little or no vacuum. Checked front wheels and they were not locked.

Got on brakes again real fast and hard - wheels locked. The 'fast' seems to have to be with the hard - I will try this out later. Got 9/16 ready to loosen mc - checked wheels - not locked.

Locked it up a couple of times - just letting it set for a minute or so causes the lock to let loose. Thinking back to my hitting - the letup could have been happenstance.

I am tied up Monday but Tuesday I plan to put 1/16 washers to offset mc and see if I can cause lockup. I figure this would be equivalent to loosing mc to see if lockup lets loose.

[lutronjim]

Finally got time to get back to truck.

I added 2 1/16" washers between booster and MC. Wheels still hang on hard braking.

To be sure I doubled them - same thing.

The damn thing will release by itself before I have time to loosen a brake line.

I drove truck to get some sod and had a idiot pull right in front of me. After hitting the brakes I felt the drag of the hangup. Jiggling the brake petal made it release.

Any ideas of what to try?

Thanks

[bloo]

Well, it cant be the booster adjustment then. Since both sides stick, and it's the front, it must be either the prop valve or the master cylinder. What a strange failure.

[RichardJ]

>>Jiggling the brake petal made it release.<<

Need to take a closer look at that.

[Minkota]

Did you install a Master Cylinder for an all Drum Brake application? A Master for an all Drum Brake application will hold residual pressure where a Disc Brake Master Cylinder will not.

[lutronjim]

Master cylinder came with disk kit. Ran OK for 'bout a year so I am sure its for disk/drum.