Front brakes keep locking up on my 73 Charger

[Charger1973]

If I bleed the system they seem to work fine for a little bit, but then they lock up in the front.  I bled them out, drove the car around for about 5 minutes (with no noticeable problems), came home and parked it.  Got in it to leave again later and the brakes felt like they were grabbing, so I went a few blocks to see what would happen.  Within about 6 blocks the front wheels were completely locked up and the back wheels just spun like I was doing a brakestand.  I cracked a front bleeder valve and they instantly released and I drove home with no problems.  Ive had the same thing happen 2 times in a row now.  The front brakes are all new (calipers, rotors, pads, lines, even the wheel bearings).  The back brakes seem fine and all I have replaced back there are the wheel cylinders.  Any ideas what might be wrong here? 

P.S.  My brake light on the dash does work and its not lit up.  It was on before I bled the air out of the lines but now its not. 

[chgr500]

I've seen similar concerns on other cars that were caused by the rubber flex line...you say you replaced the lines...including the flex hoses?....you didn't clamp any pliers on the flex hoses at all..this will damage the hoses.....what happens the hoses become damaged on the insides and create a small flapper valve...when you press the brakes the fluid flows good in one direction,  but when released,  the fluid can't return to the master causing the brakes not able to release....that or proportioning valve or master cylinder problem....I'd be leaning to a flex hose concern because you say they're OK after bleedingd the brakes...often the flapper valve can be internmittent...sometimes releases pressure and sometime not....just a thought...good luck!

[Charger1973]

The rubber lines are the ones I replaced...  I started with replacing the calipers because I thought they were just old and sticking...  The problem seems to have gotten worse since replacing the old parts though.  Im thinking it has to be in the master cylinder or proportioning valve like you said but I was hoping to narrow it down...  A friend told me today he thought maybe my master cylinder push rod could be adjusted wrong...  Is that a possibility?  Ive never actually driven this car on a long drive, mostly just short test drives and its been this way since I got it, so its hard to say what was done by the previous owner. 

[Charger1973]

you didn't clamp any pliers on the flex hoses at all

Definately not 

[Charger-Bodie]

id say you are on the right track with the master cyl or prop valve unless the booster is somhow acting up try diving it with the booster unhooked (vaccuum).

[Musicman]

It sounds as though you have a Residual Pressure Valve installed in the Front Brake outlet of your master cylinder. This would cause your brakes to drag and eventually freeze up. This would be extremely odd however, unless your vehicle was once equipped with drum brakes front and rear, and someone converted it to disc brakes in the front and forgot to remove the valve.

[Nacho-RT74]

I had similar stuff on a 93 Buick Regal of a friend of mine and ended being hoses clogged

[Charger1973]

I think i'll just replace the master cylinder first and see what happens.  It seems like the most logical source of my problems...  And it definately wont hurt anything to get a new one in there. 

It sounds as though you have a Residual Pressure Valve installed in the Front Brake outlet of your master cylinder. This would cause your brakes to drag and eventually freeze up. This would be extremely odd however, unless your vehicle was once equipped with drum brakes front and rear, and someone converted it to disc brakes in the front and forgot to remove the valve.

In 73 they came standard with front discs so I dont think that would be the case. 

[Steve P.]

Before changing anything you can make a check in your driveway.

Jack up the front end and put it on stands. This will allow you to turn the front rotors by hand.

No need to start the car up, just have someone turn the wheels and you hit the brakes. Let off and see if the brakes release. You may have to do this a few times to get them to stick. Once the wheels will NOT turn loosen up the master cylinder a few turns on every nut so that the master is loose and see if the wheels spin free again..

If the wheels loosen right up your push rod is too long.. I have seen this on cars that have had conversions from drum to disc and also from power to non-power. 

Good luck...

[Charger1973]

Thanks I will try that and i'll try the vacuum hose on the booster too.   The car still has the original power disc brakes though so I dont think theres anything out of place but we will see...

[Steve P.]

Sorry, I understood it to have STANDARD, non-power brakes. I read that wrong... 

[Charger1973]

I didnt think about that before but I can see how that could have been confusing...     But I do have power brakes from the factory so thats why I think its a long shot that the push rod could be too long.  I did wonder if it was adjustable though and possibly out of adjustment? 

[John_Kunkel]

The next time the brakes lock up, loosen all four nuts that hold the MC to the booster several turns each and let the MC slide forward on the studs. If the brakes release, the booster pushrod is too long or something else is preventing the rear piston in the MC from fully returning.

[Musicman]

Just for squirts and wiggles, you might also want to check to make sure that someone didn't cross up the lines from the master cylinder. Make sure that the cylinder output for the Front brakes (the one closest to the firewall) is actually going to the front brakes and not the back. The cylinder output for the Back brakes (the forward one) probably has a 10psi residual valve in it which would cause the problems you described. Someone could have crossed them up in the past, not knowing that there was a difference.

[chgr500]

Is the brake pedal moving freely..up at the bushing area....maybe sticking and not releasing fully...has this car always had this problem....my brake pedal becomes stiff after winter storage and with some lube seems to move easier.....just a thought

[Charger1973]

Ill try loosening the master cylinder and see what happens before I actually remove it but Im planning to go ahead and replace it either way now. 

My brake pedal seems fine, I have never noticed a problem with it.  This car is still fairly new to me though so im still working the bugs out.  I picked it up in March and havent really driven it more than a few times, just worked on it non stop.

[Charger1973]

Just for squirts and wiggles, you might also want to check to make sure that someone didn't cross up the lines from the master cylinder. Make sure that the cylinder output for the Front brakes (the one closest to the firewall) is actually going to the front brakes and not the back. The cylinder output for the Back brakes (the forward one) probably has a 10psi residual valve in it which would cause the problems you described. Someone could have crossed them up in the past, not knowing that there was a difference.

Thats a good idea but im pretty sure that isnt the case because I took note of the fluid levels while bleeding them out and they are hooked up right.