Hello all I have a 1974 Dodge D300 i bought this truck in the ongoing hope's of returning it back to it's former glory but I am having on going issue's with the Brake's. Ok when driving down the road after about 3- 5 mile's on a stright road using no brake's the brake's will slowly start to engane.Then completly lock to the rotor's to the point the motor will die from the strees to "push" itself. At this point i'm lost i have replaced the pad's, shoe's, Wheel cylinder's, Drum's, rotor's all line's front to rear top to bottom(Rubber/steel) the Master clyinder twice and Booster 3 time's, rebuilt the carb, changed the intake gasket and on and on i'm stumped my father is stummped and my friend's at the different dealer's are stummped. The brake fluid will leak out of the rubber seal on the top of the Master cylinder and the rear cup in the Master cylinder will suck dry. I've been told again and again that the booster is bad and sucking brake fluid into the carb as to why i had it rebuilt the mech said the hose from booster to carb had Brake fluid in it and the carb had brake fluid resido in it as well. So what are the odd's that the 3rd Booster/Master cylinder is bad and they are all from different part store's one napa 2 autozone 3 adavanced auto so any help or more info required just ask thank's for reading i'm still holding out hope but dad is really tired of it and ready to sell

Is this 4 wheel drums?

Front Disc Rear drum's and also i replaced the Cailper's on Driver's side 3 time's and Pass 2 time's aswell sorry forgot to metion that.

I would start to isolate things, if it starts to act up within a few miles you could set the entire rig on stands completely off the ground and run it at a higher rpm for a while in neutral or park and see if that alone causes brakes to start applying and dragging if you try to turn the wheels by hand, if all is good there then put it in gear and let it roll for a while and see which brake starts dragging you may have a brake assembly not aligned right or something along those lines.How does the booster vent? make sure that there is a vent to atmosphere I have seen this be a cause of headaches like you are explaining. Also make sure the booster "If vacuum operated" is hooked to a manifold vacuum port and not port vacuum.

We messed with the truck some today doing something like Wild and free suguested and It appers to be the Driverside front caliper that is sticking and only that one. I spoke to prevous owner and he said it started to stick a few year's back so he unhooked the rear metal hose going to the front brake's from the Master cylinder to avoid the problem as he tried to fix it aswell. He said he drove it with only rear brake's for many year's and did fine but, i wish to pull my camper with it on some accasion's to the local lake on vacation. So i need the front brake's The hose from the Booster goes down on to the back of the carb And not to the intake when placed on the intake vent the truck will die almost right away till pulled off. I am so confused with this truck And also not sure if it matter's but the truck came stock with a 440 4 barl BB and after the org owner blew it he placed a 360 2 barl SB in and left every thing else in place not sure if that affect's anything thought i'd metion it though thanks again for all ya'lls input and help well get her lined out before to long ;)

Is it possible you got a collapsed brake hose?

Well it could be but We did replace both rubber line's twice first set was stock replacement's the other's i had custom made at local napa store and was made from a high strength hydrlolic hose to elimante the chance of colapase but i i will check that tomorrow to see. See this is why i'm stummped i've changed the whole system almost twice over and even upgraded most part's with 0 sucess... But i appreicate all the info and advice so far

Just an idea and maybe completely wrong here. I was working on bleeding the brakes on a '39 chevy and had brake fluid coming out of the master cylinder from the "pedal stop assembly" which pushes on the piston. Rebuilt the master, still leaked, bought a new one and more of the same. It turns out that when we bled the brakes we were pushing the pedal too far past a port to the reservoir which allowed brake fluid past a secondary seal and it would leak out the pedal stop assembly boot. The solution we found is when bleeding the brakes we could only push the pedal about 3/4's of the way. This car does not have booster, it's all up to the driver and big wheel cylinders.So what I was thinking was this maybe your problem and brake fluid is getting in the booster and causing problems.

Just an idea and maybe completely wrong here. I was working on bleeding the brakes on a '39 chevy and had brake fluid coming out of the master cylinder from the "pedal stop assembly" which pushes on the piston. Rebuilt the master, still leaked, bought a new one and more of the same. It turns out that when we bled the brakes we were pushing the pedal too far past a port to the reservoir which allowed brake fluid past a secondary seal and it would leak out the pedal stop assembly boot. The solution we found is when bleeding the brakes we could only push the pedal about 3/4's of the way. This car does not have booster, it's all up to the driver and big wheel cylinders.

So what I was thinking was this maybe your problem and brake fluid is getting in the booster and causing problems.

Thanks for that reminder, I have also ran into this when I worked at a truck shop years ago. It actually happened several times on older rigs.

Thanks for the idea ABennin will have to try that also to add i had totaly forgot this till i ran all this info by my dad today that the brake pedal is sticking you have to pull it up with your foot after you press it down even while bleeding it. It will not just come back even when just sitting not running press it down will stick right where you stop and not come back till pulled. thank's again all hopefully after the last 3 days of snow and rain and the next three days of it when it warm's up i can get to work testing these new idea's thanks again all

Hope that works for ya. The pedal in the car would stick too when all the way to the floor when bleeding. We too got dumped on with 15 inches of wet snow.