I added power brakes to my car using a 80’s mustang booster, a dual master from an 80’s ford pickup and an adjustable proportioning valve. The car stops great but the brakes drag sometimes. I was extremely precise when setting up the adjustable pushrod but still can’t entirely rule that out. all parts are new within a few years, no used parts. It only seems to happen with the engine running. After driving you can feel the brakes start to drag. If you shut the car down the pressure seems to release over time. if I pull up on the brake pedal with the car running it releases the brakes. I can’t get it to do this with the engine off. I can pump the brakes 100 times with the engine not running and it the brakes release every time. where do I start?
Drive it until they drag, jack it up and see if it's all four wheels. If all four then check the pushrod from the pedal to the booster, and also check the adjustment on the pushrod in the booster to the master cylinder.
I had that problem. Pushrod to close, holding the MC open just slightly. Backed it off so there was the ever so slight gap between the pushrod and MC bore and that fixed it.
Yes, an overly tight pushrod is the most common cause of this problem, but on rare occasions a faulty master cylinder can have the same effect. Anything that blocks the rearmost port in the cylinder when the pedal is released will do it. Usually the pushrod, but dirt, a swollen cup, a burr in the cylinder can all cause this.
I had a 64 Lincoln that was a nice car, but hard on brakes. The shoes wore out in about 10K miles. One day while drinking beer at a buddy's house, I glanced at the Lincoln and the brake lights came on. It was parked with the engine off. That lit a bulb in my mind that expained the premature brake wear. The booster, original to the car, would apply the brakes slightly on it's own from time to time. Changed out the booster and never relined the brakes for the rest of the time I had the car.
I did a-little more trouble shooting this morning. I sat in the car in the driveway where there is a slight slope. In neutral the car would roll by itself. I pumped the brakes 25 times and then the car wouldn’t roll. I could get it to move by pushing but it was really hard to push and it would stop rolling when i stopped pushing. I pulled up on the brake pedal and felt a click and the car rolled away down the driveway without pushing it.
DON HAY for the WIN!!!!! First you either need to install a brake pedal return spring or install a stronger one and then check and adjust the pedal free play.
From my mind to your keyboard. LOL The original poster mentions in the opening post that manually lifting the pedal cures the "drag". By the time I read through the posts, my plan to mention the spring had already been suggested. There might be a minor snag in the pivot (rod, cylinder, ?) that needs addressing too. That's probably why it's just short of a full 'return' and the "click".
Any booster push rod adjustments to primary master cylinder piston should be done with full (approx 20 in/Hg) vacuum applied to the booster, either with the vehicle's engine or alternative vac source.
Not long ago I worked on something similar. Pulling up the pedal fixed the brake drag. Found the cotter pin had been bent over the rod pivot and was hanging up just enough to keep the return spring from pulling things back completely. Cut the long tails off the pin and all is well.
As usual I agree with the HAMB member’s suggestion, I had actually considered a return spring but didn’t want to suggest that myself, I wanted to hear it from others and as usual you all delivered! there is no spring on the pedal itself, the spring in the booster pushes the pedal back. Before I did this test I disconnect the pedal from the booster and checked that the pedal moved freely, and I lubricated the pivot where the booster attaches to the pedal arm when I reassembled it. As I type this I’m thinking that the brake light switch I’m using attaches to the pivot where the booster connects to the pedal arm. It could be binding the pedal and pushrod. U didn’t think to look if the brake light was staying on when the brakes were dragging.
Follow-up I added a return spring and that didn’t seem to help. I also replaced the brake light switch which mounts where the pedal arm and pushrod attach and that also made no difference. the problem appeared to get worse when the engine was hot. The master cylinder is mounted above the headers so that pointed to a problem with the master. I added the biggest spring I had and that didn’t help at all, it just made the pedal harder to push. Next I pulled the master off and took it apart. The piston did not move freely in the bore. I clamped the master in the vice and pushed the piston in with a screwdriver and it did not return all the way. I’ve installed a new master cylinder and I’ll test drive tonight after work.
Why didn't you use the Mustang master cylinder ? I don't recall seeing any cars or trucks after the 70's or so that had a return spring on the brake pedal from the factory. A;; the springs in the components return things to normal when the pressure is released.
The mustang master cylinder was not sized correctly to work with my calipers and wheel cylinders. I found one from and 80’s ford truck that was the correct sized piston and as a bonus it had a cast body and steel reservoir top. It looks a lot better under hood than the mustang’s plastic reservoir. It appears the piston was binding in the bore and the spring inside the master cylinder was not strong enough to push it back all the way.
BPN 2472 - SPRING - BRAKE PEDAL RETRACTING It is merely an anti-rattle spring. It does not help the pedal return after brake application.
The saga continues. The brakes are still sticking. if I pull up on the pedal it releases the brakes. the pushrod in to the master cylinder does not lock in. It can only push, it doesn’t pull so it can’t pull the piston back. This leads me to conclude it is not a hydraulic issue. The pedal arm moves freely. The only thing left is the booster. Is there anything inside the booster that could stick?
Yes. The valve on the booster rod could be stuck so you always have vacuum on one side and atmospheric pressure on the other. As you realease the pedal the valve should allow the pressure to equalise but if it doesn’t then the pressure difference can cause a sticky pedal.
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