Help needed, brakes gradually locking up.

[Coronet1965SWE]

Hello everyone, i have been reading quite a lot on this forum but havent had the time to write much in order to get my car driveable to the Power big meet here in Sweden.
I just made it but it took some looong days/nights the last 2 weeks, luckily i had my brothers and a few friends to help out :grin:

Now to the problem that have followed this car more or less all the time i have had it since 1991. first the specs:
Dodge coronet 440 1965
originally 318" BB but now a 400" stroked to 512"
It has the original 11" drumbrakes with power assistance.

The problem is that while im driving the brakepedal engages higher and higher until the point the brakes actually locks up and i have to open a bleedervalve to let out the excessive pressure.
I know about the residual valve in the mastercylinder and that it should hold about 10psi in the system to get less travel in the brakepedal but this build far to much pressure.

What i have done:
Several years ago i rebuilt the old mastercylinder with a new residual valve and after that it was good for 1 or 2 years then it came back but not very often and i could live with opening a bleedervalve every month or so.
But this spring i did a lot of work on the car and exchanged the mastercylinder for a new one (Dorman 1") and the problem got a lot worse, i could only drive for about 45 minutes and the brakes started locking up.
I did a proper benchbleeding of the mastercylinder and bleed the system and after the Power big meet i dissasembled the mastercylinder and checked the residual valve wich looked good (after all it was new) so i then tried a perhaps not so good method but i took a needle and poked a lot of holes on the rubber membrane and assambled/benchbled the MC and the whole system.
After that i could drive quite a bit longer but the brakes still locked up eventually :angryfire:

So, im kinda lost what to do now? Are the returnsprings to weak maybe? Could it be heat from the exhaust manifolds causing it? Maybe some air still in the system?

Any and all help would be greatly appreciated.

Here is a photo also :grin:

 

[Hemi Rebel]

When the brakes start locking up, disconnect the vacuum line from the booster. If the brakes release, you have a bad booster.

 

[MoparMikey]

Possible bad booster but more likely your front booster pin is adjusted a bit too long so it is slightly pushing the back of the master plunger in and casuing brakes to drag/lock. Usually will worsen as it gets hotter. With brakes draging, losen master bolts and pull master forward slightly, if brakes release it is the pin length. If they don't release probably the residual valve.

 

[Coronet1965SWE]

Thanks alot I will try these tips the next time they lock up.

 

[Kernel Sanders]

Is it all the brakes at all the wheels. one of the fronts or both backs?

I ask because I just ran into this problem a month back with my Caravan. Brakes worked great until they started heating up then the drivers front would start dragging...

I figured it was a caliper so I replaced the caliper but the issue continued until on my way home it dragged so bad the rim heated to the point of starting to smoke the tire!

Now i knew what it was even though I had heard about it happening I had never experienced it.... the flex line had colapsed on the inside allowing brake fluid preasure into the caliper but not back out! Replaced the flex and been problem free since.

 

[Coronet1965SWE]

Thanks alot MoparMikey, i was just out for a ride and the brakes started locking up (all 4 Kernel sanders) so i did as you said and loosened the mastercylinder and the brakes released :grin:
So i then did a quick adjustment and after that they seem to work fine except i now notice how far down the pedal goes, i need to adjust the brakeshoes quite a bit it seems.

 

[69Bee]

Nice call mike...

 

[Sixer]

I had the reverse problem-the brake pedal didn't do anything till it was damn near down to the floor, and the shoes (4 wheel drum) were adjusted up as far as I dared. While installing a front disc conversion, I discovered the master rod coming from the booster was turned all the way in, half the pedal movement was required just to engage the master. Measuring the master depth and adjusting the rod made for a nice pedal.

 

[rumblefish360]

Wow! I never heard of this and by a strange twist of date, my wife calls me today with the a complaint about a foul smell and needing to press the gas pedal down real far just to get going. No loss of power, good trans fluid level and color.

I was starting to suspect a brake problem at the rear.

 

[ArtH]

Is it all the brakes at all the wheels. one of the fronts or both backs?


Now i knew what it was even though I had heard about it happening I had never experienced it.... the flex line had colapsed on the inside allowing brake fluid preasure into the caliper but not back out! Replaced the flex and been problem free since.

I had the same problem with my GMC pick up truck years ago. The right front brake line created a one way valve that didn't allow fluid to back out when the brake pedal was released. It's the only time that it has ever happened to me, but it turns out that it's not all that uncommon on older vehicles. I replaced that flexible line, bled the brakes and the problem was solved.

 

[HT413]

My dad had the same prob in his 68 vette prob 10 years ago on a manual brake car. Honestly dont remember of it was all 4 or just the rear, but it would get to where the 400hp 327 couldnt turn the wheels. Turned out it was the master cyl.

 

[Meep-Meep]

Most likely your master cyl push rod is too long. What actually happens is the primary cup is covering the compensating port so as the fluid gets hot and expands it can't return to the reservoir. The compensating port is that little squirt you get when you hit the brakes with the reservoir cover off.

You can add some thin washers between the MC flange and firewall or booster to give a little clearance between the push rod and piston. You can do this just to prove the point then take it apart and readjust the rod. Be careful with adding washers as you don't want to run out of MC piston travel and not have enough breaks to stop the car.