Front Brakes Locking On After Car Idles.

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alldodge426

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Just checking to see if anyone else has experienced this. Condition is after you start car and let sit and idle for 10 minutes the front brakes will lock on. Let car cool down and brakes will release. Car is a 71 Scamp with recently installed 360, with stock manifolds, power front drum brakes. Before the 360 it had a slant 6 with headers. Never had this problem with that setup. Any suggestions?
 
My guess is that a brake line is too close to a heat source and the fluid expands to lock the brakes.
 
That is what I thought and I built a heat shield under master cylinder to reflect manifold heat and the brake lines are OE in the factory location? Just wondering if anyone else has experienced this and how they corrected it.
 
Never seen or heard of that problem. But, I'd have to eliminate the brake booster from the equation first by unhooking the vacuum line to it. You'll still have brakes they'll just be hard as hell. Remember to cap the vacuum Line to engine.
 
We did that and condition remained. You don't even have to drive it, just start it and let it idle. I have owned a lot of Mopar and have never experienced this before. The car belongs to my son and I convinced him to use manifolds over headers to avoid heat soak and leaking issues and that seems to be the problem with the stock manifolds.
 
Then I'd have to agree the expansion of the fluid under heat is the cause. Is all the air out of the lines?
 
I have been thinking that I may have to move prop valve into wheel well and run lines from there to get away from manifold and pipes.
I have a factory 340 Duster here and comparing both, nothing is different.
 
Check your brake lines. If they are old, they could be deteriorating internally. That will cause blockage and can keep fluid in the wheel cylinders under pressure. This happens a lot on cars with disc brakes and old brake lines. Also check your booster. If your leaking internally or it's connected to something other than vacuum, or it's on it's way out, it could be applying pressure. Disconnect the vacuum line to the booster and see it it still does it.
 
Brakes have been bled and master is a rebuilt unit. Nothing in brake system has been changed since car was driven with /6 setup. My son has driven this car to the Dreamcruise (5 hours from our house) 4 times last 6 years. It has been driven to the Tri-State Show in Penn 3 times as well as all over here in Ontario with no issues. This is the puzzling part. I appreciate everyone's input as many minds will likely solve this issue.
 
We have changed front hoses, bled system(probably enough to have flushed old fluid out) unhooked booster vacuum to remove it from equation, I fabricated an 18 qauge heat shied fashioned from photos of factory shields to reflect heat from master, I adjusted drums in case they were adjusted to high, it just seems fluid is expanding and locking them on when engine gets warmed up.
 
Got tired of messing with it last summer and put it away. Spring is around the corner and it will become a priority soon so any suggestions/experiences are appreciated.
 
Put disc brakes on the front. Especially since your son will be driving. Don't care what anyone says about drums being perfectly fine. If that was the truth, cars would still have front drum brakes. You can get the job done under $500.00. That's cheap insurance and peace of mind IMO.
 
When it locks up have you opened the bleeders to see if the pressure lets off? If so is it one wheel or both? Did you compare the rebuilt master cylinder to the old one and did you check the depth of the hole in the piston where the pushrod from the booster goes to see if it was the same? There is a spec and adjustment on the pushrod from the booster to the master cylinder. If the compensating ports in the master cylinder are not being allowed to return far enough the fluid from the lines will not return to the reservoir. What it sounds like to me is you have one of a few things going on. Yes the fluid expands under heat, but you either have a bad master cylinder (whether the bore in in the piston isn't deep enough, or some internal issue) or the adjustment of the pushrod from the booster is not correct and preventing the master cylinder piston from fully returning to expose the compensating ports. I don't have my manual handy but I but will have it here in a few and I will look up that information on the rod.
 
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Ok I don't want to spread bad information, I have seen an adjustment on the push rod from the booster but I just went thru my FSM and I don't see a spec. I have done this on a B body and the spec was in an old Motors manual. So you could maybe call Cass at Dr Diff and see what he says. Also follow advice others gave about loosening the m/c mounts when this happens and see if they release. As I said I don't want to give bad info. Sorry.
 
I forgot about my old 72 town &country I had to loosen the master.
Ok I don't want to spread bad information, I have seen an adjustment on the push rod from the booster but I just went thru my FSM and I don't see a spec. I have done this on a B body and the spec was in an old Motors manual. So you could maybe call Cass at Dr Diff and see what he says. Also follow advice others gave about loosening the m/c mounts when this happens and see if they release. As I said I don't want to give bad info. Sorry.
 
The fluid isn't able to return to the master cylinder when there is no pressure applied causing the fluid to get trapped and when it expands from the engine heat it locks your brakes.
Usually too long of a brake actuator rod or over adjusted pedal travel.
The last posts hit on this, and you can prove this is the problem by loosening the master cylinder from the booster while it's locked and it should release.
If it does, then something is holding the master cylinder part way on blocking the fluid return ports inside the master.(usually too long of a rod between the booster and master cylinder plunger)
 
When this happens both front wheels lock and loosening bleeders releases brakes. I understand about pushrods adjustment, however, why did this problem occur when we did the V8 conversion. We didn't do anything to the braking system until we experienced this problem. That's when we started changing hoses and adjusting the brakes. Master was changed long before problem showed up. We did the loosening of m/c under suggestion of mopar mechanic and even put washers in to move m/c out, but to no avail. It just seems for some reason the brake system is getting heat soaked. Have considered the line insulation you can buy and putting it over the lines to insulate them. Don't worry about misinformation, more theories/suggestions the better.
 
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